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Māori have worse health outcomes than their European counterparts.[[1–3]] Ethnic inequalities which exist in Aotearoa New Zealand, particularly those between Māori and non-Māori, are reported consistently.[[4]] Reasons for these inequalities are multifactorial and complex, but likely reflect the consequences of colonisation that continue to affect Māori health.[[5]] There are differences in access to healthcare, and in the quality of healthcare delivery, for Māori compared to non-Māori.[[6]] There are also differences in broader societal issues that affect health, including housing, education, employment and socioeconomic status.[[4]] Despite such complexities, understanding the nature of inequalities within their own area of the health system is the first step for clinicians when developing and implementing system changes to improve health outcomes for Māori.

Māori made up 15.6% of the New Zealand population in 2013. However, the proportion of patients admitted to New Zealand intensive care units (ICUs) who are Māori has not been reported. Within ICUs, highly specialised nursing and medical teams provide healthcare for immediately life-threatening but potentially treatable acute medical and surgical conditions. ICU staff also provide post-operative care for major surgery, particularly cardiac surgery. New Zealand has a network of ICUs, including large ICUs in major cities as well as regional and rural ICUs associated with smaller hospitals.[[3]] It is unknown whether the outcomes of Māori and European ICU patients admitted to New Zealand ICUs differ. For ICU patients, illness severity, based on the degree of physiological derangement, as well as age, sex and chronic comorbidities, are key determinants of outcome.[[7]] An emergency ICU admission is generally associated with higher mortality risk than an elective post-surgical admission, and the ICU admission diagnosis is also an important outcome predictor.[[7]] In addition, outcomes for otherwise similar ICU patients often vary depending on the hospital they are admitted to.[[7]] Although a high status of deprivation does not appear to be associated with increased in-hospital mortality, it is associated with longer-term mortality[[8]] and is an important confounding variable to consider in an analysis evaluating outcomes by ethnicity.

In this study, we sought to describe outcomes for Māori and European patients admitted to New Zealand ICUs between 2009 and 2018. We undertook a series of analyses adjusting for potential confounders with a view to describing possible contributors to observed inequalities in health outcomes.

Methods

Study design and setting

We undertook a retrospectively designed prospective cohort study using data from the New Zealand Ministry of Health National Minimum Dataset (NMD) matched to the Australian New Zealand Intensive Care Society Centre for Outcome and Resource Evaluation Adult Patient Database (ANZICS CORE APD). This study was submitted to the Health and Disability Ethics Committee of New Zealand (20/CEN/86) and deemed out of scope due to minimal risk. The NMD is a centralised data collection system containing all New Zealand hospital admissions. The NMD is organised using patients’ National Health Index numbers and administered by the New Zealand Ministry of Health.[[9]] The ANZICS CORE APD is an established bi-national voluntary ICU registry, which has been described previously.[[10]]

We used data relating to ICU admissions to New Zealand hospitals from 1 July 2009 until June 30 2018 inclusive and focused on admissions to the 17 ICUs in 16 hospitals that contributed to the ANZICS CORE APD throughout the period of interest. These included major tertiary, regional and rural public hospital ICUs in New Zealand.

Patients

All patients aged 18 years or older who were admitted to one of the 17 participating New Zealand ICUs were eligible for inclusion. Matching of patients included in the two databases of interest was performed based on six variables that were common to both databases. These variables were the name of the admission hospital, the date of hospital admission, the date of hospital discharge, age, sex and the in-hospital mortality. To account for situations where patients were transferred from one ICU to another, we linked ICU admission episodes where a particular patient was discharged from one ICU and then readmitted to another ICU on the same day. In these circumstances, descriptive baseline data were obtained from the first ICU admission and outcome data were obtained from the last ICU admission. Where a patient had multiple ICU admissions within the study period, only the first ICU admission the patient had was included. Accordingly, all ICU admissions included in our final dataset were from unique patients. Since illness severity is the key determinant of outcome for ICU patients,[[7]] we excluded patients where illness severity data were not available in the ANZICS CORE APD. This analysis focused on comparing the outcomes of Māori and European patients. For the purposes of this analysis, we defined patients whose ethnicity was coded as “NZ European,” “European not further defined” and “Other European” as European. Patients who were neither European nor Māori were excluded from the current analysis because we plan to report outcomes for other ethnic groups separately. We ascertained long-term mortality outcomes using data from the New Zealand death registry up until June 2020.

Outcomes

The primary outcome of interest for this study was day-180 mortality. Secondary outcomes were ICU mortality, hospital mortality, discharge to home, ICU length of stay, hospital length of stay and survival time.

Outcome predictors, potential confounders and effect modifiers

To explore the extent to which Māori ethnicity is a predictor of adverse outcomes in the New Zealand healthcare system, we compared ethnic groupings of Māori and European using “prioritised” ethnicity classification, where each patient is allocated to a single ethnic group using prioritisation tables as used in the New Zealand Ministry of Health NMD.[[11]]

We specified a number of variables as potential confounders of the relationship between ethnicity and outcome. These variables were deprivation status, age, sex, site of admission, year of admission, chronic comorbidities as measured using the Charlson comorbidity index,[[12]] admission diagnosis, admission type (elective versus emergency) and source (operating theatre, emergency department, ward, transfer from another hospital and unknown), ventilation status and illness severity. Deprivation status was defined using the New Zealand Index of Deprivation (NZDep), as included in the New Zealand Ministry of Health NMD.[[13]] This NZDep uses data associated with postcodes obtained from the 2013 census as a surrogate for the deprivation status of individual patients. The NZDep categorises patients into deciles, from 1 (least deprived) to 10 (most deprived). Age and year of admission were calculated at the date of hospital admission. Age, sex, site of admission and year of admission we included in both study databases. However, when minor date discrepancies were encountered during merging, we used data obtained from the ANZICS CORE APD for reporting purposes. The Charlson comorbidity index[[12]] was calculated using pre-existing comorbidities based on ICD-10 codes included in the New Zealand Ministry of Health NMD as previously described.[[14]] To obtain a measure of illness severity that was independent of other potential confounders, we evaluated illness severity using only the physiological parameters included in the Acute Physiology and Chronic Health Evaluation (APACHE) III score.[[15]]

Statistical methods

All baseline characteristics were summarised by ethnic group using means and standard deviations for normally distributed variables, medians and interquartile ranges for other continuous variables, and counts and percentages for categorical variables. Comparisons of baseline variables by ethnic group were undertaken using Student’s t-test for normally distributed variables, and Wilcoxon rank sum tests otherwise. Categorical variables were compared using chi-square tests for equal proportions.

For outcome comparisons, we evaluated the association between Māori ethnicity and outcome using European ethnicity as the reference category. For the four binomial outcomes (day-180 mortality, ICU mortality, hospital mortality and discharge home) we used logistic regression and adjusted for known covariates and baseline imbalance. These variables included: site, deprivation status, sex, year of admission, Charlson comorbidity index,[[12]] age, admission source and type, admission diagnosis, ventilation status and illness severity. To ascertain the individual impact of each covariate, we fitted these sequentially, with the resulting risk for Māori ethnicity (versus European) reported as odds ratios (OR), with a 95% confidence interval for each stage. An OR of more than one corresponded to a greater risk of an adverse outcome for Māori compared to their European counterparts. ICU and hospital length of stay were evaluated using a competing risk analysis adjusted for the competing risk of death, using an analogous approach to that described above with results reported at each stage as hazard ratios (HR), along with 95% CI and presented as cumulative incidence plots. For these analyses, a HR of greater than one corresponds to a shorter time to discharge alive for Māori compared to their European counterparts. Survival time to 180 days was compared using Cox Proportion Hazards regression in accordance with the approach described above, with results reported as HRs (95%CI) for each stage of model development. Proportionality assumptions for ethnicity were visually assessed using log-cumulative hazard plots. For the survival analysis, a hazard ratio of more than one corresponds to a worse outcome for Māori compared to their European counterparts because it equates to a shorter time to death.

Analyses were conducted using SAS statistical software, version 9.4 (SAS Institute).

Results

Patients

A total of 52,552 patients from 17 ICUs were included in this study (Figure 1). (The ICUs that contributed data are listed in the acknowledgments section.) A comparison of Māori and European patients in the New Zealand Ministry of Health NMD who could be matched to the ANZICS CORE APD with those could not be matched to the ANZICS CORE APD is shown in Supplementary Table 1. Of the patients included in this analysis, 9,681 (18.4%) were Māori. When patients from other (non-European) ethnic groups were included, Māori made up 15.6% of all ICU admissions. Compared to European patients, Māori patients were on average 13 years younger at ICU admission. They were more often female, had higher deprivation status, were more likely to be admitted to ICU in an emergency, and had more severe illness. (Table 1). Māori also had higher rates of some comorbidities, most notably diabetes and renal disease, and were more likely to have a Charlson comorbidity index of three or more than European patients (Table 1). Additional data on comorbidities for Māori and European patients are shown in Supplementary Table 2. Deciles of deprivation by ethnicity are shown in Supplementary Table 3. Data on admission site and year by ethnicity are shown in Supplementary Table 4.

