Journal of the New Zealand Medical Association, 23-March-2007, Vol 120 No 1251
Abdominal aortic aneurysm surveillance: application of the UK Small Aneurysm Trial to a New Zealand tertiary hospital
Timothy Buckenham, Justin Roake, David Lewis, Malcolm Gordon, Isabel Wright
Abdominal aortic aneurysm surveillance has become an established policy in many vascular units following the publication of the UK Small Aneurysm Trial, which demonstrated no significant difference in mortality at 9 years, if patients with aneurysms less than 55 mm were observed with regular ultrasound surveillance and intervention was restricted to aneurysms reaching the size threshold or becoming symptomatic. The outcome of this trial was that 25% of the observation surveillance cohort did not require repair of their aortic aneurysms over 9 years. The Aneurysm Detection and Management (ADAM) group in the United States performed a similar randomised controlled trial that drew similar conclusions. We started an abdominal aortic aneurysm surveillance programme based on the findings of the UK Small Aneurysm Trial.
We present the 5-year results of this programme. We have attempted to benchmark our data to that of the UK Small Aneurysm Trial, and we also discuss some of the issues that arose when trying to implement a surveillance programme in New Zealand.
The abdominal aortic aneurysm (AAA) surveillance programme at Christchurch Hospital was commenced in November 2000. For the purpose of this audit, we studied the period from November 2000 to September 2005 inclusive. During this time, the protocol at our institution was as follows:
The infrarenal aorta was considered aneurysmal when the dilated segment was 1.5 times greater in diameter than the adjacent normal segment or when the infrarenal aorta exceeded 3 cm in maximum AP diameter.4
Population screening for AAA did not take place in our region during the audit period. Patients with a known AAA of ≤55 mm (discovered incidentally on imaging for other indications or on physical examination either by GP or hospital staff) were clinically reviewed by the Vascular Service prior to inclusion into the surveillance programme. Only patients that were deemed fit enough to undergo elective AAA repair were included in surveillance. Upon entry to the programme, patients were set an individual threshold for review, namely the AAA diameter at which that patient would undergo vascular surgical review with a view to repair.
Technique and equipment—All patients were scanned on a GE Logiq 700 ultrasound scanner, using the 546L linear probe (imaging frequency set at 2.5, 3.0 or 4.0 MHz as deemed appropriate) using the ‘virtual convex’ setting to provide a convex field of view, or 384c curvilinear probe (imaging frequency set at 2.5, 3.0 or 4.0 MHz as deemed appropriate).
The entire aorto-iliac segment was imaged (abdominal aorta, common iliac arteries [CIA], external iliac arteries [EIA], and proximal internal iliac arteries if visualised) from the level of the coeliac axis to the inguinal ligament. Key images of aneurysmal disease were recorded: in transverse section; the maximum antero-posterior (AP) external diameter was measured from the B-mode image of the AAA; and the same measurement taken from the CIA and EIA bilaterally. (This measurement has been shown to be the most repeatable and accurate;5 we do not take transverse/ “side-to-side” measurements).
The distance of the AAA from the superior mesenteric artery (SMA) origin was measured from a longitudinal B-mode image, and the external AP diameter of the aorta at the level of one of the renal artery origins was measured from a transverse B-mode image if possible. If the suprarenal aorta was suspected to be aneurysmal, its external AP diameter in the transverse plane was measured, at the level of the coeliac axis origin if possible.
Ultrasound scan recall frequency—The recall frequency was determined by the most recently measured maximum AP diameter in the transverse plane. Table 1 demonstrates recall frequency in relation to the AAA diameter. Prior to June 2004, all patients followed the male recall schedule. Thereafter females followed a different recall schedule (see Table 1). A few selected patients had a different recall frequency as requested by the referring vascular surgeon, for example, one female patient with a 33 mm AAA was recalled annually rather than every two years due to anxiety about her AAA following the death of her husband from AAA rupture.
