Dynamic variability of binding of antiarrhythmic drugs during the evolution of acute myocardial infarction

Classification and pharmacology of antiarrhythmic drugs

Despite the emergence of several forms of nonpharmacologic therapy for cardiac arrhythmias, antiarrhythmic drugs continue to play an important role in the management of patients with this common clinical problem. The key to the proper use of antiarrhythmic drugs is a thorough knowledge of their mode of action and pharmacology. The pharmacology of antiarrhythmic drugs is particularly important because patients with cardiac arrhythmias frequently have multiorgan disease, which may influence the metabolism and elimination of antiarrhythmic drugs. The accumulation of toxic amounts of these agents can lead to dire effects including, but not limited to, ventricular proarrhythmia and malignant bradycardia. The goals of pharmacologic therapy of cardiac arrhythmia are to provide the maximum benefit in terms of arrhythmia suppression while maintaining patient safety. To accomplish these goals, a knowledge of the pharmacology of several antiarrhythmic drugs is mandatory.

Inhibition by local anesthetics of Ca2+ channels in rat anterior pituitary cells

The characteristics of local anesthetic inhibition of voltage-dependent Ca2+ channels in a rat pituitary clonal cell line were investigated by whole-cell voltage clamp and compared with inhibition by the dihydropyridine Ca2+ channel antagonist, nicardipine. With extracellular Ba2+ (10 mM) as the current carrier, depolarization above -40 mV evoked a slowly inactivating I(Ba). Extracellularly applied lidocaine inhibited I(Ba) without changing the activation threshold, the voltage of peak current, or the reversal potential. Inhibition was greater at a holding potential of -60 mV (IC50 = 1.2 mM) than at -80 mV (IC50 = 2.6 mM). This depolarization-induced potentiation in I(Ba) inhibition developed over 0.1-10 s after membrane depolarization began. Nicardipine also dose-dependently inhibited I(Ba) with an IC50 = 90 nM (at a holding potential = -80 mV). Both lidocaine and nicardipine shifted the I(Ba) steady-state inactivation (availability) curves to the left. Double-pulse protocols revealed that lidocaine (1 mM) accelerated the depolarization-induced inhibition (inactivation) of I(Ba) over the rate in drug-free solutions, but had no effect on the hyperpolarization-induced removal of channel inactivation. Nicardipine also accelerated the depolarization-induced inactivation of I(Ba) but, in addition, it slowed the hyperpolarization-induced inactivation removal. The relative inhibitory action of lidocaine in suppressing I(Ba) was unchanged in the presence of nicardipine. These results suggest that lidocaine has a direct action on membrane Ca2+ channels, similar to the voltage-dependent action of dihydropyridine, but acting at a separate and independent site.

Quinidine pharmacodynamics in patients with arrhythmia: effects of left ventricular function

OBJECTIVES

This study was undertaken to determine whether quinidine pharmacodynamics are altered in the presence of left ventricular dysfunction.

BACKGROUND

Left ventricular function is an independent predictor of antiarrhythmic drug efficacy. However, the effects of left ventricular dysfunction on the pharmacodynamics of antiarrhythmic drugs have not been studied extensively.

METHODS

Signal-averaged electrocardiograms were obtained and quinidine plasma concentrations measured during 24-h quinidine washout in 22 patients.

RESULTS

Linear quinidine concentration-effect relations were observed for QRS and QT intervals corrected for heart rate. The slopes of the concentration-effect relation describing changes in the corrected QT (QTc) interval were significantly higher in the group with left ventricular ejection fraction > or = 0.35 ([mean +/- SD] 29.5 +/- 11.2 ms/micrograms per ml) than in the group with a low left ventricular ejection fraction (15.7 +/- 9.7 ms/micrograms per ml, p = 0.001). The QRS concentration-effect relations were not different in the two groups. A significant linear correlation was observed between the slopes of the concentration-effect relations describing changes in QTc intervals and left ventricular ejection fraction (r = 0.7, p < 0.001). Nineteen patients with inducible ventricular tachycardia underwent serial electrophysiologic studies for evaluation of quinidine efficacy. Ventricular tachycardia could not be induced during quinidine therapy in eight patients. The slopes of the quinidine concentration-effect relations for QTc intervals were significantly higher in quinidine responders than in nonresponders (p < 0.05).