Compared to European patients, Māori were markedly more likely to be admitted to the ICU after trauma, with sepsis, with neurological disorders and with metabolic disorders, such as complications of diabetes (Table 2).

Figure 1: Flow diagram. View Figure 1.

Table 1: Baseline characteristics by ethnicity. View Table 1.

Table 2: ICU admission diagnostic categories by ethnicity.

Primary outcome

A total of 1,550 Māori (16.0%) and 6,407 (14.9%) European patients had died within 180 days of ICU admission (OR: 1.08, 95% CI, 1.02 to 1.15) (Figure 2, Table 3 and Supplementary Table 5). The effect of sequentially adjusting for potential confounding variables is shown in Table 3. Deprivation status, sex, year of admission, diagnosis and whether the patient was ventilated did not appear to be important confounders of the association between Māori ethnicity and day-180 mortality. Site had some effect, but the strongest confounder was age. Inclusion of age in the regression model increased the OR for day-180 mortality for Māori versus European patients substantially. Adjustment for the Charlson comorbidity index and for illness severity both reduced the OR for day-180 mortality for Māori versus European patients. In the final model incorporating adjustment for all specified variables, Māori ethnicity was not associated with day-180 mortality (OR 1.01, 95% CI 0.92 to 1.10).

Table 3: Key outcomes by ethnicity. View Table 3.

Figure 2: Kaplan-Meier survival plot.*

* The unadjusted hazard ratio for survival to day 180 for Māori vs European patients was 1.08 (95% CI, 1.02 to 1.14), with a hazard ratio of more than one corresponding to a worse outcome for Māori compared to their European counterparts because it equates to a shorter time to death. Data showing hazard ratios adjusted for potential confounding variables are shown in the Supplementary Table 5.

Secondary outcomes

A total of 702 Māori (7.3%) and 2,525 (5.9%) European patients died prior to ICU discharge (OR: 1.25, 95% CI, 1.15 to 1.36), with 1,103 (11.4%) and 4,662 (10.9%) respectively dying prior to hospital discharge (OR: 1.05, 95% CI, 0.98 to 1.13). Similar proportions of Māori and European patients were discharged home (Table 3). For these outcomes, the effect of confounding variables on the associations between Māori ethnicity and outcome was similar to that observed when evaluating the relationship between Māori ethnicity and day-180 mortality, with age observed to be consistently the strongest confounder (Table 3). Site (ie, the ICU the patient was admitted to) appeared to be an important confounder of the association between Māori ethnicity and ICU mortality (Table 3). The ICU length of stay was a median of 1.5 days (IQR, 0.8–2.9 days) and 1.2 days (IQR, 0.9–2.8 days) for Māori and European patients respectively (Supplementary Figure 1). The median hospital length of stay was 7.5 days (IQR, 4.0–14.1 days) and 8.3 days (IQR, 5.1–15.0) for Māori and European patients respectively (Supplementary Figure 2). Age was the strongest confounding variable when evaluating the association between Māori ethnicity and ICU and hospital length of stay (Supplementary Table 6). In the final models, which adjusted for all specified confounders, Māori ethnicity was not an independent predictor of ICU mortality, in-hospital mortality, discharge home, ICU or hospital length of stay or survival time (Table 3, Supplementary Table 5 and Supplementary Table 6.)

Discussion

Statement of principal findings

In this retrospectively designed prospective cohort study, we compared the outcomes of Māori and European patients admitted to New Zealand ICUs between 1 July 2009 and 30 June 2018. We observed that Māori were more likely than European to die within 180 days of ICU admission. In particular, when accounting for the fact they were on average 13 years younger at ICU admission, Māori patients had a markedly higher risk of death and of other adverse outcomes. This increased risk of adverse outcomes for Māori patients appeared, in part, to be accounted for by Māori having more comorbidities, being more likely to be admitted to ICU in an emergency and having more severe illness than European patients. Deprivation status, sex, year of admission, diagnosis and whether the patients were ventilated did not appear to be important confounders of the associations between Māori ethnicity and outcomes.

Relationship to previous studies

We have previously reported that the outcomes for Māori patients enrolled in a large scale randomised controlled trial conducted in the ICU were similar to those of European patients,[[17]] and outcomes were also similar by ethnicity in a single ICU during the 2020 COVID-19 lockdown.[[18]] However, these populations are unlikely to be representative of the patients who receive ICU care in New Zealand. This study is the first large-scale national study to compare outcomes for Māori and European patients admitted to New Zealand ICUs. Our findings are consistent with a recent single-centre study conducted at Waikato Hospital that reported Māori were more likely than non-Māori to be admitted to ICU with sepsis and after major trauma.[[19]] Previous studies have also highlighted the high burden of serious infections[[20]] and diabetes[[21]] among Māori patients hospitalised in New Zealand. Our findings are similar to those of an Australian study, where adjusted long-term mortality and median number of potential life years lost after ICU admission were higher for Indigenous than non-Indigenous patients.[[22]] Our finding that Māori have higher illness severity at ICU admission than European patients has parallels with a previous study, where high illness acuity was observed in Indigenous Australians requiring ICU admission.[[23]]

Implications of study findings

Despite their younger age, Māori have more comorbidities and more severe illness than their European counterparts. Barriers to accessing intensive care for Māori might potentially contribute their higher illness severity by the time of ICU admission. However, the observation that Māori ICU patients are much younger than European ICU patients and yet still have more chronic comorbidities implies that an unequal burden of underlying conditions is one contributor to inequality in ICU outcomes.

Strengths and weaknesses

Our study had a number of strengths. It included data from more than 50,000 patients admitted to all major tertiary, regional and rural public hospital ICUs over a 10-year period. The only ICUs that were not included in this study were private ICUs and some small ICUs in rural and regional centres. We conducted analyses that adjusted for important variables that might potentially contribute to health inequalities in Māori patients admitted to the ICU including deprivation status and comorbid conditions. We were also able to conduct analyses that included robust adjustment for illness severity.

Our study had some limitations. Despite its large size, it only included the subset of ICU admission episodes for which we we could match patient data from the New Zealand Ministry of Health NMD to the ANZICS CORE APD. Accordingly, the patient group we studied is not representative of all patients admitted to New Zealand ICUs. We used the New Zealand Ministry of Health NMD prioritised ethnicity categories to define ethnic groups and cannot preclude the possibility that different methods for categorising ethnicity would result in different findings. Our method of categorisation of deprivation was based on data related to the post codes of patients’ residences obtained from the 2013 census. Although our study did include data from ICU admissions in 2013, it is possible that the reliability of the categorisation of deprivation may be lower for patients admitted in other years. Although we captured deaths that occurred beyond hospital discharge, we only captured those deaths that were registered in New Zealand. It is possible some patients died overseas within 180 days of an ICU admission. We choose day-180 mortality as the primary end point for this study, as deaths occurring as a consequence of an acute illness episode beyond this point are rare.[[24]] However, our findings may have been different if we had evaluated mortality rates at a different time point. Although we undertook analyses that adjusted for ICU illness severity, data on illness severity at the time of hospital admission were not available and are likely to be a key determinant of mortality risk.

Conclusions

Compared to European patients, Māori were markedly more likely to be admitted to the ICU after trauma or with sepsis. Despite Māori being on average 13 years younger at ICU admission than their European counterparts, they had more co-morbidities, higher illness severity and a higher risk of dying within 180 days of an ICU admission.

Supplementary material

View Supplementary material.

  • Supplementary Table 1: A comparison of matched versus unmatched patients.*
  • Supplementary Table 2: Additional details of comorbidities by ethnicity.*
  • Supplementary Table 3: Deprivation categories by ethnicity.*
  • Supplementary Table 4: Admission site and year by ethnicity.
  • Supplementary Table 5: Cox Proportional Hazards Regression for survival to day 180.
  • Supplementary Table 6: ICU length of stay and hospital length of stay by ethnicity.*
  • Supplementary Figure 1: Time to ICU discharge by ethnicity category.
  • Supplementary Figure 2: Time to hospital discharge by ethnicity category.

Summary

Abstract

Aim

To describe characteristics and outcomes of Māori and European patients admitted to New Zealand intensive care units (ICUs) between 2009 and 2018.

Method

A retrospectively designed prospective cohort study. New Zealand Ministry of Health National Minimum Dataset matched to the Australia New Zealand Intensive Care Society Centre for Outcome and Resource Evaluation Adult Patient Database. The primary outcome was day-180 mortality. Secondary outcomes were ICU mortality, hospital mortality, discharge to home, ICU length of stay, hospital length of stay and survival time. We report associations between Māori ethnicity and each outcome, with European as the reference category, using regression analyses to adjust sequentially for site, deprivation status, sex, year of admission, the Charlson comorbidity index, age, admission source and type, ICU admission diagnosis, ventilation status and illness severity based on physiological parameters.

