Heart weight affects spontaneous defibrillation but not ventricular fibrillation threshold

Among the factors involved in the induction, generalization, and reversion of cardiac fibrillation, the amount of tissue is a determinant factor. One question is whether an increase in myocardial mass would or would not require the same electrical threshold. Accordingly, one objective was to determine if ventricular weight (VW) has any effect on the fibrillation thresholds (FTs). A second objective was to find a possible relationship between spontaneous defibrillation (SPDE) and heart weight. Fifty mongrel dogs of both sexes were used, of which 26 were 2- to 10-week-old puppies. The rest were adult animals of undetermined age. Fibrillation was induced with two types of stimuli directly applied to the heart. The stimuli were a train of rectangular pulses (TP) not synchronized with the ECG, and single pulses (SP) synchronized with and delayed from the R wave. TP type was used in one group with paddle electrodes; and the TP and SP types were used in a second group with hook electrodes. For both types, stimulation started at a low current and was increased until fibrillation was triggered. Once defibrillated, either spontaneously or by electrical shock, the procedure was repeated. Correlation coefficients between FT and VW were low (< 0.4), and scaling of the thresholds to VW resulted in hyperbolic relationship, for all cases, thus suggesting independence of the two parameters. SPDE fell sharply with weight. For values higher than 12 grams it was essentially nonexistent. VW does not have any effect on the FT but it dramatically influences the capacity of the myocardium to revert the arrhythmia by itself.