Carbon monoxide diffusing capacity in asthmatic patients with mild airflow limitation.

The aim of this study was to test the hypothesis that carbon monoxide diffusing capacity (DCO) is elevated in asthmatic patients with minimal airflow limitation and/or hyperinflation; the latter factors should reduce the possibility of technical errors in the measurement of DCO. In ten asthmatic and ten healthy subjects, DCO and its components, membrane diffusing capacity (Dm) and pulmonary capillary blood volume (Qc) were measured by the single-breath method. Values were normalized for alveolar volume (VA). The mean DCO/VA was higher in the asthma groups as was the Qc/VA. The Dm/Qc was also higher in the asthma group. In the asthmatic but not the healthy subjects, both DCO/VA and Qc/VA were negatively correlated with the forced expiratory flow at 50 percent of vital capacity and peak inspiratory flow rate. Thus, DCO/VA may be increased in asthmatic patients with only mild airflow limitation; this may be due to an elevated capillary blood volume.