Intrinsic heart rate in children and young adults: an index of sinus node function isolated from autonomic control.

Standard evaluation of children with sinus node (SN) dysfunction cannot distinguish abnormal autonomic tone from intrinsic SN disease. This distinction has potentially important therapeutic and prognostic implications. Intrinsic heart rate (IHR)--the peak heart rate (HR) measured during pharmacologic combined autonomic blockade--reflects intrinsic SN function. The purpose of this study was to evaluate the use of IHR--and its relationship with resting heart rate (RHR)--in distinguishing autonomic influence from intrinsic SN disease among children with SN dysfunction. IHR was determined in the electrophysiology laboratory using intravenous propranolol, 0.2 mg/kg, followed by intravenous atropine, 0.04 mg/kg; the peak HR recorded was the IHR. IHR was measured in two groups. Seven control subjects, defined as patients with normal noninvasive SN testing, had IHR mean of 128 +/- 24 beats/min; this was greater than RHR mean of 89 +/- 16 beats/min (p less than 0.01). Eight patients with abnormal noninvasive SN testing had IHR mean of 103 +/- 6 beats/min; this was greater than RHR mean of 71 +/- 9 beats/min (p less than 0.01). We therefore reached the following conclusions. (1) Among this particular group of patients with abnormal noninvasive SN testing, IHR was consistently in the normal range for age and greater than RHR, suggesting that excess vagal tone can play a significant role in the expression of SN dysfunction. (2) Among normal individuals, IHR is age-related, decreasing with advancing age. IHR greater than RHR suggests that vagal tone predominates in the normal resting state as the net increase in HR during combined autonomic blockade is due to blockade of vagally mediated chronotropic inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)