Key result
Oscillatory lower body negative pressure below 0.02 Hz was optimally regulated by the neural system to maintain arterial pressure, whereas higher frequencies caused increasingly lagged responses.
Human cardiovascular regulation optimally maintains arterial pressure during hypotensive stress at frequencies below 0.02 Hz, with efferent responses lagging at higher frequencies.
Neural pressure regulation is frequency-optimized below 0.02 Hz in healthy adults; hypothesis-generating for baroreflex models and clinical translation.
The frequency response characteristics of human cardiovascular regulation during hypotensive stress have not been determined. We therefore exposed 10 male volunteers to seven frequencies (0.004-0.1 Hz) of oscillatory lower body negative pressure (OLBNP; 0-50 mmHg). Fourier spectra of arterial pressure (AP), central venous pressure (CVP), stroke volume (SV), cardiac output (CO), heart rate (HR), and total peripheral resistance (TPR) were determined and first harmonic mean, amplitude, and 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 unstressed control spectra. Due to the biomechanics of the system, the magnitudes of oscillations in calf circumference (CC) and CVP decreased with increasing frequency; therefore, we normalized responses by these indexes of the fluid volume shifted. The ratios of oscillations in AP to oscillations in CC increased by an order of magnitude, whereas oscillations in CVP to oscillations in CC and oscillations in AP to oscillations 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 of fluid shifted increased (peaking at 0.08 Hz). The phase relationships between variables, particularly the increasing lags in SV and TPR, but 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 regulating AP; OLBNP-induced decreases in SV (by as much as 50%) were counteracted by appropriate oscillations in HR and TPR responses. As OLBNP frequency increased, SV, TPR, and HR oscillations increasingly lagged the input and became less optimally timed for AP regulation.
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Levenhagen et al. (1994) studied this question. Oscillatory lower body negative pressure (OLBNP) vs. Unstressed control was evaluated on Frequency response characteristics of cardiovascular regulation (AP, CVP, SV, CO, HR, TPR). Oscillatory lower body negative pressure below 0.02 Hz was optimally regulated by the neural system to maintain arterial pressure, whereas higher frequencies caused increasingly lagged responses.
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