Augmented chemosensitivity at altitude and after return to sea level: impact on subsequent return to altitude

Abstract

Augmentation of hypoxic (HVR) and hypercapnic (HVCR) ventilatory chemosensitivity is a component of acclimatization in lowlanders sojourning at high altitude. Previous studies suggest that the augmented ventilatory chemosensitivity declines upon return to sea level (SL) over one or more weeks. We hypothesized that some degree of ventilatory acclimatization would be retained to SL due to retention of enhanced chemosensitivity. Ventilation (VE/VCO₂), arterial oxygenation (SaO₂), HVR (VE/Sa-O₂) and HCVR (VE/PCO₂) of 11 male lowlanders were measured during rest at SL, after 1, 2 and 14 days residence at 4300 m, and at 1,3 and 7 days after return to SL and in 6 subjects during a 24 hr RA after 8 days at SL. Ventilatory acclimatization produced an increase (P< 0.005) in VE/VCO₂ (~12%), SaO₂ (~10%), HVR (~170%) and HCVR (~43%). After returning to SL, HVR and HCVR remained elevated (P<0.05) for at least 3 days. During RA, subjects demonstrated a retention of ~90 and ~67% of their acclimatization responses for VE/VCO₂ and SaO₂ respectively, even thought HVR and HCVR were no longer statistically elevated. These measurements of ventilation and arterial oxygenation during re-exposure to high altitude clearly indicate the retention of ventilatory acclimatization, thus lessening the hypoxic stress during subsequent sojourns to altitude within that time period. However, the absence of augmented chemosensitivity to either hypoxia or hypercapnia in these subjects leaves the mechanism for this enhanced ventilatory response in question.