THE MEASUREMENT OF PULMONARY DIFFUSING CAPACITY FOR CARBON MONOXIDE BY A REBREATHING METHOD*†

Abstract

A rebreathing method for the measurement of pulmonary diffusing capacity for carbon monoxide (Dlco) using stable CO and continuous analysis is described. The fall of CO concentration during rebreathing is apparently exponential. Calculated Dlco was independent of increases in apparatus dead space in normal subjects but not in emphysema patients. In 4 normal subjects Dlco decreased sharply when the volume of the lung-bag system was approximately the functional residual capacity. In 47 normal subjects Dlco was significantly correlated with height, weight, body surface area and vital capacity. Rebreathing and single breath measurements of DLCO correlated well in normal subjects, patients without airway obstruction and emphysema patients. Residual volume determined by rebreathing correlated significantly with residual volume by the closed circuit helium method. Empirically the rebreathing method is approximately valid in patients with uneven ventilation and slow mixing. The rebreathing method is rapid and simple. The rebreathing DLCO was 2/3 or less the predicted value in only 5 of 19 patients with emphysema.