Journal of the New Zealand Medical Association, 12-December-2003, Vol 116 No 1187
The practice of digoxin therapeutic drug monitoring
Murray Barclay and Evan Begg
Digoxin continues to have an important role in the control of ventricular rate with atrial fibrillation, and as a positive inotropic agent in heart failure. Therapeutic drug monitoring (TDM) for digoxin was introduced more than 30 years ago, and resulted in a marked reduction in the incidence of digoxin toxicity. However, despite a long experience of TDM with this drug, the way in which TDM is performed is often inappropriate, as highlighted in the article by Sidwell et al in this issue.1
In publications from other countries in the past 15 years, similar conclusions have been drawn, and show that inappropriate use of TDM with digoxin is quite common and not restricted to New Zealand. There is often no clear indication for monitoring,2,3 the sample is taken at the wrong time resulting in a falsely high or low concentration,3–5 or an inappropriate clinical action or inaction is taken after the result is received.2,3 These problems most likely relate to a lack of knowledge about the practice of digoxin TDM, in particular amongst the junior doctors who are most likely to request digoxin concentration measurement. In this editorial we review the most relevant aspects of the practice of TDM for digoxin, and specifically the indications for monitoring, timing of blood samples and interpretation of the results.
Indications for digoxin concentration measurement (Table 1)
Confirmation of toxicity The need to measure digoxin concentrations for confirmation of toxicity is related to the low therapeutic index of digoxin. The recommended therapeutic range (1.0 to 2.5 nmol/l) reflects the significant increase in risk of toxicity that occurs with serum concentrations over 2.6 nmol/l, and which is almost invariable once the concentration exceeds 3.8 nmol/l.6 Symptoms of toxicity include nausea, vomiting, diarrhoea, abdominal pain, confusion, dizziness, agitation, arrhythmias, heart block and various visual symptoms. Toxicity is generally a clinical diagnosis supported by an elevated digoxin concentration.
It is also apparent that clinical suspicion of toxicity correlates poorly with high concentrations. In the study by Sidwell et al, in those requests in which the indication for TDM was confirmation of toxicity, only 19% were associated with a high digoxin concentration.1 This result is not necessarily surprising given that many of the symptoms of digoxin toxicity are non-specific and are frequently present in acutely ill patients in general.
Table 1. Indications for therapeutic drug monitoring
Assessing the effect of factors altering the pharmacokinetics of digoxin A number of different factors influence the pharmacokinetics of digoxin in an individual, but renal function is by far the major contributor. Maintenance dose estimation based on calculation of the patient’s creatinine clearance, using a formula such as the Cockcroft and Gault equation, will usually result in an appropriate dose in most patients. However, renal function alone does not explain all the variance in serum digoxin concentrations. A computer programme that included renal function as a variable to predict an appropriate digoxin dose for individual patients performed only marginally better than physician attempts.7 Some of the unpredictable relationship between dose and serum concentration of digoxin may be explained by genetic polymorphism of the p-glycoprotein gene. P-glycoprotein is involved in the transport of digoxin into the body in the gastrointestinal tract, and out of the body in the renal tubules. Mutations within this gene have been shown to alter the bioavailability and renal clearance of digoxin.8 Drug interactions (Table 2) also affect serum concentrations of digoxin, usually through competitive inhibition of p-glycoprotein activity. Because sources of variability other than renal function are less predictable or measurable, adjusting an individual’s maintenance dose on the basis of their creatinine clearance remains the best starting point for dose individualisation.
Table 2. Drug interactions for digoxin
Therapeutic failure Digoxin TDM is also considered to be indicated in situations of apparent therapeutic failure, although the validity of the therapeutic range in terms of efficacy is unclear. In terms of improving rate control in chronic atrial fibrillation, a review of the literature found only a weak correlation between digoxin concentration and ventricular rate.9 This is not surprising given the number of other influences on the atrio-ventricular node, such as altered sympathetic drive with other comorbidities (eg, sepsis, hypoxia). However, TDM for individual patients may be useful to detect patients with a low digoxin concentration and who may benefit from an increase in digoxin dose, as opposed to those with higher concentrations who are likely to develop toxicity symptoms only from an increase in dose.
In congestive cardiac failure there is increasing evidence that concentrations lower than the currently recommended limit of the therapeutic range (<1.0 nmol/l) may be as efficacious as higher concentrations. Results from the PROVED and RADIANCE trials, and their subsequent re-analysis,10 suggest that digoxin concentrations between 0.6 and 1.2 nmol/l may be as efficacious, and less pro-arrythmic, than higher concentrations in patients with heart failure. These trials suggest that for heart failure at least, the lower end of the therapeutic range could be lowered to 0.6 nmol/l.