Table 1. Recall schedule for ultrasound surveillance scans
Patients completed surveillance upon their AAA reaching a threshold diameter, or if they became symptomatic or experienced rapid (>10 mm per year or >7 mm in 6 months’)3,6 growth. Prior to June 2004, the threshold for most patients was 55 mm. After that time for most patients (for reasons discussed below) the threshold for review was subsequently altered to 50 mm and 55 mm respectively for most females and males. For a patient considered at high risk from AAA repair, the referring vascular surgeon could request a customised threshold (e.g. >55 mm).
Patients could be excluded from surveillance prior to reaching ‘completion’ as defined above for several reasons.
Death—The vascular unit learned of the death of a surveillance patient either via notification from the GP or from hospital records. In every instance the cause of death was recorded, as was whether a post-mortem was performed, for audit purposes.
Ill-health—Precluding them from consideration for elective repair. Patients were reviewed in vascular or other outpatient clinics for other indications and deemed to be unfit for elective repair, or the patient’s GP had contacted the programme co-ordinator to inform the vascular unit of a patient’s new, significant co-morbidities. Upon communication of this to the programme co-ordinator and the patient, the patient was excluded from surveillance. Since mid 2005, as part of the surveillance programme, the health of each patient was assessed annually by the vascular unit by writing to the general practitioner. If the general practitioner felt that the patient’s health had deteriorated and was now a contraindication to repair, they were referred for vascular surgical opinion and thus could be removed from the programme.
Suprarenal component—The patient was found to have a suprarenal (component to their) aneurysm. Patients with such dilatation were not able to be part of the surveillance programme, as the UK Small Aneurysm Trial only included infrarenal abdominal aortic aneurysms. This group was managed by clinical review by the combined vascular and cardiothoracic surgical departments and decisions made as to appropriateness of surgery or continued monitoring.
Patient lost to system—It was possible, as had happened with our other surveillance programmes, that the occasional patient may have moved home and, usually due to their GP not knowing their whereabouts, become lost to recall.
Patient relocates—As had happened with our other surveillance programmes, a patient may relocate to outside our catchment area. When informed of this, we would contact the patient’s new GP requesting that surveillance be continued and supplied relevant imaging from our institution.
The entire cohort comprised 221 patients (162 males, 59 females) of age 73 (38–85) years [median (range)] at inclusion. The baseline (initial scan) AAA diameter was 45 (28–78) mm [median (range)] and the length of follow-up was 20.2 (0.0–59.5) months [median (range)].
The expansion rate was 0.5 (0.0–15.5) mm per year [median (range)]. For analysis purposes, we divided the patients into three groups:
Group 1 comprised 198 patients aged 72 (38-85) years [median (range)]. There were 148 males and 50 females in this group. Baseline AAA diameter at entry to surveillance is shown in Figure 1; it was 45 (28-78) mm [median (range)]. Length of follow-up was 19.7 (0.0-59.5) months [median (range)]. The expansion rate for AAA in this group was 0.5 (0.0–15.5) mm per year [median (range)], assuming any apparently negative growth represented inter-observer error and was therefore considered as no change in diameter.
Figure 1. Distribution of baseline AAA diameter for Group 1
Outcomes—In this group there were 28 patients who underwent elective surgical (n=24) or endoluminal (n=4) exclusion of their abdominal aortic aneurysm—3 additional patients who underwent urgent repair, 10 patients awaiting treatment, 128 on active surveillance, and 11 patients who (on clinical review), having reached their threshold, were too ill to proceed to treatment (Figure 2).
Of the urgent repairs, all three patients had reached their threshold but had become symptomatic. None of these aneurysms were ruptured. All three patients survived their urgent repair. The 30-day mortality for elective and for urgent repair was 0%. There were no emergency repairs in this group.
Figure 2. Status of patients in Group 1
All-cause mortality—23 deaths occurred in this group of 198 patients (11.6% all-cause mortality over the audit period); 16 patients died during surveillance, 2 died having completed surveillance and awaiting repair, and 5 who had been excluded due to ill health (untreated) died.