Results

Māori admitted to ICU were on average 13 years younger than European patients. A total of 968 of 9,681 (10%) Māori and 2,732 of 42,871 (5.2%) European patients were admitted after trauma, and 740 of 9,681 (7.6%) and 2,318 of 42,871 (4.4%) were admitted with sepsis respectively. A total of 1,550 of 9,681 (16.0%) Māori and 6,407 of 42,871 (14.9%) European patients died within 180 days of ICU admission; odds ratio (OR) 1.08; 95% CI, 1.02 to 1.15. When adjusted for age, the OR for day-180 mortality for Māori versus European patients increased substantially. The OR decreased after adjustment for admission source and type, and after accounting for Māori having a higher comorbidity index and more severe illness than European patients. In the final model, incorporating adjustment for all specified variables, Māori ethnicity was not associated with day-180 mortality (adjusted OR 1.01; 95%CI, 0.92 to 1.10). Findings were similar for all secondary outcomes.

Conclusion

Compared to European patients, Māori were markedly more likely to be admitted to the ICU after trauma or with sepsis. Despite Māori being on average 13 years younger at ICU admission than their European counterparts, they had more co-morbidities, higher illness severity and a higher risk of dying within 180 days.

Author Information

Alice L Reid: Research Fellow, Medical Research Institute of New Zealand, Wellington. Michael Bailey: Statistician. Australian and New Zealand Intensive Care Research Centre, Monash University, Melbourne, Victoria, Australia; Statistician University of Melbourne, Parkville, Victoria, Australia. Matire Harwood: Māori Health Programme Director, Medical Research Institute of New Zealand, Wellington, New Zealand; Associate Professor, University of Auckland, Auckland, New Zealand. James E Moore: Intensive Care Specialist, Wellington Hospital Intensive Care Unit, Wellington Hospital, Wellington, New Zealand; Senior Research Fellow, Medical Research Institute of New Zealand, Wellington, New Zealand. Paul J Young: Deputy Director, Medical Research Institute of New Zealand, Wellington, New Zealand; Co-clinical leader, Intensive Care Unit, Wellington Hospital, Wellington; Associate Professor, University of Melbourne, Melbourne, Victoria, Australia.

Acknowledgements

We acknowledge the ANZICS Centre for Outcome and Resource Evaluation (ANZICS CORE), who provided the data for this study and the staff from Christchurch Hospital ICU, Dunedin Hospital ICU, Tauranga Hospital ICU, Wellington Regional Hospital ICU, Southland Hospital ICU, Taranaki Health ICU, Timaru Hospital ICU, Hawke’s Bay Hospital ICU, Nelson Hospital ICU, Waikato Hospital ICU, Rotorua Hospital ICU, Whangārei Area Hospital ICU, Hutt Hospital ICU, Auckland City Hospital cardiac and vascular ICU, Auckland City Hospital Department of Critical Care Medicine, North Shore Hospital ICU and Middlemore Hospital ICU, who collected the data and submitted them to ANZICS CORE. We also acknowledge Chris Lewis who provided the data from the New Zealand Ministry of Health NMD for this study. This research was conducted during the tenure of a Health Research Council Clinical Practitioner Fellowship held by Paul Young. The Medical Research Institute of New Zealand is supported by independent research organisation funding from the Health Research Council of New Zealand.

Correspondence

Dr Young at the Intensive Care Unit, Wellington Hospital, Private Bag 7902, Wellington South, New Zealand.

Correspondence Email

paul.young@ccdhb.org.nz

Competing Interests

Nil.

1) Ministry of Health. 2019. Wai 2575 Māori Health Trends Report. Wellington: Ministry of Health.

2) Ministry of Health. 2015. Tatau Kahukura: Māori Health Chart Book 2015 (3rd edition). Wellington: Ministry of Health.

3) Reid P, Robson B. 2006. The state of Màori health. In: M Mulholland (ed) State of the Māori Nation: Twenty-First Century Issues in Aotearoa. Auckland: Reed A P.

4) Robson B, Harris R. (eds). 2007. Hauora: Māori Standards of Health IV. A study of the years 2000-2005. Wellington: Te Rōpū Rangahau Hauora a Eru Pōmare.

5) Reid P, Cormack D, Paine SJ. Colonial histories, racism and health-The experience of Maori and Indigenous peoples. Public Health. 2019;172:119-24.

6) Selak V, Rahiri JL, Jackson R, Harwood M. Acknowledging and acting on racism in the health sector in Aotearoa New Zealand. N Z Med J. 2020 Sep 4;133(1521):7-13.

7) Paul E, Bailey M, Pilcher D. Risk prediction of hospital mortality for adult patients admitted to Australian and New Zealand intensive care units: development and validation of the Australian and New Zealand Risk of Death model. J Crit Care. 2013;28:935-41.

8) Ho KM, Dobb GJ, Knuiman M, Finn J, Webb SA. The effect of socioeconomic status on outcomes for seriously ill patients: a linked data cohort study. Med J Aust. 2008;189:26-30.

9) National Health Board. 2014. National Minimum Dataset (Hospital Events) Data Dictionary. Wellington: Ministry of Health.

10) Hart GK, Outcomes ACF, Resources Evaluation Management C. The ANZICS CORE: an evolution in registry activities for intensive care in Australia and New Zealand. Crit Care Resusc. 2008;10:83-8.

11) Ministry of Health. HISO 10001:2017 Ethnicity Data Protocols. Wellington: Ministry of Health, 2017.

12) D'Hoore W, Bouckaert A, Tilquin C. Practical considerations on the use of the Charlson comorbidity index with administrative data bases. J Clin Epidemiol. 1996;49:1429-33.

13) Salmond CE, Crampton P. Development of New Zealand's deprivation index (NZDep) and its uptake as a national policy tool. Can J Public Health. 2012 May 9;103(8 Suppl 2):S7-11.

14) R [Internet]. [cited 2021 Mar 11]. Available from: https://cran.r-project.org/web/packages/comorbidity/vignettes/comorbidityscores.html.

15) Knaus WA, Wagner DP, Draper EA, et al. The APACHE III prognostic system. Risk prediction of hospital mortality for critically ill hospitalized adults. Chest. 1991;100:1619-36.

16) Pilcher D, Paul E, Bailey M, Huckson S. The Australian and New Zealand Risk of Death (ANZROD) model: getting mortality prediction right for intensive care units. Crit Care Resusc. 2014;16:3-4.

17) Reid AL, Chapman MJ, Peake SL, et al. Energy-dense vs routine enteral nutrition in New Zealand Europeans, Maori, and Pacific Peoples who are critically ill. N Z Med J. 2020;133:72-82.

18) Young PJ, Gladwin B, Psirides A, Reid A. Unplanned admissions to the Wellington Hospital intensive care unit before, during and after New Zealand's COVID-19 lockdown. N Z Med J. 2020;133:95-103.

19) Slim MAM, Lala HM, Barnes N, Martynoga RA. Maori health outcomes in an intensive care unit in Aotearoa New Zealand. Anaesth Intensive Care. 2021;49:292-300.

20) Baker MG, Barnard LT, Kvalsvig A, et al. Increasing incidence of serious infectious diseases and inequalities in New Zealand: a national epidemiological study. Lancet. 2012;379:1112-9.

21) Yu D, Zhao Z, Osuagwu UL, et al. Ethnic differences in mortality and hospital admission rates between Maori, Pacific, and European New Zealanders with type 2 diabetes between 1994 and 2018: a retrospective, population-based, longitudinal cohort study. Lancet Glob Health. 2021;9:e209-e17.

22) Mitchell WG, Deane A, Brown A, et al. Long term outcomes for Aboriginal and Torres Strait Islander Australians after hospital intensive care. Med J Aust. 2020;213:16-21.

23) Secombe P, Brown A, McAnulty G, Pilcher D. Aboriginal and Torres Strait Islander patients requiring critical care: characteristics, resource use, and outcomes. Crit Care Resusc. 2019;21:200-11.

24) Taori G, Ho KM, George C, et al. Landmark survival as an end-point for trials in critically ill patients--comparison of alternative durations of follow-up: an exploratory analysis. Crit Care. 2009;13:R128.

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Māori have worse health outcomes than their European counterparts.[[1–3]] Ethnic inequalities which exist in Aotearoa New Zealand, particularly those between Māori and non-Māori, are reported consistently.[[4]] Reasons for these inequalities are multifactorial and complex, but likely reflect the consequences of colonisation that continue to affect Māori health.[[5]] There are differences in access to healthcare, and in the quality of healthcare delivery, for Māori compared to non-Māori.[[6]] There are also differences in broader societal issues that affect health, including housing, education, employment and socioeconomic status.[[4]] Despite such complexities, understanding the nature of inequalities within their own area of the health system is the first step for clinicians when developing and implementing system changes to improve health outcomes for Māori.

