Cardiopulmonary baroreflexes: left ventricular effects.

Systemic vascular resistance of dogs perfused at constant arterial pressure by a pump oxygenator was used as a measure of left ventricular baroreflexes initiated by imposing pressure changes on the beating, as well as on the fibrillating, ventricle. In the unloaded beating heart mean left atrial and systolic ventricular pressures averaged 0.3 and 24 mmHg, respectively. Afterloads and preloads that increased systolic pressure to 63 mmHg while raising mean atrial pressures to 3.3 and 1.7 mmHg, respectively, had no effect on resistance. However, pressurization of the fibrillating ventricle caused vasodepressor responses proportional to the stimulus. The ventricular baroreflex had a significantly lower sensitivity and higher threshold than the vasodepressor reflex observed when simultaneously pressurizing the fibrillating ventricle, beating atrium, and lung vessels. On the basis of wall tension, the results are consistent with other reports that have shown ventricular baroreflexes to be initiated by systolic pressure in the range 100-150 mmHg, but not by diastolic pressure in the range 10-20 mmHg. A more sensitive reflex, able to respond to changes of ventricular filling pressure, arises from the left atrium and pulmonary veins.