In total, 9 of these 23 deaths were AAA-related and they are discussed below. The other causes of death were myocardial infarction (MI) (5), cerebrovascular accident (CVA) (2), cancer (2), heart failure (1), biliary sepsis (1), pneumonia (1), unknown (1), and ruptured thoracic aortic dissection (1), with all but the latter not confirmed by post-mortem.
Group 2 comprised 18 patients aged 77 (44–82) years [median (range)]. There were 9 males and 9 females in this group. Baseline AAA diameter at entry to surveillance is shown in Figure 3; it was 42 (36–56) mm [median (range)]. Length of follow-up was 23.3 (0.7–51.3) months [median (range)]. The expansion rate for AAA in this group was 0.8 (0.0–5.1) mm per year [median (range)], assuming any apparently negative growth represented inter-observer error and was therefore considered as no change in diameter.
Figure 3. Distribution of baseline AAA diameter for Group 2
Outcomes—This is the group that had been deemed unsuitable for surgical or endoluminal exclusion of their abdominal aortic aneurysm prior to their reaching threshold. This was either due to ill-health (12 patients) or due to discovery of a suprarenal component to the patient’s abdominal aortic aneurysm (n=6). No patients were lost to the programme, nor did any relocate to outside the catchment area.
All-cause mortality—Within the group of 12 patients excluded due to ill health, there were 3 deaths, one from myocardial infarction and two from cancer. These all occurred in the community without post-mortem.
In the suprarenal group (n=6), there were two deaths (n=2) despite these patients being monitored by the combined cardiothoracic and vascular surgical team: one death was AAA-related and is discussed below, and the other from a ruptured thoracic aortic aneurysm. (The latter patient died 2 weeks post-AAA repair awaiting repair of his thoracic aortic aneurysm; no post-mortem was performed). Hence for Group 2, the all-cause mortality was 27.8% over the audit period.
Group 3 comprised 5 patients aged 75 (70–79) years [median (range)]; all were male. Baseline AAA diameter at entry to surveillance was 52 (49–57) mm [median (range)]. Length of follow-up was 25.0 (0.0–41.7) months [median (range)]. The expansion rate for AAA in this group was 2.3 (1.3–2.4) mm per year [median (range)], assuming any apparently negative growth represented inter-observer error and was therefore considered as no change in diameter.
Outcomes—These patients chose to decline surgical or endoluminal repair on reaching their threshold diameter and completing surveillance, after discussion with a vascular surgeon.
All-cause mortality—Two of these patients subsequently died. Both deaths occurred in the community without post-mortem; one death was AAA-related whilst the other cause of death was unknown. Hence for Group 3, all-cause mortality was 40.0% over the audit period.
Hence for the entire cohort, all-cause mortality was 13.6% over the audit period.
Group 1—In those patients fit for treatment, there were five aneurysm-related deaths. Three deaths were from ruptures of abdominal aortic aneurysms during surveillance, i.e. between scheduled surveillance scans. The threshold for review for all three AAAs was 55 mm.
Of these, the first patient, a female, had a 50-mm abdominal aortic aneurysm; she presented to the Emergency Department with a suspected rupture, no operation was performed and this patient died, with the rupture confirmed on post-mortem. The second patient, also female, died in the community with a 51-mm ruptured aneurysm confirmed on post-mortem. The third patient, a male, ruptured at 51 mm and presented to the Emergency Department, where the rupture was confirmed by imaging and after clinical assessment; no operation was performed and this patient subsequently died.
The other two deaths were of patients awaiting repair. Both patients had reached their modified threshold of 60 mm. Patient 1 was a male with a 61-mm sac, and he died within 3 days of ending surveillance. Patient 2, a female, died 6 months after completing surveillance. Both patients died in the community prior to repair; neither had post-mortem, with the cause of death being attributed to ruptured AAA. These five deaths represent 2.5 % of Group 1.