Māori made up 15.6% of the New Zealand population in 2013. However, the proportion of patients admitted to New Zealand intensive care units (ICUs) who are Māori has not been reported. Within ICUs, highly specialised nursing and medical teams provide healthcare for immediately life-threatening but potentially treatable acute medical and surgical conditions. ICU staff also provide post-operative care for major surgery, particularly cardiac surgery. New Zealand has a network of ICUs, including large ICUs in major cities as well as regional and rural ICUs associated with smaller hospitals.[[3]] It is unknown whether the outcomes of Māori and European ICU patients admitted to New Zealand ICUs differ. For ICU patients, illness severity, based on the degree of physiological derangement, as well as age, sex and chronic comorbidities, are key determinants of outcome.[[7]] An emergency ICU admission is generally associated with higher mortality risk than an elective post-surgical admission, and the ICU admission diagnosis is also an important outcome predictor.[[7]] In addition, outcomes for otherwise similar ICU patients often vary depending on the hospital they are admitted to.[[7]] Although a high status of deprivation does not appear to be associated with increased in-hospital mortality, it is associated with longer-term mortality[[8]] and is an important confounding variable to consider in an analysis evaluating outcomes by ethnicity.

In this study, we sought to describe outcomes for Māori and European patients admitted to New Zealand ICUs between 2009 and 2018. We undertook a series of analyses adjusting for potential confounders with a view to describing possible contributors to observed inequalities in health outcomes.

Methods

Study design and setting

We undertook a retrospectively designed prospective cohort study using data from the New Zealand Ministry of Health National Minimum Dataset (NMD) matched to the Australian New Zealand Intensive Care Society Centre for Outcome and Resource Evaluation Adult Patient Database (ANZICS CORE APD). This study was submitted to the Health and Disability Ethics Committee of New Zealand (20/CEN/86) and deemed out of scope due to minimal risk. The NMD is a centralised data collection system containing all New Zealand hospital admissions. The NMD is organised using patients’ National Health Index numbers and administered by the New Zealand Ministry of Health.[[9]] The ANZICS CORE APD is an established bi-national voluntary ICU registry, which has been described previously.[[10]]

We used data relating to ICU admissions to New Zealand hospitals from 1 July 2009 until June 30 2018 inclusive and focused on admissions to the 17 ICUs in 16 hospitals that contributed to the ANZICS CORE APD throughout the period of interest. These included major tertiary, regional and rural public hospital ICUs in New Zealand.

Patients

All patients aged 18 years or older who were admitted to one of the 17 participating New Zealand ICUs were eligible for inclusion. Matching of patients included in the two databases of interest was performed based on six variables that were common to both databases. These variables were the name of the admission hospital, the date of hospital admission, the date of hospital discharge, age, sex and the in-hospital mortality. To account for situations where patients were transferred from one ICU to another, we linked ICU admission episodes where a particular patient was discharged from one ICU and then readmitted to another ICU on the same day. In these circumstances, descriptive baseline data were obtained from the first ICU admission and outcome data were obtained from the last ICU admission. Where a patient had multiple ICU admissions within the study period, only the first ICU admission the patient had was included. Accordingly, all ICU admissions included in our final dataset were from unique patients. Since illness severity is the key determinant of outcome for ICU patients,[[7]] we excluded patients where illness severity data were not available in the ANZICS CORE APD. This analysis focused on comparing the outcomes of Māori and European patients. For the purposes of this analysis, we defined patients whose ethnicity was coded as “NZ European,” “European not further defined” and “Other European” as European. Patients who were neither European nor Māori were excluded from the current analysis because we plan to report outcomes for other ethnic groups separately. We ascertained long-term mortality outcomes using data from the New Zealand death registry up until June 2020.

Outcomes

The primary outcome of interest for this study was day-180 mortality. Secondary outcomes were ICU mortality, hospital mortality, discharge to home, ICU length of stay, hospital length of stay and survival time.

Outcome predictors, potential confounders and effect modifiers

To explore the extent to which Māori ethnicity is a predictor of adverse outcomes in the New Zealand healthcare system, we compared ethnic groupings of Māori and European using “prioritised” ethnicity classification, where each patient is allocated to a single ethnic group using prioritisation tables as used in the New Zealand Ministry of Health NMD.[[11]]

We specified a number of variables as potential confounders of the relationship between ethnicity and outcome. These variables were deprivation status, age, sex, site of admission, year of admission, chronic comorbidities as measured using the Charlson comorbidity index,[[12]] admission diagnosis, admission type (elective versus emergency) and source (operating theatre, emergency department, ward, transfer from another hospital and unknown), ventilation status and illness severity. Deprivation status was defined using the New Zealand Index of Deprivation (NZDep), as included in the New Zealand Ministry of Health NMD.[[13]] This NZDep uses data associated with postcodes obtained from the 2013 census as a surrogate for the deprivation status of individual patients. The NZDep categorises patients into deciles, from 1 (least deprived) to 10 (most deprived). Age and year of admission were calculated at the date of hospital admission. Age, sex, site of admission and year of admission we included in both study databases. However, when minor date discrepancies were encountered during merging, we used data obtained from the ANZICS CORE APD for reporting purposes. The Charlson comorbidity index[[12]] was calculated using pre-existing comorbidities based on ICD-10 codes included in the New Zealand Ministry of Health NMD as previously described.[[14]] To obtain a measure of illness severity that was independent of other potential confounders, we evaluated illness severity using only the physiological parameters included in the Acute Physiology and Chronic Health Evaluation (APACHE) III score.[[15]]

Statistical methods

All baseline characteristics were summarised by ethnic group using means and standard deviations for normally distributed variables, medians and interquartile ranges for other continuous variables, and counts and percentages for categorical variables. Comparisons of baseline variables by ethnic group were undertaken using Student’s t-test for normally distributed variables, and Wilcoxon rank sum tests otherwise. Categorical variables were compared using chi-square tests for equal proportions.

For outcome comparisons, we evaluated the association between Māori ethnicity and outcome using European ethnicity as the reference category. For the four binomial outcomes (day-180 mortality, ICU mortality, hospital mortality and discharge home) we used logistic regression and adjusted for known covariates and baseline imbalance. These variables included: site, deprivation status, sex, year of admission, Charlson comorbidity index,[[12]] age, admission source and type, admission diagnosis, ventilation status and illness severity. To ascertain the individual impact of each covariate, we fitted these sequentially, with the resulting risk for Māori ethnicity (versus European) reported as odds ratios (OR), with a 95% confidence interval for each stage. An OR of more than one corresponded to a greater risk of an adverse outcome for Māori compared to their European counterparts. ICU and hospital length of stay were evaluated using a competing risk analysis adjusted for the competing risk of death, using an analogous approach to that described above with results reported at each stage as hazard ratios (HR), along with 95% CI and presented as cumulative incidence plots. For these analyses, a HR of greater than one corresponds to a shorter time to discharge alive for Māori compared to their European counterparts. Survival time to 180 days was compared using Cox Proportion Hazards regression in accordance with the approach described above, with results reported as HRs (95%CI) for each stage of model development. Proportionality assumptions for ethnicity were visually assessed using log-cumulative hazard plots. For the survival analysis, a hazard ratio of more than one corresponds to a worse outcome for Māori compared to their European counterparts because it equates to a shorter time to death.

Analyses were conducted using SAS statistical software, version 9.4 (SAS Institute).

Results

Patients

A total of 52,552 patients from 17 ICUs were included in this study (Figure 1). (The ICUs that contributed data are listed in the acknowledgments section.) A comparison of Māori and European patients in the New Zealand Ministry of Health NMD who could be matched to the ANZICS CORE APD with those could not be matched to the ANZICS CORE APD is shown in Supplementary Table 1. Of the patients included in this analysis, 9,681 (18.4%) were Māori. When patients from other (non-European) ethnic groups were included, Māori made up 15.6% of all ICU admissions. Compared to European patients, Māori patients were on average 13 years younger at ICU admission. They were more often female, had higher deprivation status, were more likely to be admitted to ICU in an emergency, and had more severe illness. (Table 1). Māori also had higher rates of some comorbidities, most notably diabetes and renal disease, and were more likely to have a Charlson comorbidity index of three or more than European patients (Table 1). Additional data on comorbidities for Māori and European patients are shown in Supplementary Table 2. Deciles of deprivation by ethnicity are shown in Supplementary Table 3. Data on admission site and year by ethnicity are shown in Supplementary Table 4.

Compared to European patients, Māori were markedly more likely to be admitted to the ICU after trauma, with sepsis, with neurological disorders and with metabolic disorders, such as complications of diabetes (Table 2).

Figure 1: Flow diagram. View Figure 1.

Table 1: Baseline characteristics by ethnicity. View Table 1.

Table 2: ICU admission diagnostic categories by ethnicity.

