MATERNAL HEART RATE AND STROKE VOLUME IN THE PYGMY GOAT: EFFECTS OF EXERCISE AND CARDIAC AUTONOMIC BLOCKADE

¹ Heart Research Laboratory, Division of Cardiology, Department of Medicine, Oregon Health Sciences University, Portland, OR 97201, U.S.A.

To determine the normal maternal heart rate and stroke volume response to exercise and the role of the autonomic nervous system in this response, pregnant and non-pregnant pygmy goats were studied at rest, during and following treadmill exercise, and after autonomic blockade. Resting heart rates, stroke volumes and cardiac outputs were 10%, 60%, and 76% higher in pregnant compared to non-pregnant animals (P 0·05, 0·001, and 0·001 respectively). During exercise, heart rate and stroke volume rose in both pregnant and non-pregnant animals by similar amounts. Immediately post-exercise, stroke volume in pregnant animals dropped precipitously below resting levels by 15% (P 0·005) but cardiac output was preserved by heart rate. This fall in stroke volume was not seen in control animals. Autonomic blockade lowered resting and exercise heart rates in both groups without changing stroke volume. The elevations in heart rate and stroke volume in pregnancy are not primarily mediated via the autonomic nervous system. Although the maternal cardiac output response to exercise appears normal, the post-exercise fall in stroke volume, presumed secondary to a reduction in preload, could potentially be harmful to both mother and fetus.