Group 2—One patient excluded from surveillance due to their aneurysm having a suprarenal component died from a ruptured AAA; no post-mortem was performed, but the AAA appeared ruptured on CT. None of the 12 patients excluded due to ill-health died of ruptured AAA. Hence for Group 2, the AAA-related mortality was 5.6% over the audit period.
Group 3—One death was attributed to rupture of the patient’s abdominal aortic aneurysm, although no post-mortem was performed. Hence for Group 3, the AAA-related mortality was 20.0% over the audit period.
We commenced our surveillance program in Christchurch Hospital, New Zealand, in November 2000. Christchurch Hospital is the biggest tertiary, teaching, and research hospital on the South Island, with 650 beds serving approximately 36,000 inpatients and 13,000 day patients each year.
Our approach to surveillance was based primarily on the findings of the UK Small Aneurysm Trial,1,6 although in this paper we shall also compare some of our results and experiences with the US ADAM trial3 and other smaller published studies.
Briefly, the UK Small Aneurysm Trial1 was a randomised controlled trial designed to compare the outcomes and costs of electively repairing AAA of 40–55 mm versus ultrasonographic surveillance of these lesions. It found no advantage in early repair: there was no significant difference in mortality between the two groups at 9 years, if patients with aneurysms less than 55 mm were observed with regular ultrasound surveillance and intervention was restricted to aneurysms reaching the size threshold or becoming symptomatic.
The US ADAM trial3 compared outcomes and costs of electively repairing AAA of 40–55 mm versus ultrasonographic or computed tomographic surveillance with a slightly different (almost exclusively male) cohort, but essentially drew the same conclusions: that trends in survival did not favour immediate repair and that there was no significant reduction in mortality between the two groups.
There are potential problems when adapting level one data achieved in one country to the local circumstances in another country due to several factors such as resources and service delivery; patient numbers and geography; influences of ethnicity, demography, etc upon the natural history of a particular condition; and other factors.
This audit was carried out to assess our 5-year experience of a surveillance programme, and to examine the data to identify problems that have arisen and ways to improve the surveillance programme. Our surveillance cohort (27% female) closely resembles that of the UK trial. Their trial population comprised both males and females (roughly 80% male to 20% female).
Subjects (with known AAA) were aged between 60 and 76 years; they were referred from general practitioners, non-vascular specialists, and hospital or community-based screening programmes to vascular surgeons for assessment for suitability for inclusion to the programme. The US ADAM cohort2 was predominantly (~99%) male and aged 50–79 years with incidentally-discovered as well as known or screen-discovered AAA.
The main purpose of a surveillance programme is to reduce deaths due to ruptured AAA. In relation to this we found two main issues, namely that of setting a suitable threshold for review in females, and the time between decision to repair and the operation itself. A further secondary issue was that of patients reaching their threshold for review and being (appropriately) denied elective repair due to existing comorbidities, or deciding that they no longer wanted treatment, that may have had unnecessary surveillance. Equally, it is not known whether those excluded prior to reaching threshold due to ill health were removed from the surveillance programme in a timely manner.
Considering the latter first, Group 2 partly comprised those excluded prior to reaching threshold of the clinical opinion that death was more likely from their comorbidities than from their aneurysm. These 12 patients (5.4% of cohort) were excluded due to ill-health before reaching their threshold. This is not comparable with the UK Small Aneurysm Trial since we have no record of those patients that were excluded prior to being placed on the surveillance programme. Moreover, this figure reflects only those who have become ill during the surveillance programme.
In the UK Small Aneurysm Trial, exclusion due to ill-health was performed at the time of enrolment in the trial, rather than progressively throughout the trial;7 they reported 16% of their patients were excluded due to being unfit at the time of randomisation.
Eleven patients in Group 1 (5.0% of cohort) reached their threshold for review but were denied surgery on the grounds of ill-health, hence a total of 10.4% of our patients were excluded due to ill-health. They also found that 4.8% of the early-surgery group became unfit for repair whilst on the waiting list. This figure is, however, comparable with our percentage of patients excluded during surveillance.