Primary outcome

A total of 1,550 Māori (16.0%) and 6,407 (14.9%) European patients had died within 180 days of ICU admission (OR: 1.08, 95% CI, 1.02 to 1.15) (Figure 2, Table 3 and Supplementary Table 5). The effect of sequentially adjusting for potential confounding variables is shown in Table 3. Deprivation status, sex, year of admission, diagnosis and whether the patient was ventilated did not appear to be important confounders of the association between Māori ethnicity and day-180 mortality. Site had some effect, but the strongest confounder was age. Inclusion of age in the regression model increased the OR for day-180 mortality for Māori versus European patients substantially. Adjustment for the Charlson comorbidity index and for illness severity both reduced the OR for day-180 mortality for Māori versus European patients. In the final model incorporating adjustment for all specified variables, Māori ethnicity was not associated with day-180 mortality (OR 1.01, 95% CI 0.92 to 1.10).

Table 3: Key outcomes by ethnicity. View Table 3.

Figure 2: Kaplan-Meier survival plot.*

* The unadjusted hazard ratio for survival to day 180 for Māori vs European patients was 1.08 (95% CI, 1.02 to 1.14), with a hazard ratio of more than one corresponding to a worse outcome for Māori compared to their European counterparts because it equates to a shorter time to death. Data showing hazard ratios adjusted for potential confounding variables are shown in the Supplementary Table 5.

Secondary outcomes

A total of 702 Māori (7.3%) and 2,525 (5.9%) European patients died prior to ICU discharge (OR: 1.25, 95% CI, 1.15 to 1.36), with 1,103 (11.4%) and 4,662 (10.9%) respectively dying prior to hospital discharge (OR: 1.05, 95% CI, 0.98 to 1.13). Similar proportions of Māori and European patients were discharged home (Table 3). For these outcomes, the effect of confounding variables on the associations between Māori ethnicity and outcome was similar to that observed when evaluating the relationship between Māori ethnicity and day-180 mortality, with age observed to be consistently the strongest confounder (Table 3). Site (ie, the ICU the patient was admitted to) appeared to be an important confounder of the association between Māori ethnicity and ICU mortality (Table 3). The ICU length of stay was a median of 1.5 days (IQR, 0.8–2.9 days) and 1.2 days (IQR, 0.9–2.8 days) for Māori and European patients respectively (Supplementary Figure 1). The median hospital length of stay was 7.5 days (IQR, 4.0–14.1 days) and 8.3 days (IQR, 5.1–15.0) for Māori and European patients respectively (Supplementary Figure 2). Age was the strongest confounding variable when evaluating the association between Māori ethnicity and ICU and hospital length of stay (Supplementary Table 6). In the final models, which adjusted for all specified confounders, Māori ethnicity was not an independent predictor of ICU mortality, in-hospital mortality, discharge home, ICU or hospital length of stay or survival time (Table 3, Supplementary Table 5 and Supplementary Table 6.)

Discussion

Statement of principal findings

In this retrospectively designed prospective cohort study, we compared the outcomes of Māori and European patients admitted to New Zealand ICUs between 1 July 2009 and 30 June 2018. We observed that Māori were more likely than European to die within 180 days of ICU admission. In particular, when accounting for the fact they were on average 13 years younger at ICU admission, Māori patients had a markedly higher risk of death and of other adverse outcomes. This increased risk of adverse outcomes for Māori patients appeared, in part, to be accounted for by Māori having more comorbidities, being more likely to be admitted to ICU in an emergency and having more severe illness than European patients. Deprivation status, sex, year of admission, diagnosis and whether the patients were ventilated did not appear to be important confounders of the associations between Māori ethnicity and outcomes.

Relationship to previous studies

We have previously reported that the outcomes for Māori patients enrolled in a large scale randomised controlled trial conducted in the ICU were similar to those of European patients,[[17]] and outcomes were also similar by ethnicity in a single ICU during the 2020 COVID-19 lockdown.[[18]] However, these populations are unlikely to be representative of the patients who receive ICU care in New Zealand. This study is the first large-scale national study to compare outcomes for Māori and European patients admitted to New Zealand ICUs. Our findings are consistent with a recent single-centre study conducted at Waikato Hospital that reported Māori were more likely than non-Māori to be admitted to ICU with sepsis and after major trauma.[[19]] Previous studies have also highlighted the high burden of serious infections[[20]] and diabetes[[21]] among Māori patients hospitalised in New Zealand. Our findings are similar to those of an Australian study, where adjusted long-term mortality and median number of potential life years lost after ICU admission were higher for Indigenous than non-Indigenous patients.[[22]] Our finding that Māori have higher illness severity at ICU admission than European patients has parallels with a previous study, where high illness acuity was observed in Indigenous Australians requiring ICU admission.[[23]]

Implications of study findings

Despite their younger age, Māori have more comorbidities and more severe illness than their European counterparts. Barriers to accessing intensive care for Māori might potentially contribute their higher illness severity by the time of ICU admission. However, the observation that Māori ICU patients are much younger than European ICU patients and yet still have more chronic comorbidities implies that an unequal burden of underlying conditions is one contributor to inequality in ICU outcomes.

Strengths and weaknesses

Our study had a number of strengths. It included data from more than 50,000 patients admitted to all major tertiary, regional and rural public hospital ICUs over a 10-year period. The only ICUs that were not included in this study were private ICUs and some small ICUs in rural and regional centres. We conducted analyses that adjusted for important variables that might potentially contribute to health inequalities in Māori patients admitted to the ICU including deprivation status and comorbid conditions. We were also able to conduct analyses that included robust adjustment for illness severity.

Our study had some limitations. Despite its large size, it only included the subset of ICU admission episodes for which we we could match patient data from the New Zealand Ministry of Health NMD to the ANZICS CORE APD. Accordingly, the patient group we studied is not representative of all patients admitted to New Zealand ICUs. We used the New Zealand Ministry of Health NMD prioritised ethnicity categories to define ethnic groups and cannot preclude the possibility that different methods for categorising ethnicity would result in different findings. Our method of categorisation of deprivation was based on data related to the post codes of patients’ residences obtained from the 2013 census. Although our study did include data from ICU admissions in 2013, it is possible that the reliability of the categorisation of deprivation may be lower for patients admitted in other years. Although we captured deaths that occurred beyond hospital discharge, we only captured those deaths that were registered in New Zealand. It is possible some patients died overseas within 180 days of an ICU admission. We choose day-180 mortality as the primary end point for this study, as deaths occurring as a consequence of an acute illness episode beyond this point are rare.[[24]] However, our findings may have been different if we had evaluated mortality rates at a different time point. Although we undertook analyses that adjusted for ICU illness severity, data on illness severity at the time of hospital admission were not available and are likely to be a key determinant of mortality risk.

Conclusions

Compared to European patients, Māori were markedly more likely to be admitted to the ICU after trauma or with sepsis. Despite Māori being on average 13 years younger at ICU admission than their European counterparts, they had more co-morbidities, higher illness severity and a higher risk of dying within 180 days of an ICU admission.

Supplementary material

View Supplementary material.

  • Supplementary Table 1: A comparison of matched versus unmatched patients.*
  • Supplementary Table 2: Additional details of comorbidities by ethnicity.*
  • Supplementary Table 3: Deprivation categories by ethnicity.*
  • Supplementary Table 4: Admission site and year by ethnicity.
  • Supplementary Table 5: Cox Proportional Hazards Regression for survival to day 180.
  • Supplementary Table 6: ICU length of stay and hospital length of stay by ethnicity.*
  • Supplementary Figure 1: Time to ICU discharge by ethnicity category.
  • Supplementary Figure 2: Time to hospital discharge by ethnicity category.

Summary

Abstract

Aim

To describe characteristics and outcomes of Māori and European patients admitted to New Zealand intensive care units (ICUs) between 2009 and 2018.

Method

A retrospectively designed prospective cohort study. New Zealand Ministry of Health National Minimum Dataset matched to the Australia New Zealand Intensive Care Society Centre for Outcome and Resource Evaluation Adult Patient Database. The primary outcome was day-180 mortality. Secondary outcomes were ICU mortality, hospital mortality, discharge to home, ICU length of stay, hospital length of stay and survival time. We report associations between Māori ethnicity and each outcome, with European as the reference category, using regression analyses to adjust sequentially for site, deprivation status, sex, year of admission, the Charlson comorbidity index, age, admission source and type, ICU admission diagnosis, ventilation status and illness severity based on physiological parameters.

Results

Māori admitted to ICU were on average 13 years younger than European patients. A total of 968 of 9,681 (10%) Māori and 2,732 of 42,871 (5.2%) European patients were admitted after trauma, and 740 of 9,681 (7.6%) and 2,318 of 42,871 (4.4%) were admitted with sepsis respectively. A total of 1,550 of 9,681 (16.0%) Māori and 6,407 of 42,871 (14.9%) European patients died within 180 days of ICU admission; odds ratio (OR) 1.08; 95% CI, 1.02 to 1.15. When adjusted for age, the OR for day-180 mortality for Māori versus European patients increased substantially. The OR decreased after adjustment for admission source and type, and after accounting for Māori having a higher comorbidity index and more severe illness than European patients. In the final model, incorporating adjustment for all specified variables, Māori ethnicity was not associated with day-180 mortality (adjusted OR 1.01; 95%CI, 0.92 to 1.10). Findings were similar for all secondary outcomes.

