Gas exchange at rest during simulated altitude in patients with chronic lung disease

To characterize the gasometric and oximetric response to simulated altitudes of 3,100 m and sea level of patients with Chronic Obstructive Pulmonary Disease (COPD) and Interstitial Lung Disease (ILD) studied at 2,240 m above sea level.

Consecutive stable patients with COPD and ILD were studied at the National Institute of Respiratory Diseases, a referral center for pulmonary diseases in Mexico City, and a healthy control group. The patients breathed room air (FIO2 = 0.21), for at least 15 min, then, a hypoxic mixture (FIO2 = 0.18, simulating 3,100 m), and finally, a hyperoxic mixture (FIO2 = 0.28, simulating sea level). Arterial blood gases and oxygen saturation were measured by a pulse oximeter at the end of each stage.

Twelve patients with COPD, 13 patients with ILD and 11 healthy controls were studied. The PaCO2 and pH were constant in the three study stages in both groups of patients and controls. A slope of PaO2 vs. altitude of 9 Torr per Km was found for each of the study's patients, either by simple linear regression or multiple regression, which is identical to that previously obtained at sea level with COPD patients (Gong et al.). Oxygen desaturation per Km of altitude change was alinear, higher for the hypoxic than for the hyperoxic challenge and more severe for the most hypoxic patients.

Exposure tests to simulated altitudes are safe, and orient the physician concerning the patient's condition at altitudes different from the place where the measurement is done. Alveolar ventilation remains constant despite hypoxia or hyperoxia during the challenges. A computer model of the lung reproduces many of the findings in the challenges of this study.