Circulatory and respiratory responses to lower body negative pressure in man.

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Abstract
Circulatory and ventilatory responses to lower body negative pressure (LBNP) were simultaneously investigated in 8 healthy men before, during, and after the application of -20, -40, and -60 mmHg pressure. Minute ventilation (.ovrhdot.VE) decreased during LBNP due to a fall in respiratory frequency with sustained tidal volume. The cardiac output (.ovrhdot.Q) was reduced in proportion to the applied LBNP exposure, while .ovrhdot.QE decreased to almost the same level at all LBNP applications. In spite of decreased .ovrhdot.VE, end-tidal PO2 and PCO2 were increased and decreased, respectively, indicating a relative alveolar hyperventilation. The ventilation equivalent for O2 (.ovrhdot.VE/.ovrhdot.VO2) increased, while the cardiac output equivalent for O2 (.ovrhdot.Q/.ovrhdot.VO2) decreased. The relation beteween SS.DELTA.+VE/.ovrhdot.VO2 and .ovrhdot.Q/.ovrhdot.VO2 showed a significant negative correlation (r = -0.93, p < 0.01). The veno-arterial CO2 concentration difference (C.hivin.vCO2-CaCO2) increased with LBNP, due to a fall in CaCO2 with constant C.hivin.vCO2. The constant C.hivin.vCO2 indicated a constant tissue acid-base balance. These observations suggest the existence of a ventilatory mechanism improving the efficiency of respiration in order to compensate for the sustained LBNP depression of $SD+Q at a given gas exchange.

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