Autonomic Influences on Cardiac Function in the Newborn Lamb

The influences of reflexly induced changes of sympathetic outflow to the heart on left ventricle contractility, heart rate, and systemic vascular resistance were studied in 26 technically successful experiments with newborn lambs ranging in age from 1 to 11 days. Reduction of brachiocephalic artery perfusion pressure was associated with a large increase of heart rate and of systemic vascular resistance. Under conditions of constant aortic pressure, heart rate and cardiac output, cephalic hypotension in most preparations induced a rapid onset of a fall of left ventricular end-diastolic pressure and an increase in the maximal rate of rise of left ventricular pressure (dp/dt max). Ventricular function curves obtained under conditions of constant aortic pressure and heart rate were shifted to the left. The average increase of stroke volume at left ventricular end-diastolic pressure of 10 cm H 2 O (SV 10 ) was 23.3%, and of dp/dt max was 30.7% in the reactive preparations. The average of the largest responses for each preparation was an increase of SV 10 of 42%, and for left ventricular dp/dt max, 48.8%. Three preparations previously subjected to carotid sinus nerve sectioning showed similar responses. No evidence for diminution of the response following parasympathetic blockade with atropine was found. In all preparations, ganglionic blockade virtually or completely eliminated the response.