Conclusion

Compared to European patients, Māori were markedly more likely to be admitted to the ICU after trauma or with sepsis. Despite Māori being on average 13 years younger at ICU admission than their European counterparts, they had more co-morbidities, higher illness severity and a higher risk of dying within 180 days.

Author Information

Alice L Reid: Research Fellow, Medical Research Institute of New Zealand, Wellington. Michael Bailey: Statistician. Australian and New Zealand Intensive Care Research Centre, Monash University, Melbourne, Victoria, Australia; Statistician University of Melbourne, Parkville, Victoria, Australia. Matire Harwood: Māori Health Programme Director, Medical Research Institute of New Zealand, Wellington, New Zealand; Associate Professor, University of Auckland, Auckland, New Zealand. James E Moore: Intensive Care Specialist, Wellington Hospital Intensive Care Unit, Wellington Hospital, Wellington, New Zealand; Senior Research Fellow, Medical Research Institute of New Zealand, Wellington, New Zealand. Paul J Young: Deputy Director, Medical Research Institute of New Zealand, Wellington, New Zealand; Co-clinical leader, Intensive Care Unit, Wellington Hospital, Wellington; Associate Professor, University of Melbourne, Melbourne, Victoria, Australia.

Acknowledgements

We acknowledge the ANZICS Centre for Outcome and Resource Evaluation (ANZICS CORE), who provided the data for this study and the staff from Christchurch Hospital ICU, Dunedin Hospital ICU, Tauranga Hospital ICU, Wellington Regional Hospital ICU, Southland Hospital ICU, Taranaki Health ICU, Timaru Hospital ICU, Hawke’s Bay Hospital ICU, Nelson Hospital ICU, Waikato Hospital ICU, Rotorua Hospital ICU, Whangārei Area Hospital ICU, Hutt Hospital ICU, Auckland City Hospital cardiac and vascular ICU, Auckland City Hospital Department of Critical Care Medicine, North Shore Hospital ICU and Middlemore Hospital ICU, who collected the data and submitted them to ANZICS CORE. We also acknowledge Chris Lewis who provided the data from the New Zealand Ministry of Health NMD for this study. This research was conducted during the tenure of a Health Research Council Clinical Practitioner Fellowship held by Paul Young. The Medical Research Institute of New Zealand is supported by independent research organisation funding from the Health Research Council of New Zealand.

Correspondence

Dr Young at the Intensive Care Unit, Wellington Hospital, Private Bag 7902, Wellington South, New Zealand.

Correspondence Email

paul.young@ccdhb.org.nz

Competing Interests

Nil.

1) Ministry of Health. 2019. Wai 2575 Māori Health Trends Report. Wellington: Ministry of Health.

2) Ministry of Health. 2015. Tatau Kahukura: Māori Health Chart Book 2015 (3rd edition). Wellington: Ministry of Health.

3) Reid P, Robson B. 2006. The state of Màori health. In: M Mulholland (ed) State of the Māori Nation: Twenty-First Century Issues in Aotearoa. Auckland: Reed A P.

4) Robson B, Harris R. (eds). 2007. Hauora: Māori Standards of Health IV. A study of the years 2000-2005. Wellington: Te Rōpū Rangahau Hauora a Eru Pōmare.

5) Reid P, Cormack D, Paine SJ. Colonial histories, racism and health-The experience of Maori and Indigenous peoples. Public Health. 2019;172:119-24.

6) Selak V, Rahiri JL, Jackson R, Harwood M. Acknowledging and acting on racism in the health sector in Aotearoa New Zealand. N Z Med J. 2020 Sep 4;133(1521):7-13.

7) Paul E, Bailey M, Pilcher D. Risk prediction of hospital mortality for adult patients admitted to Australian and New Zealand intensive care units: development and validation of the Australian and New Zealand Risk of Death model. J Crit Care. 2013;28:935-41.

8) Ho KM, Dobb GJ, Knuiman M, Finn J, Webb SA. The effect of socioeconomic status on outcomes for seriously ill patients: a linked data cohort study. Med J Aust. 2008;189:26-30.

9) National Health Board. 2014. National Minimum Dataset (Hospital Events) Data Dictionary. Wellington: Ministry of Health.

10) Hart GK, Outcomes ACF, Resources Evaluation Management C. The ANZICS CORE: an evolution in registry activities for intensive care in Australia and New Zealand. Crit Care Resusc. 2008;10:83-8.

11) Ministry of Health. HISO 10001:2017 Ethnicity Data Protocols. Wellington: Ministry of Health, 2017.

12) D'Hoore W, Bouckaert A, Tilquin C. Practical considerations on the use of the Charlson comorbidity index with administrative data bases. J Clin Epidemiol. 1996;49:1429-33.

13) Salmond CE, Crampton P. Development of New Zealand's deprivation index (NZDep) and its uptake as a national policy tool. Can J Public Health. 2012 May 9;103(8 Suppl 2):S7-11.

14) R [Internet]. [cited 2021 Mar 11]. Available from: https://cran.r-project.org/web/packages/comorbidity/vignettes/comorbidityscores.html.

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21) Yu D, Zhao Z, Osuagwu UL, et al. Ethnic differences in mortality and hospital admission rates between Maori, Pacific, and European New Zealanders with type 2 diabetes between 1994 and 2018: a retrospective, population-based, longitudinal cohort study. Lancet Glob Health. 2021;9:e209-e17.

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Māori have worse health outcomes than their European counterparts.[[1–3]] Ethnic inequalities which exist in Aotearoa New Zealand, particularly those between Māori and non-Māori, are reported consistently.[[4]] Reasons for these inequalities are multifactorial and complex, but likely reflect the consequences of colonisation that continue to affect Māori health.[[5]] There are differences in access to healthcare, and in the quality of healthcare delivery, for Māori compared to non-Māori.[[6]] There are also differences in broader societal issues that affect health, including housing, education, employment and socioeconomic status.[[4]] Despite such complexities, understanding the nature of inequalities within their own area of the health system is the first step for clinicians when developing and implementing system changes to improve health outcomes for Māori.

Māori made up 15.6% of the New Zealand population in 2013. However, the proportion of patients admitted to New Zealand intensive care units (ICUs) who are Māori has not been reported. Within ICUs, highly specialised nursing and medical teams provide healthcare for immediately life-threatening but potentially treatable acute medical and surgical conditions. ICU staff also provide post-operative care for major surgery, particularly cardiac surgery. New Zealand has a network of ICUs, including large ICUs in major cities as well as regional and rural ICUs associated with smaller hospitals.[[3]] It is unknown whether the outcomes of Māori and European ICU patients admitted to New Zealand ICUs differ. For ICU patients, illness severity, based on the degree of physiological derangement, as well as age, sex and chronic comorbidities, are key determinants of outcome.[[7]] An emergency ICU admission is generally associated with higher mortality risk than an elective post-surgical admission, and the ICU admission diagnosis is also an important outcome predictor.[[7]] In addition, outcomes for otherwise similar ICU patients often vary depending on the hospital they are admitted to.[[7]] Although a high status of deprivation does not appear to be associated with increased in-hospital mortality, it is associated with longer-term mortality[[8]] and is an important confounding variable to consider in an analysis evaluating outcomes by ethnicity.

In this study, we sought to describe outcomes for Māori and European patients admitted to New Zealand ICUs between 2009 and 2018. We undertook a series of analyses adjusting for potential confounders with a view to describing possible contributors to observed inequalities in health outcomes.

Methods

Study design and setting

We undertook a retrospectively designed prospective cohort study using data from the New Zealand Ministry of Health National Minimum Dataset (NMD) matched to the Australian New Zealand Intensive Care Society Centre for Outcome and Resource Evaluation Adult Patient Database (ANZICS CORE APD). This study was submitted to the Health and Disability Ethics Committee of New Zealand (20/CEN/86) and deemed out of scope due to minimal risk. The NMD is a centralised data collection system containing all New Zealand hospital admissions. The NMD is organised using patients’ National Health Index numbers and administered by the New Zealand Ministry of Health.[[9]] The ANZICS CORE APD is an established bi-national voluntary ICU registry, which has been described previously.[[10]]

We used data relating to ICU admissions to New Zealand hospitals from 1 July 2009 until June 30 2018 inclusive and focused on admissions to the 17 ICUs in 16 hospitals that contributed to the ANZICS CORE APD throughout the period of interest. These included major tertiary, regional and rural public hospital ICUs in New Zealand.

