Increased carbon monoxide in exhaled air of critically ill patients

Heme oxygenase produces carbon monoxide (CO) during breakdown of heme molecules primarily in liver and spleen. Recent data suggest that CO is also produced in the lungs. CO is excreted by exhalation via the lungs. A number of inflammatory agents induce the expression of heme oxygenase, possibly leading to increased CO production. To investigate whether critical illness results in increased CO production we measured the CO concentration in exhaled air in 30 critically ill patients and in healthy controls (n = 6). Critically ill patients showed a significantly higher CO concentration in exhaled air (median 2.4 ppm, 95% CI 1.0-7.0 ppm vs median 1.55 ppm, 95% CI 1.2-1.7 ppm, P = 0.01) as well as total CO production (median 20 ml/min, 95% CI 8 to 90 ml/min vs median 13.5 ml/min, 95% CI 11 to 19 ml/min, P = 0.026) compared to healthy controls. No correlation was found between CO concentration in exhaled air and carboxyhemoglobin concentration in arterial and central venous blood (P > 0.05). The increase of CO concentration in exhaled air in critical illness suggests an induction of inducible heme oxygenase (HO-1) and might reflect the severity of illness.