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

The aim of this study was to quantify the influence of head-up-tilt (HUT) o n the isocapnic hypoxic ventilatory response (HVR) in man, and to investiga te the effect of orthostatic blood shifts separately from other gravitation al effects by the application of lower-body negative pressure (LBNP) with s ubjects in a horizontal position. HVR was measured in 15 subjects during pa ssive HUT from 0 degrees to 85 degrees as well as during -7 degrees head-do wn-tilt and while they were in a sitting position. In a subgroup of eight s ubjects the effect of 85 degrees HUT was compared to a corresponding LBNP o f -70 mbar on HVR. Moreover, by imposing graded HUT (7 degrees, 15 degrees, 30 degrees, 50 degrees) and LBNP (-15, -30 mbar) we studied the effect of low-level orthostatic stress on HVR. Ventilation, end-tidal partial pressur e of CO2, heart rate and blood pressure were recorded continuously for 1 mi n before, and during HVR. HVR was significantly increased by congruent to 5 0% through both 85 degrees HUT and -70 mbar LBNP as compared to 0 degrees a nd 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 i ncrease may be attributable to caudal blood shifts (i.e., central hypovolem ia). This HVR increase requires a pronounced hypovolemic stress but no decr ease 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.