Impairment of autonomically mediated heart rate control in pa- tients with cardiac dysfunction

Patients with cardiac disorders have defective parasympathetic control of heart rate. To evaluate the possibility of similar changes in sympathetic control of heart rate, we compared reflex chronotropic responses to 80 ° upright tilt and nitroglycerin-induced hypotension in 31 cardiac patients and 7 normal individuals before and after partial parasympathetic blockade with atropine. Tilting revealed an attenuation of the normal heart rate increase in patients; the magnitude of this defect was greatest in patients with more severe symptoms (class HI) and evidence of left ventricular dysfunction (the heart rate increase averaged 25 ± 3 beats/min in normal subjects, 12 ± 2 beats/min in class I—II patients, and 7 ± 1 beats/min in class HI patients). Class m symptoms due to mechanical causes (mitral stenosis), however, were not associated with this defect. A marked reduction in heart rate rise with hypotension was seen only in those class HI patients without mitral stenosis (0.4 ± 0.1 beats min"1 mm Hg~ ' vs. 3.0 ± 0.5 beats min~l mm Hg"1 in normal subjects). This abnormality also persisted after atropine administra-tion, thus confirming a defect in the sympathetic as well as the parasympathetic compo