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 maint enance of ventricular fibrillation in canine pups, we have documented that at about the third-week of age, hearts reach a critical point whe re ventricular fibrillation may become both inducible and sustainable, thus forming the basis for cardiac arrhythmic death. Since age-relate d variations of cardiac mass may account for these findings, this stud y was conducted to systematically investigate the role of changing hea rt mass on the induction and maintenance of ventricular fibrillation i n the canine heart, during the early postnatal development. Repetitive determinations of ventricular fibrillation threshold and individual i ncidence 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 pos itively with ventricular weight (VFTmA=3.30+1.27V(wtg), r=0.71). Howev er the slopes were steeper and correlations were stronger for the firs t, second and sixth week and nonsignificant in the fourth and fifth we eks. The ventricular fibrillation to ventricular weight ratio also var ied with age (P<0.01). Spontaneous defibrillation occurred at least on ce 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 inciden ce to increasing weight was weak (SDF %=48.6-0.90 Vwtg, r=0.106). Spon taneous defibrillation was not observed at any age or weight in two li tters, totaling 9 puppies. These findings suggest that ventricular mas s influences the vulnerability of the neonatal dog heart to electrical induction, as well as maintenance of ventricular fibrillation. Howeve r, the ability for spontaneous defibrillation is only partially depend ent on ventricular mass. Other developmental and genetic factors appea r to play an equally important role and require further study.