Animal models of the long QT syndrome: relation to clinical features

Torsades de pointes: unanswered questions

Torsade de pointes (Tdp) is a polymorphic ventricular tachycardia (VT) in which the axis of the QRS complex changes direction after a certain number of complexes as if the complex rotated around the baseline. Tdp is usually associated with QT prolongation, and dispersion of ventricular repolarlization (DR). Experimental models of tdp are usually associated with induction of early after depolarizations (EADs). Several aspects of the pathogenesis of tdp are incompletely understood. The purpose of this article is to propose the directions in research that may increase our current understanding of the factors responsible for tdp. The most plausible hypotheses requiring further supporting evidence are: 1. The occurrence of tdp requires the presence of DR i.e. tdp does not occur in the absence of DR. 2. EADs appear to play an important role as a trigger to tdp in the animal models, but more evidence are needed at the clinical level. 3. EADs may be responsible for arrhythmias other than tdp. 4. The greater incidence of tdp in the females than in tha males may be attributed to differences in the duration of the QT interval and different morphology of the ST-segment and the T-wave. The above gender differences may be caused by the effects of gonadal hormones which modulate some of the membrane currents flowing during early ventricular repolarization.

Proarrhythmic effects of intravenous quinidine, amiodarone, D-sotalol, and almokalant in the anesthetized rabbit model of torsade de pointes

The proarrhythmic effects of four antiarrhythmic agents were examined during alpha1-adrenoceptor stimulation in chloralose-anesthetized rabbits. Each dose of almokalant (26, 88, and 260 microg/kg), D-sotalol, quinidine, or amiodarone (each 3, 10, and 30 mg/kg) was infused i.v. over 5 min and there was a 20-min interval between each infusion. D-sotalol and almokalant evoked torsade de pointes (TdP) and other arrhythmics, frequently. The incidences of TdP were 0, 50, and 40% after administering the first, second, and third doses of the nonselective I(Kr) inhibitor D-sotalol, respectively. Similarly, these values were 20, 40, and 33% after administering the first, second, and third doses, respectively, of the selective I(Kr) inhibitor almokalant. Quinidine elicited only a few arrhythmics, but not TdP. Quinidine, D-sotalol, and almokalant evoked conduction blocks in a dose-related manner (p < 0.05) and prolonged QT and QT(c) intervals (p < 0.05). Amiodarone neither prolonged QT and QT(c) nor evoked ventricular tachyarrhythmias, blocks, or other proarrhythmias. In conclusion, these results show no direct correlation between the occurrence of TdP and the infusion rate or dose of anti-arrhythmics. Furthermore, the lack of TdP with quinidine warns of false-negative results in the applied model.

Electropharmacological characterization of cardiac repolarization in German shepherd dogs with an inherited syndrome of sudden death: abnormal response to potassium channel blockers

OBJECTIVES

This study sought to determine whether abnormal ventricular repolarization is implicated in cardiac arrhythmias of German shepherd dogs with inherited sudden death.

BACKGROUND

Moïse et al. (9) have identified German shepherd dogs that display pause-dependent lethal ventricular arrhythmias.

METHODS

Ventricular repolarization was studied both in vivo using electrocardiogram recordings on conscious dogs and in vitro with a standard microelectrode technique performed on endomyocardial biopsies and Purkinje fibers. Pharmacological manipulation was used to evaluate the role of potassium channels.

RESULTS

In control conditions, electrocardiogram parameters were similar in both groups of dogs, except for the PR interval (18% longer in affected dogs, p < 0.05). Injection of d,l-sotalol (2 mg/kg) prolonged QT interval more in affected dogs (+14%, n = 9) than it did in unaffected dogs (+6%, n = 6, p < 0.05) and increased the severity of arrhythmias in affected dogs. In vitro, in control conditions, action potential duration (APD90) of endomyocardial biopsies and Purkinje fibers were significantly longer in affected dogs (respectively 209 +/- 3 ms, n = 30 and 352 +/- 15 ms, n = 17) than they were in unaffected dogs (197 +/- 4 ms, n = 25 and 300 +/- 9 ms, n = 30) at a pacing cycle length (PCL) of 1,000 ms. This difference increased with PCL. The kinetics of adaptation of APD90 to a change in PCL was faster in affected dogs. D,l-sotalol (10(-5) and 10(-4)M) increased APD90 in both groups of dogs, but this increase was greater in affected dogs, with the occurrence of triggered activity on Purkinje fibers. E-4031 (10(-7) and 10(-6) M), an I(Kr)-blocker, increased APD90 similarly in both groups of dogs. Chromanol 293B (10(-6) and 10(-5)M), an I(Ks)-blocker, increased significantly APD90 in unaffected dogs but had no effect in affected dogs.

CONCLUSIONS

These results support the hypothesis of an abnormal cardiac repolarization in affected dogs. The effects of 293B suggest that I(Ks) may be involved in this anomaly.