Simultaneous measurement of cardiac output and pulmonary diffusing capacity for CO by a rebreathing method

A rebreathing method was developed to obtain the pulmonary diffusing capacity for CO, DL(CO) and the cardiac output, Q, simultaneously at rest and during exercise. From DL(CO) and Q, contact time of the red blood cell (RBC) through the lung capillary, tc, was estimated by referring to a reaction rate factor of CO with RBC, Fc(CO). The DL(CO) and Q were calculated from the changes in CO, O2, and CO2 concentrations in rebreathing air. The Fc(CO) was evaluated so as to attain a proportional relation between the DL(CO) and Fc(CO) by varying a transfer coefficient for CO across the RBC boundary including the capillary wall. The PO2 dependence of Q was also taken into account in the above analyses. The tc was determined by dividing the DL(CO) by Fc(CO), Q, and fractional hematocrit. The DL(CO) and Q measured in five normal subjects in a sitting position were 26.6 +/- 2.9 ml X min-1 X Torr-1 and 5.9 +/- 0.6 l X min-1 (mean +/- S.D.), respectively, which increased during moderate exercise up to 34.6 +/- 3.9 ml X min-1 X Torr-1 and 13.0 +/- 1.2 l X min-1. The tc was 0.70 +/- 0.06 s at rest and 0.40 +/- 0.05 s during exercise. The rebreathing method for measuring DL(CO) was more advantageous than the single breath method, because the tc could be obtained as well as the Q and the influence of the dead space and ventilation-perfusion uneveness on DL(CO) was reduced.