Effects of bepridil and diltiazem on ventricular repolarization in angina pectoris

Clinical relevance and management of drug-related QT interval prolongation

Much attention recently has focused on drugs that prolong the QT interval, potentially leading to fatal cardiac dysrhythmias (e.g., torsade de pointes). We provide a detailed review of the published evidence that supports or does not support an association between drugs and their risk of QT prolongation. The mechanism of drug-induced QT prolongation is reviewed briefly, followed by an extensive evaluation of drugs associated with QT prolongation, torsade de pointes, or both. Drugs associated with QT prolongation are identified as having definite, probable, or proposed associations. The role of the clinician in the prevention and management of QT prolongation, drug-drug interactions that may occur with agents known to affect the QT interval, and the impact of this adverse effect on the regulatory process are addressed.

Influence of dofetilide on QT-interval duration and dispersion at various heart rates during exercise in humans

BACKGROUND

The objective of this study was to assess the influence of heart rate on QT-interval duration and dispersion during administration of the new selective potassium-channel blocker dofetilide in normal subjects.

METHODS AND RESULTS

Dofetilide 0.25 and 0.75 mg was administered for 4 days to 12 subjects in a randomized-sequence, double-blind, three-period, placebo-controlled, crossover study. QT-RR pairs were measured on study day 4 over a wide range of RR intervals obtained at rest and during an exercise test. QT-interval durations were calculated at seven predetermined RR intervals ranging from 400 ms (150 bpm) to 1000 ms (60 bpm) by use of monoexponential nonlinear curve fitting. QTmax and QTmin were calculated similarly, and QT-interval dispersion was measured as QTmax-QTmin at each predetermined RR interval. Minimal effects were found with 0.25 mg dofetilide. Two hours after administration of 0.75 mg dofetilide, QT interval was prolonged by 16.7 +/- 8.7% at a heart rate of 60 bpm (P < .01) and by 7.4 +/- 8.2% at a heart rate of 150 bpm (P < .05). QT prolongation at a heart rate of 150 bpm was less pronounced than at lower heart rates. Neither placebo nor dofetilide at either dose significantly increased QT-interval dispersion at any heart rate.

CONCLUSIONS

Dofetilide increases QT-interval duration but does not increase QT-interval dispersion in healthy subjects. QT-interval prolongation remains significant at high heart rates, although some degree of reverse rate dependence is observed at high concentrations.

Rate-corrected QT interval: techniques and limitations

The duration of the QT interval on the surface electrocardiogram represents the time required for all ventricular depolarization and repolarization processes to occur. Among the many physiologic and pathologic factors that contribute to the QT interval, heart rate plays a major role. Several approaches have been used to correct the QT interval, all of which take into account the heart rate at which the interval is measured. The simplest and most common approach to correcting the QT interval is to divide its value by the square root of the preceding RR interval expressed in seconds, i.e., by using Bazett's formula. This calculation provides a corrected QT (QTc) interval that represents the QT interval normalized for a heart rate of 60 beats/min. However, several studies have shown that Bazett's correction formula is not optimal. Fridericia's cube-root formula has been shown to perform better in correcting the QT interval for heart rate. Other formulas require the measurement of several QT-RR pairs at various heart rates to obtain a reliable QTc interval and are therefore not easily usable. Any correction formula is likely to introduce an error in assessing the QTc interval. Although the importance of this error should not be minimized, the corrected QT interval remains useful in assessing the effects of drugs on the duration of repolarization. For this purpose, Fridericia's cube-root formula is preferable to Bazett's square-root formula.(ABSTRACT TRUNCATED AT 250 WORDS)

Proarrhythmia of nonantiarrhythmic drugs

Proarrhythmic effects of nonantiarrhythmic drugs have not been as extensively studied or reported compared with the effects of antiarrhythmic drugs. The proarrhythmic incidence of many of these agents is not accurately known. In some instances, the facilitation of arrhythmias may be the result of compounding clinical factors. Many agents, however, share structural similarities to antiarrhythmics and manifest the same arrhythmic tendencies. Many reports of proarrhythmia may represent toxic rather than proarrhythmic effects, and in vitro studies to elicit the underlying mechanisms may be warranted for the more common drugs. This report summarizes reported arrhythmic effects of a variety of commonly utilized nonantiarrhythmic drugs. The incidence and mechanism of the proarrhythmia is not always clear. The clinician, however, should be aware of reported events to appropriately diagnose and treat the arrhythmia.

