AGE-DEPENDENCE OF THE DEVELOPMENT OF VENTRICULAR ARRHYTHMIAS IN A CANINE MODEL OF SUDDEN CARDIAC DEATH

Objectives: The age-dependence of the development of ventricular arrhy thmias was studied in German shepherd dogs with inherited ventricular arrhythmias and sudden death. Background: A colony of German. shepherd dogs has been established that exhibit inherited ventricular arrhythm ias and sudden death. The incidence of arrhythmias increases with age. Because ventricular tachycardia is associated with bradycardia, it wa s hypothesized that the increased incidence of arrhythmias was related to age-dependent slowing of heart rate. Methods: Arrhythmia counts an d RR intervals were measured from serial ambulatory ECG recordings obt ained in 71 dogs (1-48 weeks). In addition, 19 dogs were challenged wi th phenylephrine (10 mu g/kg i.v.) at 15, 28, and 45 weeks of age, 10 dogs were challenged with epinephrine (1 mu g/kg i.v.) at 3, 5, 7, 9, 11, 18, and 28 weeks of age, and 10 dogs were challenged at 28 weeks w ith epinephrine (2.5 mu g/kg i.v.), before and after propranolol (0.5 mg/kg i.v.). Results: The incidence and severity of ventricular arrhyt hmias increased between 7 and 28 weeks of age and decreased between 28 and 44 weeks of age. The age-dependent increase in the incidence of v entricular tachycardia was associated with age-dependent reductions in sinus rate. Baroreflex-mediated slowing of the heart rate unmasked ar rhythmias in young animals that did not spontaneously display arrhythm ias and exacerbated existing arrhythmias in older animals. However, th e magnitude of baroreflex-induced bradycardia was similar from 7-18 we eks of age, yet the incidence of arrhythmias increased progressively. Moreover, the waning of ventricular arrhythmias in older animals was n ot associated with more rapid sinus rates. Conclusion: The risk for su dden death in dogs with inherited ventricular arrhythmias increases wi th age in part because of age-dependent slowing of heart rate and in p art because of other heart-rate-independent factors. The correspondenc e between the development of ventricular tachycardia and sinus pauses is consistent with the hypothesis that ventricular arrhythmias are ini tiated by early afterdepolarization-induced triggered activity.