Exercise alveolar oxygen extraction rate reflects ventilatory efficiency and predicts outcomes in idiopathic pulmonary fibrosis.

BACKGROUND

Oxygen uptake (V'O2) obtained from expiratory gas analysis is generally calculated using minute ventilation (V'E) and the inspired‒expired mean oxygen (O2) concentration difference (ΔFO2) during cardiopulmonary exercise testing (CPET). We have reported that ΔFO2, which is associated with ventilatory efficiency, is independent of V'E at peak exercise and affects exercise tolerance in respiratory diseases other than idiopathic pulmonary fibrosis (IPF). We hypothesized that similar results are obtained in IPF, and that ΔFO2 is a prognostic factor for survival in IPF.

METHODS

Forty-three patients with IPF, who underwent CPET with blood gas analysis were enrolled from our database.

RESULTS

At peak exercise, ΔFO2 was strongly correlated with variables related to ventilatory efficiency, i.e., V'E/carbon dioxide output (V'CO2) ratio at the nadir during exercise (r=‒0.91) and correlated well with peak V'O2 (r = 0.67), but it was independent of V'E (r = 0.24) at peak exercise. Two multivariate Cox proportional hazards models with adjustment for age, including the previously reported prognostic factors, showed that ΔFO2 at peak exercise was a stronger predictor of survival than (1) peak V'O2, V'E at peak exercise in a first analysis (hazard ratio: 0.195, 95% CI 0.070 to 0.500; p = 0.0005) and (2) than tidal volume at peak exercise, body mass index, and arterial oxygen tension (PaO2)-slope, i.e., the decrease in PaO2 per the increase in V'O2 during exercise in a second analysis (hazard ratio: 0.437, 95% CI 0.201 to 0.958; p = 0.0389).

CONCLUSIONS

These results show that ∆FO2 at peak exercise, which is correlated with ventilatory efficiency related to carbon dioxide clearance, is independent of ventilatory ability and is a stronger prognostic factor for survival than physiological ventilatory impairments with hypoxemia in IPF. CPET is essential for evaluating exercise alveolar O2 extraction and guiding the optimal management of patients with IPF.