1. We have compared rebreathing, breath-hold and mean alveolar methods of measuring alveolar carbon monoxide (CO), at levels similar to those found in smokers, as a preliminary to using them as indirect measures of carboxyhaemoglobin levels. In the present study alveolar CO levels were raised by rebreathing a 2% CO mixture.
2. Breath-hold CO was measured after breath-hold times of 0–35 s in 5 s increments. Using generalized linear models, the maximum value for breath-hold CO was estimated to occur at 23 s. Breath-hold CO after a 20 and 25 s breath-hold were similar to and significantly greater than those of less than 20 s or greater than 25 s.
3. As expired CO increased, the difference between breath-hold and mean alveolar CO became proportionally larger. On average, breath-hold CO was 24% larger than mean alveolar CO.
4. Rebreathing, breath-hold and mean alveolar CO were compared at four different inspired oxygen concentrations. Expired CO increased significantly with increasing oxygen for all three methods. At end-tidal oxygen levels of less than 25%, breath-hold and rebreathing CO were similar, however, the overall mean difference between the three methods was significant.
5. While rebreathing CO was unaffected by changes in ventilation/perfusion of the lung, induced by change in body posture, both breath-hold and mean alveolar CO showed a significant fall with change from the supine to erect posture.
6. We conclude that under normoxic conditions, rebreathing and breath-hold CO (20 or 25 s breath-hold) were similar, whereas the mean alveolar method produced significantly lower values, presumably due to lack of equilibration. Altering ventilation/perfusion of the lung caused no mean change in the measurement of rebreathing CO but did affect the other methods.