CONCLUSIONS

The effects of quinidine on ventricular repolarization are linearly related to left ventricular ejection fraction. Quinidine concentration-effect relations describing ventricular repolarization are associated with antiarrhythmic efficacy in patients with ventricular tachycardia.

Selecting appropriate first-line antiarrhythmic agents: comparative pharmacological profiles

An understanding of the pharmacological issues presented here will help the physician treat arrhythmias in a manner that will meet the therapeutic goals of relieving clinical symptoms, reducing the risk of sudden death, and prevention of treatment-related adverse effects. Most of the issues presented here are qualitative. It usually is not necessary to memorize quantitative data, e.g., to know the exact clearance or exact volume of distribution of a drug in a given setting, but we do need to know qualitatively what kind of changes to expect and alter dosing or drug selection accordingly. Armed with this knowledge, we can more appropriately and safely treat patients with cardiac arrhythmias.

Poisoning due to class IA antiarrhythmic drugs. Quinidine, procainamide and disopyramide

Quinidine, procainamide and disopyramide are antiarrhythmic drugs in the class 1A category. These drugs have a low toxic to therapeutic ratio, and their use is associated with a number of serious adverse effects during long term therapy and life-threatening sequelae following acute overdose. Class 1A agents inhibit the fast inward sodium current and decrease the maximum rate of rise and amplitude of the cardiac action potential. Prolonged Q-T interval and, to a lesser extent, QRS duration may be observed at therapeutic concentrations of quinidine. With increasing plasma concentrations, progressive depression of automaticity and conduction velocity occur. 'Quinidine syncope' (a transient loss of consciousness due to paroxysmal ventricular tachycardia, frequently of the torsade de pointes type) occurs with therapeutic dosing, often in the first few days of therapy. Extracardiac adverse effects of quinidine include potentially intolerable gastrointestinal effects and hypersensitivity reactions such as fever, rash, blood dyscrasias and hepatitis. Procainamide produces electrophysiological changes that are similar to those of quinidine, although Q-T interval prolongation with the former is less pronounced at therapeutic concentrations. Hypersensitivity reactions including fever, rash and (more seriously) agranulocytosis are associated with procainamide, and a frequent adverse effect requiring cessation of therapy is the development of systemic lupus erythematosus. Of the 3 drugs, disopyramide has the most pronounced negative inotropic effects, which are especially significant in patients with pre-existing left ventricular dysfunction. As with quinidine, unexpected 'disopyramide syncope' at therapeutic concentrations has been described. Anticholinergic side effects are common with this drug and may require cessation of therapy. Disopyramide therapy may unpredictably induce severe hypoglycaemia. Severe intoxication with the class 1A agents may result from acute accidental or intentional overdose, or from accumulation of the drugs during long term therapy. Acute overdose can result in severe disturbances of cardiac conduction and hypotension, frequently accompanied by central nervous system toxicity. Decreased renal function can cause significant accumulation of procainamide and its active metabolite acecainide (N-acetyl-procainamide), resulting in severe intoxication. Mild to moderate renal dysfunction is less likely to lead to quinidine or disopyramide intoxication, unless renal failure is severe or concurrent hepatic dysfunction is present. Management of acute intoxication with class 1A drugs includes gut decontamination with provision of respiratory support and treatment of seizures as needed. Hypertonic sodium bicarbonate, by antagonising the inhibitory effect of quinidine on sodium conductance, may reverse many or all manifestations of cardiovascular toxicity.(ABSTRACT TRUNCATED AT 400 WORDS)

