Pulmonary diffusing capacity measured at multiple intervals during a single exhalation in man

Abstract

In 6 healthy subjects and 12 patients with airway obstruction the lung volume and carbon monoxide and He concentrations were measured continuously during a single enhalation at a constant flow rate. Rather than calculating the single-breath diffusing capacity (DLCO SB)) during the period of breath holding, the diffusing capacities (DLCO (exhaled)) were measured within each 10% decrement in the lung volume from 20-80% of the vital capacity. In seated healthy subjects, DLCO (exhaled) was unchanged as lung volume decreased, at rest and during exercise (P > 0.5). At rest the DLCO (exhaled) was unaffected by inhalation time, prior breath-holding time and lung volume. DLCO (exhaled) was unaffected by the exhaled flow rate when the breath-holding time exceeded 5 s (P > 0.5). In the prone position DLCO (exhaled) increased as lung volume decreased (P < 0.05). When the subject exhaled forcibly through a 3 mm orifice, DLCO (exhaled) decreased as lung volume decreased markedly as lung volume decreased in the small group of patients studied with chronic obstructive pulmonary disease. This test may prove useful in the early detection of diffusion abnormalities in disease, and avoids the assumptions of the standardized single-breath diffusing capacity.