Single breath diffusing capacity for carbon monoxide in stable asthma.

BACKGROUND

Single breath diffusing capacity for carbon monoxide (Dco) is commonly used as a simple method of assessing overall pulmonary gas exchange properties. Studies of Dco in bronchial asthma have yielded conflicting results.

OBJECTIVE

To study Dco and to determine the factors influencing Dco in patients with asthma.

METHODS

Dco was prospectively measured in 80 consecutive never-smoker patients with uncomplicated stable asthma. The topographic distribution of lung perfusion was determined in 10 asthmatics and 10 controls, with a 133Xe radionuclide scan.

RESULTS

The mean (SD) value of Dco was increased to 117 (17) percent of predicted values; individual values were either within or above normal limits; diffusion was also elevated at 116 (19) percent after correction for alveolar volume (transfer coefficient, D/VA). The Dco was not correlated with atopic status, duration of asthma, or results of spirometric tests; there was a weak negative correlation between D/VA and FEV1 or residual volume. There was a better perfusion of the upper zones of the lungs in asthmatics as compared with controls. Among the asthmatics, there was a strong positive correlation between Dco and the apex to base perfusion ratio (r = 0.975).

CONCLUSIONS

Dco is normal or high among never smoker patients with uncomplicated asthma; elevated Dco may be attributed to a better perfusion of the apices of teh lungs; the latter could result from two mutually nonexclusive mechanisms: an increase in pulmonary arterial pressure and/or a more negative pleural pressure generated during inspiration as a consequence of bronchial narrowing. The unexpected finding of high Dco should raise the possibility of bronchial asthma in patients with otherwise undiagnosed conditions.