In the US randomised trial, patients underwent cardiac thallium scanning before randomisation to reduce the difference in coronary artery disease management between the two groups and to reduce the number of patients to be found unfit for surgery after randomisation.2
Regarding Group 3, it is unclear as to why these patients declined treatment when at the threshold; they represent 2.3% of the cohort. Unsurprisingly, 40% (2/5) of this group have died; neither had post-mortem, with the cause of death being ruptured AAA (1) and unknown (1). The Chichester study found that 5 (3.3%) of the 153 AAA patients they reviewed declined surgery upon reaching threshold; 3 died “having refused screening recalls or surgery”.8
Hence 16 patients underwent surveillance (7.2% of the cohort) and reached their threshold without receiving treatment. Whilst the patient clearly has a right to refuse treatment if correctly informed, from a fiscal perspective it would be preferable to exclude patients no longer suitable for/desirable of elective treatment before they reached threshold. As we were not able to clearly identify why these patients refused treatment, after accepting the possibility initially, we were not able to develop a strategy for predicting such refusal at an earlier stage in their surveillance. An additional cost is the potential psychological one to those patients reaching threshold and then being denied repair.
Indeed, following the end of this audit, we have introduced an annual letter to each surveillance patient’s GP asking if they are aware of any comorbidities that would preclude the patient from elective treatment and hence surveillance. We shall audit these results in the future to ascertain whether this is an efficient use of resources.
The above problems aside, of most concern are the patients who would have been fit for elective repair but who died before it was performed. These fall into two categories—those who died during surveillance and those during the wait for repair. Both groups contributed to the AAA-related mortality of our cohort. In the latter group, there were two ruptures that occurred after completion of surveillance but prior to open surgical or endoluminal exclusion, representing 4.9% of the subset of Group 1 comprising patients awaiting or having had repair (n=41).
One of these patients suffered a ruptured abdominal aortic aneurysm 2 days after completing surveillance whilst awaiting repair, and one patient ruptured 6 months after completion of surveillance. Given that the annual risk of rupture of a 55-mm abdominal aortic aneurysm is approximately 6–10%9 and that the median time for surgical or endoluminal exclusion of these aneurysms at our institution is 6 months, one would anticipate 3–5% of patients having completed surveillance but waiting for surgery to rupture, with a mortality approaching 90% (as has been our experience).
There is little published data on the time to repair in other papers. The Huntingdon (UK) aneurysm screening programme, whereby men in the district over 50 years deemed fit for elective repair are routinely screened for AAA, found 4 (0.5%) of its 768 screen-detected AAA over a 9-year period ruptured whilst being worked up.10 They do not state their average waiting time for repair nor the time-lag for all four cases, but cite a patient dying whilst waiting 2 months for repair and one who was apparently lost to their system with a 72 mm AAA who died 3 years after being scheduled for repair.
In the Chichester (UK) study8 (a randomised trial of ultrasonographic screening for men over 65 years with a threshold for review of 60 mm), 3 (1.8%) of 170 reviewed AAAs ‘ruptured before outpatient appointment’ having reached this threshold. Whilst the UKSMAT1 was a multicentre study and hence the time to treat will vary with centre, this will have contributed to the final mortality results, and knowledge of their median waiting time would assist in applying the recommendations. The message from this paper is that once the patient has reached threshold they should be repaired expeditiously, as they are at the point of where the balance of risk favours surgery.
At a public hospital institution such as ours, if 6 months is the necessary time needed to effect surgical repair then it is important to identify the factors contributing to this and we intend to publish further on this topic. We are unsure as to whether our waiting list is excessive or average and are considering restructuring the surveillance schedule or reducing review thresholds to incorporate a 6-month wait for repair.
Finally in our first and largest group, those intended for treatment or who have completed treatment, there were three fatal ruptures (1.4% of cohort, followed for on average 1.7 years, giving annual rate of death due to rupture of 0.8%) in between routine scheduled surveillance scans.
