Increased hypoxic ventilatory response during hypovolemic stress imposed through head-up-tilt and lower-body negative pressure

Abstract The aim of this study was to quantify the influence of head-up-tilt (HUT) on the isocapnic hypoxic ventilatory response (HVR) in man, and to investigate the effect of orthostatic blood shifts separately from other gravitational effects by the application of lower-body negative pressure (LBNP) with subjects in a horizontal position. HVR was measured in 15 subjects during passive HUT from 0° to 85° as well as during −7° head-down-tilt and while they were in a sitting position. In a subgroup of eight subjects the effect of 85° HUT was compared to a corresponding LBNP of −70 mbar on HVR. Moreover, by imposing graded HUT (7°, 15°, 30°, 50°) and LBNP (−15, −30 mbar) we studied the effect of low-level orthostatic stress on HVR. Ventilation, end-tidal partial pressure of CO2, heart rate and blood pressure were recorded continuously for 1 min before, and during HVR. HVR was significantly increased by ≅50% through both 85° HUT and −70 mbar LBNP as compared to 0° and 0 mbar, respectively, at unchanged mean arterial pressure. Low-level HUT and LBNP had no effect on HVR. It was concluded that the orthostatic HVR increase may be attributable to caudal blood shifts (i.e., central hypovolemia). This HVR increase requires a pronounced hypovolemic stress but no decrease in arterial blood pressure. It is suggested that a central interaction of arterial and cardiopulmonary baroreceptors is underlying this response. Their separate contribution remains to be assessed.