Rates of carbon monoxide elimination in males and females

The purpose of this study was to verify the previously reported shorter half-time of elimination (t½) of carbon monoxide (CO) in females compared to males. Seventeen healthy subjects (nine men) completed three sessions each, on separate days. For each session, subjects were exposed to CO to raise the carboxyhemoglobin percentage (COHb) to ~10%; then breathed in random order, either (a) 100% O2 at poikilocapnia (no CO2 added), or (b) hyperoxia while maintaining normocapnia using sequential gas delivery, or (c) voluntary hyperpnea at~4x the resting minute ventilation. We measured minute ventilation, hemoglobin concentration [Hb] and COHb at 5 min intervals. The half-time of reduction of COHb (t½) was calculated from serial blood samples. The total hemoglobin mass (HbTOT) was calculated from [Hb] and estimated blood volume from a nomogram based on gender, height, and weight. The t½ in the females was consistently shorter than in males in all protocols. This relationship was sustained even after controlling for alveolar ventilation (P < 0.05), with the largest differences in t½ between the genders occurring at low alveolar ventilation rates. However, when t½ was further normalized for HbTOT, there was no significant difference in t½ between genders at alveolar ventilation rates between 4 and 40 L/min (P = 0.24). We conclude that alveolar ventilation and HbTOT are sufficient to account for a major difference in CO clearance between genders under resting (nonexercising) conditions.