Comparison of our data with the randomised trials from the UK and United States has been made. The ADAM trial,3 reported an annual risk of rupture of 0.6% in the surveillance cohort, although it was ~99% male and a substantial number of AAA-related deaths in our group were female. The UK Small Aneurysm Trial reported 3.8% of deaths in its surveillance group to be attributable to AAA rupture (including AAA >55 mm, but excluding those deemed unfit for surgery and those who refused surgery) over a median follow-up of 4.6 years—yielding an annual death rate of 0.8%.6
In this Christchurch study, these five ruptures (2.5% of cohort) over a median follow-up of 1.7 years yield an annual death rate of 1.4% which (given the relatively small size of our sample) appears similar to that in the UK trial.
The three ruptures during surveillance highlighted the issues surrounding surveillance of women. Whilst one was an 82-year-old male with a 51-mm AAA, two of these patients were female (aged 78 and 76 years, with 50- and 51-mm AAA respectively). We initially applied the 55-mm threshold, but following the two female ruptures mentioned above we reviewed the 55-mm threshold in women and reduced it to 50 mm. This practice is clearly not evidence-based, and the problem of setting an appropriate intervention threshold is one of the main issues surrounding the management of AAA in women.
The prevalence of AAA in women is approximately one-quarter to one-sixth of that in men.11,12 There is some evidence in comparison to men that female aortas have a proclivity to rupture at a slightly lesser diameter and this may be due to having a smaller aorta initially.13 A study in Norway found more rapid expansion in AAA in women than men (2.43 vs 1.65 mm per year).14 The UK Small Aneurysm Trial1 found the risk of rupture in women to be four times that of men. A Finnish study of ruptured AAA found that the diameter was less than 55 mm in 24% of females with rupture.15
There is also evidence that once the AAA has ruptured, women are less likely to be considered for emergency repair than men16 and that they have a lower survival rate post-emergent repair.17
The UK Small Aneurysm Trial was not powered to make recommendations about females with abdominal aortic aneurysms.1 Unlike the US ADAM trial,2 it did include a significant proportion of women but concludes that, whilst the 5.5 cm threshold for repair may be too high for females, the diameter at which surgery should be recommended for women cannot be ascertained from their study. No asymptomatic females have ruptured since our local change in threshold. In retrospect, had the threshold been 50 mm in women, the death from rupture may have been prevented.
We present the experience of an abdominal aortic aneurysm surveillance programme based on the UK Small Aneurysm Trial, as well as some problems that were identified. The study was attempted to confirm the feasibility of applying the UK Small Aneurysm Trial data to the local population in New Zealand.
The main problems encountered were setting a threshold for intervention in women; the interval between completion of surveillance and undergoing endoluminal or open surgical repair; and the number of exclusions from surveillance group, mainly due to ill health.
We conclude that the annual risk of death due to rupture in our New Zealand surveillance group is similar to the UK Small Aneurysm Trial (at approximately 1% per annum), but the ruptures and urgent repairs that occurred after completion of surveillance are concerning.
Competing interests: None.
Author information: Timothy Buckenham, Clinical Professor of Radiology and Vascular Radiologist, Department of Radiology, Christchurch Hospital, Christchurch; Justin Roake, Professor of Surgery, Department of Surgery, Christchurch Hospital, Christchurch; David Lewis, Vascular Surgeon, Department of Surgery, Christchurch Hospital, Christchurch; Malcolm Gordon, Vascular Surgeon, Department of Surgery, Christchurch Hospital, Christchurch; Isabel Wright, Sonographer, United Kingdom (former employee of Department of Radiology, Christchurch Hospital, Christchurch)
Correspondence: Professor Timothy Buckenham, Department of Radiology, Christchurch Hospital, Private Bag 4710, Christchurch. Fax: (03) 364 0620; email: TimB2@cdhb.govt.nz
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