Changes in arterial oxygen tension correlate with changes in end-expiratory carbon monoxide level

OBJECTIVE: Carbon monoxide (CO) and oxygen compete for haemoglobin binding sites. While the effects of increased inspiratory oxygen fractions on exhaled carbon monoxide concentrations have been studied previously, the relationships between intravascular oxygen tension, blood carboxyhaemoglobin levels and expiratory CO concentrations remain unclear. We therefore studied the effects of increases in arterial oxygen tension as crucial determinant for the displacement of carbon monoxide from its haemoglobin bond during lung passage.

METHODS: Measurements of end-expiratory CO concentrations (eCO), arterial oxygen tensions and carboxyhaemoglobin concentrations were performed in 19 patients while breathing air and oxygen.

RESULTS: With increasing PaO(2) (from 11.5 +/- 1.9 to 35.2 +/- 10.3 kPa) end-expiratory CO concentrations increased from 8.6 +/- 4.9 to 16.7 +/- 9.4 ppm, p < 0.001, with a mean increase of 0.36 ppm CO per kPa increase in PaO(2) (DeltaeCO [ppm] = 0.36 * DeltaPaO(2 )[kPa]). Increases of arterial oxygen tension correlated with increases of end-expiratory CO concentration (r(2) = 0.33). Arterial carboxyhaemoglobin concentrations decreased from 1.06 +/- 0.37 during air breathing to 0.92 +/- 0.35 % after 5 minutes of oxygen inhalation (p < 0.001).

CONCLUSIONS: Oxygen-induced increases in exhaled CO correlate with increases in arterial oxygen tensions. Furthermore, oxygen inhalation reduces carboxyhaemoglobin levels, supporting the concept of accelerated CO elimination by oxygen via the lungs.