Quantitative Evaluation of Cardiac Parasympathetic Activity in Normal and Diabetic Man

TLDR

The study aimed to assess parasympathetic control of RR variation during β‑adrenergic blockade with propranolol, independent of sympathetic effects. They measured heart rate and RR variation (standard deviation of the mean RR interval over 5 min) in fasting supine diabetic and age‑matched normal subjects during β‑adrenergic blockade. RR variation was inversely related to breathing rate, decreased while heart rate increased during β‑adrenergic stimulation and atropine, and was consistently lower in diabetic subjects—both symptomatic and asymptomatic—than in normals, indicating that supine RR variation during deep breathing and β‑adrenergic blockade is a sensitive, reproducible marker of parasympathetic activity that may be impaired early in diabetes.

Abstract

Heart rate and RR variation (the standard deviation of the mean RR interval for a 5-min period) were evaluated as measurements of cardiac parasympathetic nervous system activity in fasting supine diabetic (N = 22) and comparable age normal (N = 22) subjects. The rate of breathing did not effect heart rate, but was inversely related to the RR variation (r = 0.89, P < 0.01). Heart rate was increased (P < 0.0001) and RR variation decreased (P < 0.05) during β-adrenergic stimulation with isoproterenol and during parasympathetic blockade with atropine (both P < 0.0001). Hence, the cardiac effects of β-adrenergic stimulation may mimic the effects of diminished parasympathetic function. To evaluate parasympathetic control of RR variation, independently of possible effects of increased sympathetic activities, studies were performed during β-adrenergic blockade with propranolol. RR variation during propranolol was less both in 14 diabetic subjects without clinical symptoms of autonomic neuropathy (P < 0.005) and in 8 diabetics with clinical symptoms of autonomic neuropathy (P < 0.001) when compared with 22 age-comparable normal subjects. The measurement of RR variation was very reproducible with a day-to-day coefficient of variation of 9.7 ± 2.8﹪ (x̄ ± SEM) in diabetic subjects with stable hyperglycemia. It is concluded that supine RR variation during a deep respiratory rate and during β-adrenergic blockade is a sensitive, quantitative, and reproducible method to evaluate parasympathetic nervous activity in normal and diabetic subjects. Furthermore, cardiac parasympathetic activity may be diminished in diabetic subjects before clinical symptoms of autonomic neuropathy are evident.