Appropriate sampling time
Digoxin concentrations should be measured at least eight hours following an oral dose of digoxin and ideally when concentrations have reached steady-state. Understanding of the reasons behind these recommendations requires an understanding of the pharmacokinetic profile of digoxin. Digoxin is well absorbed, with peak serum concentrations occurring within one hour. A large volume of distribution (4–7 l/kg) reflects that digoxin concentrates in the tissues, with the active site being within myocardial and other cells. Redistribution from serum to tissue takes at least eight hours (Figure 1). Samples taken within eight hours of a dose will falsely imply elevated tissue concentrations, and inappropriate dose reduction may result. Our department has been consulted in a number of dramatic cases where an apparently very high digoxin concentration was an artifact of early sampling.
Figure 1. Digoxin concentration profile following an oral dose
Digoxin elimination is predominantly renal in nature (the fraction excreted unchanged in the urine is 0.6 to 0.9) and is dependent upon glomerular filtration and p-glycoprotein-mediated active tubular secretion. A long half-life of at least 30 h (in normal renal function) results in steady-state concentrations taking at least five days to be achieved (it takes four half-lives to achieve >90% of steady-state concentrations). In the elderly and in patients with renal impairment, elimination is diminished and the half-life prolonged. In these cases, steady-state may take several weeks to achieve. Measurement of concentrations before steady-state is reached results in a falsely low estimate of the steady-state concentration, and inappropriate dose increases may result.
Serum digoxin concentrations should be interpreted within the clinical context. It is generally accepted that when the concentration is above the therapeutic range, the dose should be reduced even in the absence of obvious toxicity. This is because the patient is at risk of arrhythmia and there is probably no additional efficacy associated with a high concentration. On the other hand, toxicity can occur with concentrations within the therapeutic range. This may result from several known factors (Table 3) that change tissue sensitivity to digoxin and alter the therapeutic index.
Table 3. Factors altering digoxin sensitivity and the likelihood of toxicity
The period of time that digoxin should be withheld following an episode of toxicity depends upon how high the concentration is, and the half-life of digoxin in that patient. In a patient with normal renal function (half-life approximately 30 h) and a concentration of 3.0 nmol/l, the digoxin should be withheld for 1–2 days before restarting at the appropriately altered dose, as this will allow the concentration to drop to within the therapeutic range. In renal impairment with a prolonged digoxin half-life, doses may need to be withheld for several days.
When the measured digoxin concentration is low, options include stopping treatment, increasing the dose or making no change. If the indication for therapy is rate control, and the current ventricular rate is appropriate in the presence of low digoxin concentrations, a trial without digoxin may be appropriate. If ventricular rate is not controlled, a dose increase is usually indicated. However, poor rate control may be related to other acute illness processes, and treatment of the underlying condition may be all that is required. If the concentration is above 0.6 nmol/l and the indication is heart failure that is now controlled, the dose may not need to be adjusted for reasons already discussed.
Dose adjustment for apparent therapeutic failure should ideally only be performed following a digoxin concentration measured at steady-state. A change in dose will normally result in a proportional change in digoxin concentration, eg, doubling the dose will double the digoxin concentration, and halving the dose will halve the concentration (assuming stable renal function and no new drug interactions). In situations where there is changing renal function, the adjustment can be estimated by calculating the change in creatine clearance using the Cockcroft and Gault formula.11 For example, a halving of the patient’s renal function from baseline means that only half of the initial maintenance dose will be required to maintain the same steady-state concentration.
Digoxin remains a classical drug for which therapeutic drug monitoring may be useful. It has a narrow therapeutic index, complex pharmacokinetics, and a dose-response relationship, at least for toxicity. However, therapeutic drug monitoring is only useful if performed correctly. Particular attention needs to be paid to the timing of sampling with respect to dosing, the presence or otherwise of steady-state conditions, and the half-life and its consequences for dosing in the individual patient.
Author information: Murray L Barclay, Clinical Pharmacologist; Evan J Begg, Clinical Pharmacologist, Department of Clinical Pharmacology, Christchurch Hospital, Christchurch
Correspondence: Dr Murray Barclay, Department of Clinical Pharmacology, Christchurch Hospital, Private Bag 4710, Christchurch. Fax: (03) 364 1003; email: MurrayB@cdhb.govt.nz
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