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

Objective: To assess the feasibility of exhaled carbon monoxide (CO) measur ements in mechanically ventilated critically in adult patients and to deter mine 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 v entilator. We assessed the stability of exhaled carbon monoxide concentrati on over a 4-hour period and determined its course during a 7-hour period af ter inspired oxygen fraction had been abruptly increased from baseline to 1 . Carbon monoxide was detected in exhaled breath in each patient at a highe r concentration than in inspired gas (0.64 +/- 0.1 ppm vs 0.25 ppm, approxi mately). Exhaled carbon monoxide did not vary during a 4-hour period in fiv e hemodynamically stable patients. When inspired oxygen fraction was increa sed from baseline (0.52 +/- 0.04) to 1, exhaled carbon monoxide concentrati on 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 with in 7 h. Conclusion: CO was easily detected in the exhaled breath of mechanically ve ntilated patients and CO lung excretion was markedly but transiently depend ent on inspired oxygen fraction. Other studies are warranted in order to de termine the different factors that might influence CO lung excretion in cri tically ill patients.