Pulmonary diffusing capacity for carbon monoxide by rebreathing in awake dogs.

In order to reduce the effects of functional inhomogeneities on the determination of pulmonary diffusing capacity (transfer factor) for CO (DLCO), DLCO was measured by a rebreathing method in 5 awake chronically tracheostomized dogs (mean body weight 28 kg) during high ventilation (15-25 times above normal) induced by hypoxia, hypercapnia, exercise, or their combinations. The animals rebreathed for 15 s a mixture containing 1% He, 0.1% C18O, with CO2 and O2 concentrations adjusted to maintain end-tidal PCO2 and PO2 close to their prerebreathing values. Gas partial pressures in the trachea were continuously monitored by mass spectrometry. DLCO was calculated from C18O equilibration kinetics, effective ventilation (obtained from He mixing kinetics) and lung volume (obtained from He dilution). Each of the factors, hypoxia, hypercapnia and exercise, contributed to increasing DLCO. During exercise in combined hypoxia and hypercapnia, DLCO (mean +/- SE) was 71.7 +/- 1.8 ml/(min X Torr). The mechanisms for increases in DLCO might have involved improvement of diffusion conditions, decrease of functional inhomogeneities and reduction of the effects of functional inhomogeneities. The highest DLCO values were close to values derived from morphometry.