The clinical implication of changing unbound quinidine levels

Pharmacodynamic and pharmacokinetic aspects pertinent to the potential clinical application of unbound quinidine levels were studied. Following heparin administration during electrophysiologic testing in 10 patients receiving quinidine, there were significant increases in the mean (+/- SD) right ventricular effective refractory period (266 +/- 24 versus 279 +/- 23; p less than 0.025), free fatty acid concentration (515 +/- 213 versus 1071 +/- 359 mmol/L; p less than 0.001), and unbound quinidine concentration (0.3 +/- 0.1 to 0.6 +/- 0.1 microgram/ml; p less than 0.001) but no changes in heart rate, corrected QT interval, or total plasma quinidine concentration. Ten control patients showed no change in the right ventricular effective refractory period following heparin administration. These findings were consistent with a heparin-induced increase in unbound drug concentration and activity that was limited to the vascular compartment. Eleven patients studied on day 3 (+/- 1) and day 10 (+/- 3) during an acute myocardial infarction showed a significant decrease in unbound quinidine fraction (12 +/- 4% versus 9 +/- 4%; p less than 0.02) accompanied by a decrease, rather than the predicted increase, in half-life (7.1 +/- 2.7 versus 6.3 +/- 2.1 hours; p less than 0.02). Volumes of distribution remained stable while the mean quinidine clearance tended to increase. Half-life correlated with albumin changes (r = -0.71; p less than 0.02). Apparently, improvement in clinical status (assumed) and drug clearance (measured) negated the direct effects of the decrease in unbound quinidine fraction. Although unbound drug concentrations should correlate best with drug dynamic and kinetic information, full knowledge of the clinical context of such measurements is needed for appropriate interpretation.

Initial dosage selection of antiarrhythmic therapy

The success or failure of antiarrhythmic drug treatment depends, in part, on the selection of the initial dosage. Too low a dosage can lead to unnecessary (and frequently life-endangering) delays in achievement of arrhythmia suppression. Conversely, an excessively high dosage can lead to intolerable toxicity and cessation of treatment. The recommended approach to therapy is to begin with a relatively low dosage, i.e., the lowest dosage with a reasonable chance of producing a favorable response, and titrating the dose upward as needed. Dose titration should be guided by clinical response and, when appropriate, concentrations of the drug and any active metabolites in the plasma. In situations frequently encountered in practice, however, the initial dosage must be modified because of interindividual differences in drug disposition. These changes in drug pharmacokinetics can arise from a variety of factors, including disease processes (e.g., congestive heart failure, cirrhosis and renal failure), concomitant medications (e.g., hepatic enzyme inducers such as phenytoin and inhibitors such as amiodarone), drug formulation, protein binding and inherited drug metabolism capacity. Knowledge of these factors can help the clinician to avoid potential pitfalls in initial dosage selection and can enhance the changes of successful drug treatment of arrhythmias.

Procainamide pharmacokinetics in patients with acute myocardial infarction or congestive heart failure

Abnormal procainamide pharmacokinetics (prolonged half-life and decreased volume of distribution) and pharmacodynamics (decreased threshold for the suppression of premature ventricular complexes) have been suggested in patients with acute myocardial infarction or congestive heart failure, or both. To better define procainamide kinetics, 37 patients in the acute care setting received intravenous procainamide (25 mg/min, median dose 750 mg) with peak and hourly blood samples taken over 6 hours. Compared with the 10 control patients, the 12 patients with acute myocardial infarction and the 15 patients with congestive heart failure had normal procainamide pharmacokinetics with respect to half-life (2.3 +/- 1.0, 2.5 +/- 0.9 and 2.6 +/- 0.8 hours, respectively), volume of distribution (1.9 +/- 0.7, 1.8 +/- 0.4 and 1.8 +/- 0.5 liters/kg, respectively), clearance (11.3 +/- 7.5, 9.3 +/- 3.6 and 9.1 +/- 3.5 ml/min per kg, respectively) and unbound drug fraction (66 +/- 9, 66 +/- 9 and 69 +/- 4%, respectively). Low thresholds for greater than 85% premature ventricular complex suppression were confirmed in these patients (median 4.7 micrograms/ml in patients with acute myocardial infarction and 3.3 micrograms/ml in patients with congestive heart failure). Thus, differences in the response of premature ventricular complexes to procainamide reflect electropharmacologic differences dependent on clinical setting rather than pharmacokinetic abnormalities. Furthermore, the reduction of procainamide dosing in patients with acute myocardial infarction or congestive heart failure, based solely on prior kinetic data, may result in inappropriate antiarrhythmic therapy.