Modulation of procainamide's effect on conduction by cellular uncoupling in perfused rabbit hearts

Effects of acute global low-flow ischemia on triggered arrhythmias in d-sotalol-induced long Q-T intervals in perfused rabbit hearts

Little information is available on how acute ischemia modifies the electrophysiologic substrate associated with long Q-T interval conditions. We studied the effects of low-flow ischemia (10 min at 5.0 ml/min followed by 10 min of 2.5 ml/min) in Langendorff perfused rabbit hearts during control and in hearts 20 min after the addition to the perfusate of 92 microM d-sotalol, which reliably produced triggered activity. Epicardial electrograms, a left ventricular endocardial monophasic action potential (MAP), and simulated X and Y lead electrocardiograms were used to characterize myocardial activation and recovery during ventricular pacing. In the control hearts, conduction velocity as indicated by the mean epicardial activation time accelerated for most of the period of ischemia (maximum decrease of -9.4 +/- 7.9%). The mean activation-recovery interval, MAP duration, and Q-T interval were moderately decreased (-4.9 +/- 8.6%, -7.5 +/- 4.4%, and -4.6 +/- 2.3%, respectively). The mean standard deviation of the activation-recovery interval (epicardial heterogeneity of recovery) was increased by 34.6 +/- 23.4%. d-Sotalol had no effect on conduction but prolonged myocardial recovery time, increased heterogeneity, and produced triggered arrhythmias in all hearts. Within 2 min of ischemia triggered activity was eliminated. With d-sotalol, ischemia slowed conduction and produced relatively larger decreases in the activation-recovery interval, MAP duration, and Q-T interval (-11.8 +/- 10.3%, -13.9 +/- 12.0%, and -15.8 +/- 11.2%). The increased epicardial heterogeneity seen with d-sotalol was attenuated by ischemia. Thus ischemia superimposed on long Q-T conditions had antiarrhythmic as well as arrhythmogenic effects.

Gender and seasonally related differences in myocardial recovery and susceptibility to sotalol-induced arrhythmias in isolated rabbit hearts

INTRODUCTION

Gender differences and seasonal variations in cardiac electrophysiology and susceptibility to arrhythmias have been described clinically. The present study was undertaken to determine if there are similar gender and seasonally related differences in the electrophysiology of the rabbit heart.

METHODS AND RESULTS

We analyzed epicardial electrograms, left ventricular endocardial monophasic action potentials (MAPs), and simulated X and Y lead ECGs from 145 isolated rabbit hearts studied over a period of 41 months. Hearts from males had seasonal increases in the duration of myocardial recovery. During the months of June to September compared with October to January and February to May, epicardial activation-recovery intervals (231.6+/-23.4 vs 215.6+/-19.2 and 213.5+/-18.8 msec, P = 0.003), MAP durations (256.5+/-25.4 vs 237.0+/-19.6 and 230.7+/-26.4 msec, P < 0.001), and QT intervals (278.3+/-25.6 vs 267.3+/-11.8 and 261.3+/-13.0 msec, P = 0.037) were longer. Overall, hearts from females had shorter QT intervals than males (257.7+/-15.7 vs 270.1+/-20.3 msec, P < 0.001), and this difference was reflected in their shorter epicardial activation-recovery intervals and MAP durations. However, hearts from females showed a greater prolongation of epicardial recovery (P = 0.007) and greater incidence of arrhythmias (P < 0.001) with sotalol than males. Also, the incidence of arrhythmias was greater in the winter months October to May (P < 0.001).

CONCLUSION

The isolated rabbit heart provides a spontaneous model of gender and seasonally related differences in cardiac electrophysiology and arrhythmia susceptibility. These differences may be related to variation in the expression of or regulation of the membrane ion channels mediating repolarization.

Modulation of arrhythmias by isoproterenol in a rabbit heart model of d-sotalol-induced long Q-T intervals

Sympathetic influences have been implicated in arrhythmias associated with both congenital and acquired long Q-T intervals. We recorded epicardial electrograms, a left ventricular endocardial monophasic action potential (MAP), and a bipolar electrocardiogram in 23 isolated rabbit hearts. Spontaneous focal arrhythmias appeared within 8-18 min following 92 microM d-sotalol in 15 of 23 hearts. The epicardial activation-recovery interval was shorter at baseline and increased to a significantly greater degree after d-sotalol administration in the hearts that developed focal activity. The standard deviation of the activation-recovery interval of the epicardial sites also increased. With the addition of 0.01 microM isoproterenol, the incidence of focal activity increased, and its mean cycle length was shortened by 7%. Also, myocardial recovery time in the epicardium was shortened to a greater degree than the endocardial MAP duration. It did not alter local epicardial heterogeneity of recovery but did increase the regional dispersion between epicardial recovery times, and the endocardial MAP duration. Therefore, beta-adrenergic stimulation in the presence of d-sotalol favors the appearance of arrhythmias by increasing the propensity for closely coupled focal activity and the temporal dispersion of recovery.

Modulation of quinidine-induced arrhythmias by temperature in perfused rabbit heart

We used low temperature to slow ion channel kinetics and studied the electrophysiological effects of quinidine at different pacing rates in isolated rabbit hearts. Fifteen epicardial electrograms together with an endocardial monophasic action potential were recorded. Epicardial activation and local recovery times were measured. Arrhythmias together with the characteristics of their mode of induction and rate were analyzed by epicardial activation sequence mapping. In the presence of quinidine, arrhythmias consistent with both triggered activity and reentry were observed. At baseline, triggered activity was not inducible, even though at 25 degrees C the recovery time was greater than that in the presence of quinidine at 36 degrees C. Also, with quinidine, the incidence of triggered activity decreased at 30 and 25 degrees C. Therefore prolongation of the recovery time per se does not cause triggered activity. Quinidine's use-dependent effects on conduction and reverse use-dependent effects on recovery time were amplified by low temperatures. These findings can be understood in terms of the known temperature sensitivities of the kinetics of the membrane ion channels responsible for activation and recovery. The results demonstrate that temperature can be used as a tool to elucidate mechanisms of drug action.

Pathophysiologic substrate for sustained ventricular tachycardia in coronary artery disease

Sustained ventricular tachycardia (VT) in the presence of coronary artery disease (CAD) is almost always associated with prior infarction. Its mechanism is reentrant excitation and it can be initiated > 95% of the time. Disrupted and delayed endocardial activation and prolonged, fragmented electrograms recorded during sinus rhythm distinguish patients with VT from those with normal ventricles and those of prior infarction without VT. The extent of abnormalities of activation and number of abnormal, fragmented and late electrograms are greatest in patients with sustained VT. These abnormalities are associated with scar tissue separating the viable myocytes. Fragmented electrograms are due to discontinuous activation due to nonuniform anisotropy caused by the scar tissue. Patients with CAD demonstrate depressed excitability and prolonged relative refractory periods (ie, an upward shift in the strength-interval curve) at sites of infarction but effective refractory periods measured at 10 mA comparable to normals and dispersion of refractory periods. However the associated abnormalities of conduction and activation produce an abnormal dispersion of recovery. Intraoperative mapping of patients with CAD has shown that most of the abnormalities of endocardial activation and conduction are in the subendocardial layers and subendocardial resection of these areas cures VT and abolishes delayed, fragmented electrograms and split potentials and normalizes the electrograms recorded from the subjacent tissue. This supports the hypothesis that abnormalities of conduction are the critical pathophysiologic substrate of VT in CAD.