Patients

All patients aged 18 years or older who were admitted to one of the 17 participating New Zealand ICUs were eligible for inclusion. Matching of patients included in the two databases of interest was performed based on six variables that were common to both databases. These variables were the name of the admission hospital, the date of hospital admission, the date of hospital discharge, age, sex and the in-hospital mortality. To account for situations where patients were transferred from one ICU to another, we linked ICU admission episodes where a particular patient was discharged from one ICU and then readmitted to another ICU on the same day. In these circumstances, descriptive baseline data were obtained from the first ICU admission and outcome data were obtained from the last ICU admission. Where a patient had multiple ICU admissions within the study period, only the first ICU admission the patient had was included. Accordingly, all ICU admissions included in our final dataset were from unique patients. Since illness severity is the key determinant of outcome for ICU patients,[[7]] we excluded patients where illness severity data were not available in the ANZICS CORE APD. This analysis focused on comparing the outcomes of Māori and European patients. For the purposes of this analysis, we defined patients whose ethnicity was coded as “NZ European,” “European not further defined” and “Other European” as European. Patients who were neither European nor Māori were excluded from the current analysis because we plan to report outcomes for other ethnic groups separately. We ascertained long-term mortality outcomes using data from the New Zealand death registry up until June 2020.

Outcomes

The primary outcome of interest for this study was day-180 mortality. Secondary outcomes were ICU mortality, hospital mortality, discharge to home, ICU length of stay, hospital length of stay and survival time.

Outcome predictors, potential confounders and effect modifiers

To explore the extent to which Māori ethnicity is a predictor of adverse outcomes in the New Zealand healthcare system, we compared ethnic groupings of Māori and European using “prioritised” ethnicity classification, where each patient is allocated to a single ethnic group using prioritisation tables as used in the New Zealand Ministry of Health NMD.[[11]]

We specified a number of variables as potential confounders of the relationship between ethnicity and outcome. These variables were deprivation status, age, sex, site of admission, year of admission, chronic comorbidities as measured using the Charlson comorbidity index,[[12]] admission diagnosis, admission type (elective versus emergency) and source (operating theatre, emergency department, ward, transfer from another hospital and unknown), ventilation status and illness severity. Deprivation status was defined using the New Zealand Index of Deprivation (NZDep), as included in the New Zealand Ministry of Health NMD.[[13]] This NZDep uses data associated with postcodes obtained from the 2013 census as a surrogate for the deprivation status of individual patients. The NZDep categorises patients into deciles, from 1 (least deprived) to 10 (most deprived). Age and year of admission were calculated at the date of hospital admission. Age, sex, site of admission and year of admission we included in both study databases. However, when minor date discrepancies were encountered during merging, we used data obtained from the ANZICS CORE APD for reporting purposes. The Charlson comorbidity index[[12]] was calculated using pre-existing comorbidities based on ICD-10 codes included in the New Zealand Ministry of Health NMD as previously described.[[14]] To obtain a measure of illness severity that was independent of other potential confounders, we evaluated illness severity using only the physiological parameters included in the Acute Physiology and Chronic Health Evaluation (APACHE) III score.[[15]]

Statistical methods

All baseline characteristics were summarised by ethnic group using means and standard deviations for normally distributed variables, medians and interquartile ranges for other continuous variables, and counts and percentages for categorical variables. Comparisons of baseline variables by ethnic group were undertaken using Student’s t-test for normally distributed variables, and Wilcoxon rank sum tests otherwise. Categorical variables were compared using chi-square tests for equal proportions.

For outcome comparisons, we evaluated the association between Māori ethnicity and outcome using European ethnicity as the reference category. For the four binomial outcomes (day-180 mortality, ICU mortality, hospital mortality and discharge home) we used logistic regression and adjusted for known covariates and baseline imbalance. These variables included: site, deprivation status, sex, year of admission, Charlson comorbidity index,[[12]] age, admission source and type, admission diagnosis, ventilation status and illness severity. To ascertain the individual impact of each covariate, we fitted these sequentially, with the resulting risk for Māori ethnicity (versus European) reported as odds ratios (OR), with a 95% confidence interval for each stage. An OR of more than one corresponded to a greater risk of an adverse outcome for Māori compared to their European counterparts. ICU and hospital length of stay were evaluated using a competing risk analysis adjusted for the competing risk of death, using an analogous approach to that described above with results reported at each stage as hazard ratios (HR), along with 95% CI and presented as cumulative incidence plots. For these analyses, a HR of greater than one corresponds to a shorter time to discharge alive for Māori compared to their European counterparts. Survival time to 180 days was compared using Cox Proportion Hazards regression in accordance with the approach described above, with results reported as HRs (95%CI) for each stage of model development. Proportionality assumptions for ethnicity were visually assessed using log-cumulative hazard plots. For the survival analysis, a hazard ratio of more than one corresponds to a worse outcome for Māori compared to their European counterparts because it equates to a shorter time to death.

Analyses were conducted using SAS statistical software, version 9.4 (SAS Institute).

Results

Patients

A total of 52,552 patients from 17 ICUs were included in this study (Figure 1). (The ICUs that contributed data are listed in the acknowledgments section.) A comparison of Māori and European patients in the New Zealand Ministry of Health NMD who could be matched to the ANZICS CORE APD with those could not be matched to the ANZICS CORE APD is shown in Supplementary Table 1. Of the patients included in this analysis, 9,681 (18.4%) were Māori. When patients from other (non-European) ethnic groups were included, Māori made up 15.6% of all ICU admissions. Compared to European patients, Māori patients were on average 13 years younger at ICU admission. They were more often female, had higher deprivation status, were more likely to be admitted to ICU in an emergency, and had more severe illness. (Table 1). Māori also had higher rates of some comorbidities, most notably diabetes and renal disease, and were more likely to have a Charlson comorbidity index of three or more than European patients (Table 1). Additional data on comorbidities for Māori and European patients are shown in Supplementary Table 2. Deciles of deprivation by ethnicity are shown in Supplementary Table 3. Data on admission site and year by ethnicity are shown in Supplementary Table 4.

Compared to European patients, Māori were markedly more likely to be admitted to the ICU after trauma, with sepsis, with neurological disorders and with metabolic disorders, such as complications of diabetes (Table 2).

Figure 1: Flow diagram. View Figure 1.

Table 1: Baseline characteristics by ethnicity. View Table 1.

Table 2: ICU admission diagnostic categories by ethnicity.

Primary outcome

A total of 1,550 Māori (16.0%) and 6,407 (14.9%) European patients had died within 180 days of ICU admission (OR: 1.08, 95% CI, 1.02 to 1.15) (Figure 2, Table 3 and Supplementary Table 5). The effect of sequentially adjusting for potential confounding variables is shown in Table 3. Deprivation status, sex, year of admission, diagnosis and whether the patient was ventilated did not appear to be important confounders of the association between Māori ethnicity and day-180 mortality. Site had some effect, but the strongest confounder was age. Inclusion of age in the regression model increased the OR for day-180 mortality for Māori versus European patients substantially. Adjustment for the Charlson comorbidity index and for illness severity both reduced the OR for day-180 mortality for Māori versus European patients. In the final model incorporating adjustment for all specified variables, Māori ethnicity was not associated with day-180 mortality (OR 1.01, 95% CI 0.92 to 1.10).

Table 3: Key outcomes by ethnicity. View Table 3.

Figure 2: Kaplan-Meier survival plot.*

* The unadjusted hazard ratio for survival to day 180 for Māori vs European patients was 1.08 (95% CI, 1.02 to 1.14), with a hazard ratio of more than one corresponding to a worse outcome for Māori compared to their European counterparts because it equates to a shorter time to death. Data showing hazard ratios adjusted for potential confounding variables are shown in the Supplementary Table 5.

Secondary outcomes

A total of 702 Māori (7.3%) and 2,525 (5.9%) European patients died prior to ICU discharge (OR: 1.25, 95% CI, 1.15 to 1.36), with 1,103 (11.4%) and 4,662 (10.9%) respectively dying prior to hospital discharge (OR: 1.05, 95% CI, 0.98 to 1.13). Similar proportions of Māori and European patients were discharged home (Table 3). For these outcomes, the effect of confounding variables on the associations between Māori ethnicity and outcome was similar to that observed when evaluating the relationship between Māori ethnicity and day-180 mortality, with age observed to be consistently the strongest confounder (Table 3). Site (ie, the ICU the patient was admitted to) appeared to be an important confounder of the association between Māori ethnicity and ICU mortality (Table 3). The ICU length of stay was a median of 1.5 days (IQR, 0.8–2.9 days) and 1.2 days (IQR, 0.9–2.8 days) for Māori and European patients respectively (Supplementary Figure 1). The median hospital length of stay was 7.5 days (IQR, 4.0–14.1 days) and 8.3 days (IQR, 5.1–15.0) for Māori and European patients respectively (Supplementary Figure 2). Age was the strongest confounding variable when evaluating the association between Māori ethnicity and ICU and hospital length of stay (Supplementary Table 6). In the final models, which adjusted for all specified confounders, Māori ethnicity was not an independent predictor of ICU mortality, in-hospital mortality, discharge home, ICU or hospital length of stay or survival time (Table 3, Supplementary Table 5 and Supplementary Table 6.)

