VSTPD as a measure of ventilatory acclimatization to hypobaric hypoxia.

This study compared the ventilation response to an incremental ergometer exercise at two altitudes: 633 mmHg (resident altitude = 1,600 m) and following acute decompression to 455 mmHg (≈4,350 m altitude) in eight male cyclists and runners. At 455 mmHg, the VSTPD at RER <1.0 was significantly lower and the VBTPS was higher because of higher breathing frequency; at VOmax, both VSTPD and VBTPS were not significantly different. As percent of VOmax, the VBTPS was nearly identical and VSTPD was 30% lower throughout the exercise at 455 mmHg. The lower VSTPD at lower pressure differs from two classical studies of acclimatized subjects (Silver Hut and OEII), where VSTPD at submaximal workloads was maintained or increased above that at sea level. The lower VSTPD at 455 mmHg in unacclimatized subjects at submaximal workloads results from acute respiratory alkalosis due to the initial fall in HbO (≈0.17 pHa units), reduction in PACO (≈5 mmHg) and higher PAO throughout the exercise, which are partially pre-established during acclimatization. Regression equations from these studies predict VSTPD from VO and P in unacclimatized and acclimatized subjects. The attainment of ventilatory acclimatization to altitude can be estimated from the measured vs. predicted difference in VSTPD at low workloads after arrival at altitude.