V_ESTPD 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 V_ESTPD at RER <1.0 was significantly lower and the V_EBTPS was higher because of higher breathing frequency; at VO₂max, both V_ESTPD and V_EBTPS were not significantly different. As percent of VO₂max, the V_EBTPS was nearly identical and V_ESTPD was 30% lower throughout the exercise at 455 mmHg. The lower V_ESTPD at lower pressure differs from two classical studies of acclimatized subjects (Silver Hut and OEII), where V_ESTPD at submaximal workloads was maintained or increased above that at sea level. The lower V_ESTPD at 455 mmHg in unacclimatized subjects at submaximal workloads results from acute respiratory alkalosis due to the initial fall in HbO₂ (≈0.17 pHa units), reduction in PACO₂ (≈5 mmHg) and higher PAO₂ throughout the exercise, which are partially pre-established during acclimatization. Regression equations from these studies predict V_ESTPD from VO₂ and P_B in unacclimatized and acclimatized subjects. The attainment of ventilatory acclimatization to altitude can be estimated from the measured vs. predicted difference in V_ESTPD at low workloads after arrival at altitude.