EFFECTS OF BLOOD-VOLUME DISTRIBUTION ON THE CHARACTERISTICS OF THE CAROTID BAROREFLEX IN HUMANS AT REST AND DURING EXERCISE

Seven supine subjects were studied at rest and during mild to moderate dynamic leg exercise with and without unloading of the cardiopulmonar y baroreceptors accomplished by exposing the lower portion of the body to a subatmospheric pressure of 20 mmHg (Lower Body Negative Pressure , LBNP). The function of the cardiac branch of the carotid baroreflex was studied over its full operational range by measuring R-R intervals during application of pulse synchronous graded pressures (40 to -65 m mHg) in a neck-chamber device. Raising the carotid transmural pressure (systolic arterial pressure minus neck-chamber pressure) induced incr easing R-R intervals in all conditions. In conformity with previous re sults from our laboratories it was found that the maximal rate of chan ge in relative R-R intervals and the corresponding transmural pressure were higher during exercise than at rest, indicating that exercise in creased the carotid baroreflex sensitivity and shifted its optimal buf fering range to higher arterial pressures. LBNP did not affect the cha racteristics of the reflex at rest nor during exercise. It is conclude d that reduced central venous pressure with consequent selective cardi opulmonary receptor disengagement exerts no influence on the carotid b aroreflex control of heart rate (HR), as tested over the entire arteri al pressure-effector response relation, either at rest or during mild- moderate exercise.