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 ( SpO 2 ) 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 J anuary 2005 and A pril 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.