Determination of the Pulmonary Capillary Blood Flow in Man

A method is described for the measurement of the pulmonary capillary blood flow in human subjects with normal lungs. The method depends on the performance of a vital capacity maneuver using nitrous oxide as the test gas. It attempts to circumvent the deficiencies of previous respiratory methods by using an infrared analyzer for the continuous measurement of the concentration of N2O in expired air, by ensuring that the respiratory maneuver is completed before recirculation occurs, and by determining simultaneously the values involved in the formula used for the calculation of the flow. As a basis for the method, the pulmonary recirculation time and the rate of N2O absorption by the lungs were measured separately. The values for the pul monary capillary blood flow obtained by the N2O method correlate well with those obtained by the direct Fick method: in 12 subjects, the average value by the N2O method was 5.65 L./min. in comparison with 5.55 L./min. by the direct Pick method; the mean Percentile deviation between the values obtained by the two methods in each subject was 11 per cent. The results obtained by the N2O method were also shown to be reproducible: In 14 subjects, successive measurements on the same occasion showed a mean percentile deviation of 6 per cent.