Therapeutic drug monitoring of antiarrhythmic drugs. Rationale and current status

Therapeutic drug monitoring of antiarrhythmic drugs

Most antiarrhythmic drugs fulfil the formal requirements for rational use of therapeutic drug monitoring, as they show highly variable plasma concentration profiles at a given dose and a direct concentration-effect relationship. Therapeutic ranges for antiarrhythmic drugs are, however, often very poorly defined. Effective drug concentrations are based on small studies or studies not designed to establish a therapeutic range, with varying dosage regimens and unstandardised sampling procedures. There are large numbers of nonresponders and considerable overlap between therapeutic and toxic concentrations. Furthermore, no study has ever shown that therapeutic drug monitoring makes a significant difference in clinical outcome. Therapeutic concentration ranges for antiarrhythmic drugs as they exist today can give an overall impression about the drug concentrations required in the majority of patients. They may also be helpful for dosage adjustment in patients with renal or hepatic failure or in patients with possible toxicological or compliance problems. Their use in optimising individual antiarrhythmic therapy, however, is very limited.

Propafenone disposition during continuous venovenous hemofiltration

OBJECTIVE

To determine the extent of removal of propafenone by continuous venovenous hemofiltration (CVVH) in a critically ill pediatric patient.

CASE SUMMARY

A three-year-old white-Japanese girl was admitted to the critical care unit following cardiac surgery. Her postoperative course was complicated by the development of junctional ectopic tachycardia, requiring propafenone, and acute renal failure, which necessitated the use of CVVH. The serum and ultrafiltrate concentrations of propafenone and its 5-hydroxy metabolite were measured to determine both total and CVVH clearance.

CONCLUSIONS

The data from this case report showed that propafenone was not significantly removed by CVVH. Furthermore, the total clearance of propafenone was not affected by the patient's renal or liver function impairment.

Pharmacokinetic based adjustment of lidocaine antiarrhythmic schedule

Administration of lidocaine, 200 mg/day i.m. or 275 mg orally, decreased sudden death after myocardial infarct (from 20.7% to 10.3%) although such schedules are not considered adequate to guarantee efficient plasma levels. Inclusion of lidocaine in a polyethylene matrix assured a slow release and complete disappearance of known side effects. Lidocaine was administered 200 mg intramuscularly to hospitalized patients every 6 h or 275 mg oral tablets to healthy volunteers every 8 h and plasma levels evaluated. Plasma levels after oral administration to healthy volunteers showed a great variability, so that it was not possible to draw a statistically significant conclusion about the accumulation of lidocaine in a period of 1 week. In coronary artery disease patients, plasma levels slowly increased with time, but clinical signs indicated, in some cases, a much more rapid accumulation. The therapeutic efficiency at low repeated doses was explained as a consequence of a slow accumulation on the one hand and of the addition of the action of MEGX, the major metabolite of lidocaine, on the other hand.

Clinical relevance of cimetidine drug interactions

The excellent efficacy and tolerability profiles of H2-antagonists have established these agents as the leading class of antiulcer drugs. Attention has been focused on drug interactions with H2-antagonists as a means of product differentiation and because many patients are receiving multiple drug therapy. The main mechanism of most drug interactions involving cimetidine appears to be inhibition of the hepatic microsomal enzyme cytochrome P450, an effect which may be related to the different structures of H2-antagonists. Ranitidine appears to have less affinity than cimetidine for this system. There have been many published case reports and studies of drug interactions with cimetidine, but many of these have provided pharmacokinetic data only, with little information concerning the clinical significance of these findings. Nevertheless, the coadministration of cimetidine with drugs that have a narrow therapeutic margin (such as theophylline) may potentially result in clinically significant adverse effects. The monitoring of serum concentrations of drugs coadministered with cimetidine may reduce the risk of adverse events but does not abolish the problem. However, for most patients, concomitant administration of cimetidine with drugs possessing a wide therapeutic margin is unlikely to pose a significant problem.

Practical optimisation of antiarrhythmic drug therapy using pharmacokinetic principles

The optimisation of antiarrhythmic drug therapy is dependent on the definitions and methods of short term efficacy testing and the characteristics of those drugs used for rhythm disturbances. The choice of an initial antiarrhythmic drug dosage is highly empirical, and will remain so until the measurement of free concentrations, enantiomeric fractions and genetic phenotyping becomes routine. However, the clinician can devise an efficient initial dosage for efficacy testing procedures based on pharmacokinetic principles and disposition variables in the literature. In this regard, a nomogram for commonly used agents and dosages was constructed and is offered as a guide to accomplish this goal. Verification of the accuracy and usefulness of this nomogram in a prospective manner in patients with symptomatic tachyarrhythmias is still required. On a long term basis, dosage regimens can be modified by the use of pharmacokinetic principles and patient-specific target concentrations, in accordance with the methods used to monitor arrhythmia recurrence and drug-related side effects.