Exhaled carbon monoxide in mechanically ventilated critically ill patients:: influence of inspired oxygen fraction

Objective: To assess the feasibility of exhaled carbon monoxide (CO) measurements in mechanically ventilated critically in adult patients and to determine the influence of inspired oxygen fraction on this measurement. Design: Prospective physiologic study. Setting: Medical ICU in a community hospital. Patients: The study was performed on nine mechanically ventilated patients with varying diagnoses. Measurements and results: Carbon monoxide concentration was determined with an infrared CO analyzer on exhaled breath collected at the outlet of the ventilator. We assessed the stability of exhaled carbon monoxide concentration over a 4-hour period and determined its course during a 7-hour period after inspired oxygen fraction had been abruptly increased from baseline to 1. Carbon monoxide was detected in exhaled breath in each patient at a higher concentration than in inspired gas (0.64 +/- 0.1 ppm vs 0.25 ppm, approximately). Exhaled carbon monoxide did not vary during a 4-hour period in five hemodynamically stable patients. When inspired oxygen fraction was increased from baseline (0.52 +/- 0.04) to 1, exhaled carbon monoxide concentration increased abruptly from baseline (0.63 +/- 0.13 ppm) to a peak value of 1.54 +/- 0.16 ppm within 15 min and returned slowly to baseline values within 7 h. Conclusion: CO was easily detected in the exhaled breath of mechanically ventilated patients and CO lung excretion was markedly but transiently dependent on inspired oxygen fraction. Other studies are warranted in order to determine the different factors that might influence CO lung excretion in critically ill patients.