Role of heart mass in the developmental changes of ventricular fibrillation threshold and spontaneous defibrillation in young dogs.

In the course of studying developmental changes of induction and maintenance of ventricular fibrillation in canine pups, we have documented that at about the third week of age, hearts reach a critical point where ventricular fibrillation may become both inducible and sustainable, thus forming the basis for cardiac arrhythmic death. Since age-related variations of cardiac mass may account for these findings, this study was conducted to systematically investigate the role of changing heart mass on the induction and maintenance of ventricular fibrillation in the canine heart, during the early postnatal development. Repetitive determinations of ventricular fibrillation threshold and individual incidence of spontaneous defibrillation were obtained in 87 puppies 1-6 weeks old, from litters of varied body size breeds, studied at weekly intervals. Overall, ventricular fibrillation threshold correlated positively with ventricular weight (VFTmA = 3.30 + 1.27 Vwtg, r = 0.71). However the slopes were steeper and correlations were stronger for the first, second and sixth week and nonsignificant in the fourth and fifth weeks. The ventricular fibrillation to ventricular weight ratio also varied with age (P < 0.01). Spontaneous defibrillation occurred at least once in 68 of the puppies (78%). In general, spontaneous defibrillation was more likely to occur in hearts weighing less than 9 grams (P < 0.01), but the overall correlation of the decreasing defibrillation incidence to increasing weight was weak (SDF % = 48.6-0.90 Vwtg, r = 0.106). Spontaneous defibrillation was not observed at any age or weight in two litters, totaling 9 puppies.(ABSTRACT TRUNCATED AT 250 WORDS)