Discussion

Statement of principal findings

In this retrospectively designed prospective cohort study, we compared the outcomes of Māori and European patients admitted to New Zealand ICUs between 1 July 2009 and 30 June 2018. We observed that Māori were more likely than European to die within 180 days of ICU admission. In particular, when accounting for the fact they were on average 13 years younger at ICU admission, Māori patients had a markedly higher risk of death and of other adverse outcomes. This increased risk of adverse outcomes for Māori patients appeared, in part, to be accounted for by Māori having more comorbidities, being more likely to be admitted to ICU in an emergency and having more severe illness than European patients. Deprivation status, sex, year of admission, diagnosis and whether the patients were ventilated did not appear to be important confounders of the associations between Māori ethnicity and outcomes.

Relationship to previous studies

We have previously reported that the outcomes for Māori patients enrolled in a large scale randomised controlled trial conducted in the ICU were similar to those of European patients,[[17]] and outcomes were also similar by ethnicity in a single ICU during the 2020 COVID-19 lockdown.[[18]] However, these populations are unlikely to be representative of the patients who receive ICU care in New Zealand. This study is the first large-scale national study to compare outcomes for Māori and European patients admitted to New Zealand ICUs. Our findings are consistent with a recent single-centre study conducted at Waikato Hospital that reported Māori were more likely than non-Māori to be admitted to ICU with sepsis and after major trauma.[[19]] Previous studies have also highlighted the high burden of serious infections[[20]] and diabetes[[21]] among Māori patients hospitalised in New Zealand. Our findings are similar to those of an Australian study, where adjusted long-term mortality and median number of potential life years lost after ICU admission were higher for Indigenous than non-Indigenous patients.[[22]] Our finding that Māori have higher illness severity at ICU admission than European patients has parallels with a previous study, where high illness acuity was observed in Indigenous Australians requiring ICU admission.[[23]]

Implications of study findings

Despite their younger age, Māori have more comorbidities and more severe illness than their European counterparts. Barriers to accessing intensive care for Māori might potentially contribute their higher illness severity by the time of ICU admission. However, the observation that Māori ICU patients are much younger than European ICU patients and yet still have more chronic comorbidities implies that an unequal burden of underlying conditions is one contributor to inequality in ICU outcomes.

Strengths and weaknesses

Our study had a number of strengths. It included data from more than 50,000 patients admitted to all major tertiary, regional and rural public hospital ICUs over a 10-year period. The only ICUs that were not included in this study were private ICUs and some small ICUs in rural and regional centres. We conducted analyses that adjusted for important variables that might potentially contribute to health inequalities in Māori patients admitted to the ICU including deprivation status and comorbid conditions. We were also able to conduct analyses that included robust adjustment for illness severity.

Our study had some limitations. Despite its large size, it only included the subset of ICU admission episodes for which we we could match patient data from the New Zealand Ministry of Health NMD to the ANZICS CORE APD. Accordingly, the patient group we studied is not representative of all patients admitted to New Zealand ICUs. We used the New Zealand Ministry of Health NMD prioritised ethnicity categories to define ethnic groups and cannot preclude the possibility that different methods for categorising ethnicity would result in different findings. Our method of categorisation of deprivation was based on data related to the post codes of patients’ residences obtained from the 2013 census. Although our study did include data from ICU admissions in 2013, it is possible that the reliability of the categorisation of deprivation may be lower for patients admitted in other years. Although we captured deaths that occurred beyond hospital discharge, we only captured those deaths that were registered in New Zealand. It is possible some patients died overseas within 180 days of an ICU admission. We choose day-180 mortality as the primary end point for this study, as deaths occurring as a consequence of an acute illness episode beyond this point are rare.[[24]] However, our findings may have been different if we had evaluated mortality rates at a different time point. Although we undertook analyses that adjusted for ICU illness severity, data on illness severity at the time of hospital admission were not available and are likely to be a key determinant of mortality risk.

Conclusions

Compared to European patients, Māori were markedly more likely to be admitted to the ICU after trauma or with sepsis. Despite Māori being on average 13 years younger at ICU admission than their European counterparts, they had more co-morbidities, higher illness severity and a higher risk of dying within 180 days of an ICU admission.

Supplementary material

View Supplementary material.

  • Supplementary Table 1: A comparison of matched versus unmatched patients.*
  • Supplementary Table 2: Additional details of comorbidities by ethnicity.*
  • Supplementary Table 3: Deprivation categories by ethnicity.*
  • Supplementary Table 4: Admission site and year by ethnicity.
  • Supplementary Table 5: Cox Proportional Hazards Regression for survival to day 180.
  • Supplementary Table 6: ICU length of stay and hospital length of stay by ethnicity.*
  • Supplementary Figure 1: Time to ICU discharge by ethnicity category.
  • Supplementary Figure 2: Time to hospital discharge by ethnicity category.

Summary

Abstract

Aim

To describe characteristics and outcomes of Māori and European patients admitted to New Zealand intensive care units (ICUs) between 2009 and 2018.

Method

A retrospectively designed prospective cohort study. New Zealand Ministry of Health National Minimum Dataset matched to the Australia New Zealand Intensive Care Society Centre for Outcome and Resource Evaluation Adult Patient Database. The primary outcome was day-180 mortality. Secondary outcomes were ICU mortality, hospital mortality, discharge to home, ICU length of stay, hospital length of stay and survival time. We report associations between Māori ethnicity and each outcome, with European as the reference category, using regression analyses to adjust sequentially for site, deprivation status, sex, year of admission, the Charlson comorbidity index, age, admission source and type, ICU admission diagnosis, ventilation status and illness severity based on physiological parameters.

Results

Māori admitted to ICU were on average 13 years younger than European patients. A total of 968 of 9,681 (10%) Māori and 2,732 of 42,871 (5.2%) European patients were admitted after trauma, and 740 of 9,681 (7.6%) and 2,318 of 42,871 (4.4%) were admitted with sepsis respectively. A total of 1,550 of 9,681 (16.0%) Māori and 6,407 of 42,871 (14.9%) European patients died within 180 days of ICU admission; odds ratio (OR) 1.08; 95% CI, 1.02 to 1.15. When adjusted for age, the OR for day-180 mortality for Māori versus European patients increased substantially. The OR decreased after adjustment for admission source and type, and after accounting for Māori having a higher comorbidity index and more severe illness than European patients. In the final model, incorporating adjustment for all specified variables, Māori ethnicity was not associated with day-180 mortality (adjusted OR 1.01; 95%CI, 0.92 to 1.10). Findings were similar for all secondary outcomes.

Conclusion

Compared to European patients, Māori were markedly more likely to be admitted to the ICU after trauma or with sepsis. Despite Māori being on average 13 years younger at ICU admission than their European counterparts, they had more co-morbidities, higher illness severity and a higher risk of dying within 180 days.

Author Information

Alice L Reid: Research Fellow, Medical Research Institute of New Zealand, Wellington. Michael Bailey: Statistician. Australian and New Zealand Intensive Care Research Centre, Monash University, Melbourne, Victoria, Australia; Statistician University of Melbourne, Parkville, Victoria, Australia. Matire Harwood: Māori Health Programme Director, Medical Research Institute of New Zealand, Wellington, New Zealand; Associate Professor, University of Auckland, Auckland, New Zealand. James E Moore: Intensive Care Specialist, Wellington Hospital Intensive Care Unit, Wellington Hospital, Wellington, New Zealand; Senior Research Fellow, Medical Research Institute of New Zealand, Wellington, New Zealand. Paul J Young: Deputy Director, Medical Research Institute of New Zealand, Wellington, New Zealand; Co-clinical leader, Intensive Care Unit, Wellington Hospital, Wellington; Associate Professor, University of Melbourne, Melbourne, Victoria, Australia.

Acknowledgements

We acknowledge the ANZICS Centre for Outcome and Resource Evaluation (ANZICS CORE), who provided the data for this study and the staff from Christchurch Hospital ICU, Dunedin Hospital ICU, Tauranga Hospital ICU, Wellington Regional Hospital ICU, Southland Hospital ICU, Taranaki Health ICU, Timaru Hospital ICU, Hawke’s Bay Hospital ICU, Nelson Hospital ICU, Waikato Hospital ICU, Rotorua Hospital ICU, Whangārei Area Hospital ICU, Hutt Hospital ICU, Auckland City Hospital cardiac and vascular ICU, Auckland City Hospital Department of Critical Care Medicine, North Shore Hospital ICU and Middlemore Hospital ICU, who collected the data and submitted them to ANZICS CORE. We also acknowledge Chris Lewis who provided the data from the New Zealand Ministry of Health NMD for this study. This research was conducted during the tenure of a Health Research Council Clinical Practitioner Fellowship held by Paul Young. The Medical Research Institute of New Zealand is supported by independent research organisation funding from the Health Research Council of New Zealand.

Correspondence

Dr Young at the Intensive Care Unit, Wellington Hospital, Private Bag 7902, Wellington South, New Zealand.

Correspondence Email

paul.young@ccdhb.org.nz

Competing Interests

Nil.

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