VENTRICULAR-FIBRILLATION THRESHOLD IS INFLUENCED BY LEFT-VENTRICULAR STRETCH AND MASS IN THE ABSENCE OF ISCHEMIA

Study objective - Ischaemic heart disease is known to contribute to the development of ventricular arrhythmias. However, the role of non-ischaemic variables has been less well defined. We therefore studied the effect of myocardial stretch and ventricular mass on the vulnerability of the rat heart to ventricular fibrillation. Design and experimental material - Two groups of rat hearts were studied in an isolated buffer perfused apparatus: group I, mature female animals with an average dry left ventricular weight of 73 mg and group II, a group of retired breeders with an average left ventricular weight of 122 mg. Hearts performed isovolumetric work at either low (0 mm Hg) or high (20 mm Hg) left ventricular end diastolic pressures. Measurements and main results - Ventricular fibrillation was provoked by trains of ventricular extrastimuli delivered at increasing current until development of the arrhythmia. The current required to provoke ventricular fibrillation decreased in both groups at the high left ventricular end diastolic pressure and the larger hearts in group II were more vulnerable to ventricular fibrillation than those in group I. The decrease in ventricular fibrillation threshold occurred in the absence of an increase in myocardial lactate production or a decrease in endocardial to epicardial flow ratios. Conclusions - This study thus identified two variables, myocardial stretch and ventricular mass, which influence the development of ventricular fibrillation and which are independent of myocardial ischaemia in this animal model. These observations may be relevant to an understanding of the increased incidence of lethal arrhythmias which occur in patients with dilated cardiomyopathies.