Effect of five nights of normobaric hypoxia on cardiovascular responses to acute isocapnic hypoxia in humans: relationship to ventilatory chemosensitivity.

The aim of this study was to elucidate (1) the cardiovascular responses to acute isocapnic hypoxia stimuli following five nights of normobaric poikilocapnic hypoxia, and (2) whether the changes in the cardiovascular responses to hypoxia are correlated to the change in acute hypoxic ventilatory (AHVR) chemosensitivity. Twelve male subjects [26.6 +/- 4.1 (SD) years] slept 8-9 h day(-1) overnight for five consecutive days at a simulated altitude of 4300 m (FiO2 = approximately 13.8%). Using the technique of dynamic end-tidal forcing, the AHVR was assessed twice, prior to and immediately after the hypoxic exposure. During each AHVR test, mean arterial blood pressure (MAP) and heart rate (HR) were measured continuously using finger photoplethysmography and an ECG monitor, respectively. Immediately following the exposure, AHVR and MAP sensitivities were increased by 1.80 +/- 1.30 l min(-1) %(-1) (p < 0.01) and 0.69 +/- 0.40 mmHg %(-1) (p < 0.05), respectively, and there were significant correlations between the increases in the AHVR and MAP sensitivities (r = 0.67; p < 0.05). In summary, following five nights of normobaric hypoxia, there is an enhanced MAP response to hypoxic stimuli. The relationship between the enhanced AHVR and MAP sensitivity raises the possibility of a common pathway in the regulation of peripheral chemosensitivity and MAP responses during periods of isocapnic hypoxia.