The partial pressure of carbon monoxide in human tissues calculated using a parallel capillary-tissue cylinder model

Tissue P_CO values have not been previously estimated under conditions where the blood carboxyhemoglobin % saturation ([COHb]) is at a normal level or increased. Tissue P_CO values are not known for conditions when [COHb] is increased during CO therapy or during CO poisoning. Using a modified Krogh parallel capillary-tissue model, mean tissue P_CO was calculated for when [COHb] was 1, 5, 10, and 15% saturation, relevant to CO therapy, and 20, 30, and 40% saturation, relevant to CO poisoning. Calculations were made for the time during which CO was being inhaled, after cessation of CO uptake, and for different O₂ extractions from blood flowing in the model capillary. The T_1/2 of relevant CO reactions was used in these calculations. When the [COHb] increased to 5 to 10% saturation, mean tissue P_CO values increased to 500 to 1,100% of values when the [COHb] was 1% saturation. When the [COHb] increased to 20 to 40% saturation, mean tissue P_CO values increased to 2,300 to 5,700% of the 1% saturation value. Results indicate the utility of the modified Krogh model in furthering understanding the physiology of determinants of tissue P_CO and should facilitate future studies of in vivo CO binding to different extravascular heme proteins during CO therapy and during CO poisoning. NEW & NOTEWORTHY Tissue P_CO levels resulting from carboxyhemoglobin concentrations achieved during CO therapy or during CO poisoning have not been previously estimated. Results published here show that at carboxyhemoglobin levels achieved during CO therapy there are 500 to 1,100% increases in mean tissue P_CO values. With carboxyhemoglobin increases associated with toxic effects, there are 2,300 to 5,700% increases in the mean tissue P_CO. These differences suggest a basis for understanding the therapeutic and toxic effects of CO.