Normo or hypobaric hypoxic tests: propositions for the determination of the individual susceptibility to altitude illnesses

Assessment of individual susceptibility to altitude illnesses and more particularly to acute mountain sickness (AMS) by means of tests performed in normobaric hypoxia (NH) or in hypobaric hypoxia (HH) is still debated. Eighteen subjects were submitted to HH and NH tests (PIO₂=120 hPa, 30 min) before an expedition. Maximal and mean acute mountain sickness scores (AMSmax and mean) were determined using the self-report Lake Louise questionnaire scored daily. Cardio-ventilatory (f, V_T, PetO₂ and PetCO₂, HR and finger pulse oxymetry SpO₂) were measured at times 5 and 30 min of the tests. Arterial (PaO₂, PaCO₂, pH, SaO₂) and capillary haemoglobin (Hb) measurements were performed at times 30 min. Hypoxic ventilatory (HVR) and cardiac (HCR) responses, peripheral O₂ blood content (CpO₂) were calculated. A significant time effect is found for ΔSpO₂ (P = 0.04). Lower PaCO₂ (P = 0.005), SaO₂ (P = 0.07) and higher pH (P = 0.02) are observed in HH compared to NH. AMSmax varied from 3 to12 and AMSmean between 0.6 and 3.5. In NH at 30 min, AMSmax is related to PetO₂ (R = 0.61, P = 0.03), CpO₂ (R = −0.53, P = 0.02) and in HH to CpO₂ (R = −0.57, P = 0.01). In NH, AMSmean is related to Δf (R = 0.46, P = 0.05), HCR (R = 0.49, P = 0.04), CpO₂ (R = −0.51, P = 0.03) and, in HH at 30 min, to V_T (R = 0.69, P = 0.01) and a tendency for CpO₂ (R = −0.43, P = 0.07). We conclude that HH and NH tests are physiologically different and they must last 30 min. CpO₂ is an important variable to predict AMS. For practical considerations, NH test is proposed to quantify AMS individual susceptibility using the formulas: AMSmax = 9.47 + 0.104PetO₂(hPa)–0.68CpO₂ (%), (R = 0.77, P = 0.001); and AMSmean = 3.91 + 0.059Δf + 0.438HCR–0.135CpO₂ (R = 0.71, P = 0.017).