Exhaled carbon monoxide levels during treatment of acute asthma.

Carbon monoxide is known to be present in measurable quantities in the exhaled air of normal subjects and at higher concentrations in asthmatic patients not treated with glucocorticoids. To examine whether exhaled CO is useful in monitoring asthma control, time course changes in peak expiratory flow rate (PEFR) and exhaled CO concentration before and after treatment of acute asthma exacerbations were measured in 20 asthmatic patients. Exhaled CO was measured in triplicate by a portable CO analyser. Exhaled CO was reproducible at all time points. Asthma exacerbations caused a fall in PEFR and a rise in exhaled CO (towards an average of 3.3 parts per million (ppm)) in all patients, and treatment with oral glucocorticoids reversed these changes in both parameters. An improvement of PEFR was closely associated with a reduction of exhaled CO (to an average of 1.5 ppm) after treatment. The maximal exhaled CO concentration significantly correlated with recovery time of PEFR after treatment with oral glucocorticoids (p<0.01). The present study suggests that exhaled CO may be a useful noninvasive means of monitoring the control of asthma.