Vital capacity and oxygen saturation at rest and after exercise predict hypoxaemia during hypoxic inhalation test in patients with respiratory disease

BACKGROUND AND OBJECTIVE: The hypoxic inhalation test (HIT) accurately predicts air travel hypoxaemia, but availability is limited. We assessed whether oxygen saturation (SpO2) during HIT can be predicted by simple measures of lung function and exercise-induced hypoxaemia.

METHODS: Analysis of all patients who had HIT and lung function testing performed between January 2005 and April 2010. Prior to HIT, SpO(2) on room air was measured at rest and after 2 min of moderate exercise. HIT was conducted using fractional inspired oxygen concentration 0.14 and SpO(2) was measured after equilibration of ≥ 5 min. The relationships between SpO(2) during HIT (HIT SpO(2)) and resting SpO(2), post-exercise SpO(2), gas exchange (diffusing capacity for carbon monoxide) and ventilatory capacity were examined. Linear regression was performed to identify significant predictors of HIT SpO(2).

RESULTS: 137 patients were studied: 70 had chronic obstructive pulmonary disease, 43 interstitial lung disease and 20 extra-pulmonary restriction. All patients with HIT SpO(2) ≤ 85% had post-exercise SpO(2) < 95% (n = 24 (18%)). HIT SpO(2) correlated best with post-exercise SpO(2) overall (r = 0.69, P < 0.01). Linear regression identified forced vital capacity % predicted, resting SpO(2) and post-exercise SpO(2) as independent predictors of HIT SpO(2).

CONCLUSIONS: In patients with respiratory disease, post-exercise SpO(2) ≥ 95% on room air may be used to exclude the need for HIT to assess oxygen requirement for air travel. HIT SpO(2) can be estimated from forced vital capacity % predicted, resting SpO(2) and post-exercise SpO(2) using a linear regression model.