Effects of lower body positive pressure on muscle sympathetic nerve activity

To elucidate the role of vasomotor sympathetic nerve activity response to lower-body positive pressure (LBPP), muscle sympathetic nerve activity (MSNA) along with blood pressure, electrocardiogram, transthoracic impedance and echocardiogram were recorded in ten healthy young men at 10, 20 and 30 mmHg LBPP in the supine position. We found that MSNA was suppressed (27%) at both 10 and 20 mmHg LBPP but tended to increase at 30 mmHg LBPP. Mean arterial pressure (11%) and total peripheral resistance (36%) increased despite a tendency toward decreased stroke volume and cardiac output at 30 mmHg LBPP. Heart rate remained unchanged. The left atrial dimension significantly increased during 10-30 mmHg LBPP, indicating an increase in cardiac filling. These results suggest that LBPP at levels less than 20 mmHg suppressed MSNA by the translocation of blood from the lower body to the thorax, thus stimulating the cardiopulmonary baroreceptors which in turn inhibited vasomotor sympathetic nerve activity. However, LBPP at 30 mmHg tended to enhance MSNA through the activation of intramuscular (pressure-sensitive) receptors which could counteract the inhibitory effect of cardiopulmonary baroreflexes on MSNA.