CARDIOVASCULAR REGULATION IN HUMANS IN RESPONSE TO OSCILLATORY LOWER-BODY NEGATIVE-PRESSURE

The frequency response characteristics of human cardiovascular regulat ion during hypotensive stress have not been determined. We therefore e xposed 10 male volunteers to seven frequencies (0.004-0.1 Hz) of oscil latory lower body negative pressure (OLBNP; 0-50 mmHg). Fourier spectr a of arterial pressure (AP), central venous pressure (CVP), stroke vol ume (SV), cardiac output (CO), heart rate (HR), and total peripheral r esistance (TPR) were determined and first harmonic mean, amplitude, an d phase angles with respect to OLBNP are presented. AP was relatively well regulated as demonstrated by small oscillations in half amplitude (3.5 mmHg) that were independent of OLBNP frequency and similar to un stressed control spectra. Due to the biomechanics of the system, the m agnitudes of oscillations in calf circumference (CC) and CVP decreased with increasing frequency; therefore, we normalized responses by thes e indexes of the fluid volume shifted. The ratios of oscillations in A P to oscillations in CC increased by an order of magnitude, whereas os cillations in CVP to oscillations in CC and oscillations in AP to osci llations in CVP both tripled between 0.004 and 0.1 Hz. Therefore, even though the amount of fluid shifted by OLBNP decreased with increasing frequency, the magnitude of both CVP and AP oscillations per volume o f fluid shifted increased (peaking at 0.08 Hz). The phase relationship s between variables, particularly the increasing lags in SV and TPR, b ut not CVP, indicated that efferent responses with lags of 5-6 s could account for the observed responses. We conclude that, at frequencies below 0.02 Hz, the neural system of humans functioned optimally in reg ulating AP; OLBNP-induced decreases in SV (by as much as 50%) were cou nteracted by appropriate oscillations in HR and TPR responses. As OLBN P frequency increased, SV, TPR, and HR oscillations increasingly lagge d the input and became less optimally timed for AP regulation.