Rate-dependence of class III actions in the heart

The pharmacodynamics of many antiarrhythmic drugs are altered by heart rate. The ability of sodium channel blockers to decrease conduction velocity (class I action) is more pronounced with rapid heart rates. Drugs with class III action increase action potential duration and refractoriness in the heart. Most drugs with class III actions, currently being developed, produce their action by blocking one or several of the potassium channels responsible for repolarization. In vitro and in vivo studies have shown that their ability to increase repolarization time is less pronounced, or even disappears, at rapid pacing or heart rates. This so called 'inverse' rate-dependence of class III action is a characteristic of all drugs currently used in man except amiodarone, for which prolongation of repolarization time persists to a limited extent with rapid heart rates. It has been suggested that one possible mechanism of the inverse rate-dependence of class III action is related to the preferential binding of drugs to the potassium channels in the closed, polarized state. An inverse rate-dependence of class III action has also been found on prolongation of refractoriness. However, preliminary studies suggest that the positive inotropism of class III drugs not only persists but may increase with rapid heart rates. The clinical consequences of this phenomenon remain unclear, especially in view of the fact that the rate-dependence of class III action on dispersion of repolarization has not been specifically studied and that class III actions tend to decrease in ischemic tissues. However, the increase of action prolongation at slow heart rates may contribute to the bradycardia-dependent development of torsades de pointes arrhythmias.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of drug-induced QT interval modifications in dynamic electrocardiography: the case of bepridil

Drug-induced modifications of QT interval are usually assessed through formulae defining the corrected QT interval "QTc". They are all based on the assumption that the correction is adequate, and that drug-induced heart rate variations and rate-dependent QT changes are proportional. Holter ECG allows to study the repolarization in selected RR cycles while controlling environmental rate-related and circadian influences. Repolarization duration was evaluated in 15 normal individuals and 13 patients with stable coronary artery disease and no heart failure who did not differ in terms of 24-hour heart rate, age and sex. The effects of a 3-month treatment with bepridil were assessed in the latter. Using the conventional evaluation through the corrected QT (Bazett formula), no difference was found between the two groups at baseline, and bepridil induced a non-significant 5% prolongation of QTc. At Holter recordings, the QTa (Q-T apex) duration was linearly correlated with the heart rate over 24 hours. To specifically study day-to-night variations and to exclude the rate-dependent and short-term autonomic influences. QTa was studied in populations of averaged QRS-T selected according to i) the last RR cycle length and ii) an identical mean RR interval during the preceding minute. Both RR values were fixed at 800 ms to obtain the "QTa-800" measured directly or extrapolated from linearly correlated, other RR values.(ABSTRACT TRUNCATED AT 250 WORDS)

Safety profile of bepridil determined from clinical trials in chronic stable angina in the United States

Bepridil is a unique calcium antagonist with a broad pharmacologic profile and demonstrated efficacy as an antianginal agent. Data from US clinical trials of bepridil in 840 patients (820 with chronic stable angina; 20 with vasospastic angina) indicated that the drug is well tolerated. The most frequent adverse reactions were of gastrointestinal or neurologic origin. Although bepridil has negative inotropic potential, a contributory role of the drug could not be precisely determined in 24 cases of congestive heart failure. Bepridil (median dose, 300 mg/day) lengthened the corrected QT (QTc) interval in most patients. The mean increase was 7.7%. Torsades de pointes occurred in 7 patients, an incidence of approximately 1%. Prolonged QT/QTc interval, low serum potassium level, or use of a potassium-wasting diuretic was present in each case. A life-table analysis involving 712 patients showed a 1-year overall survival rate of 97.1%. A 1-year survival rate of 97% was also determined by comparing 652 bepridil-treated patients with 1,361 matched control subjects from a data base of greater than 10,000 patients referred to the Duke University Medical Center for cardiac catheterization. Based on US clinical trials, the overall safety results indicate an acceptable safety profile of bepridil when this calcium antagonist is used in the treatment of chronic stable angina.