Measurement of continuous distributions of ventilation-perfusion ratios: theory

Abstract

measuring virtually continuous distributions of venti-lation-perfusion ratios (VA/Q) based on the steady-state elimination of six gases of different solubilities. The method is applied here to 12 normal subjects, aged 21-60. In nine, the distributions were compared breathing air and 100 % oxygen, while in the remaining three, effects of changes in posture were examined. In four young semirecumbent subjects (ages 21-24) the distributions of blood flow and ventilation with respect to VA/Q were virtually log-normal with little dispersion (mean log standard deviations 0.43 and 0.35, respec-tively). The 95.5 % range of both blood flow and ventilation was from VA/Q ratios of 0.3-2.1, and there was no intrapulmonary shunt (VA/Q of 0). On breathing oxygen, a shunt developed in three of these subjects, the mean value being 0.5 % of the cardiac output. The five older subjects (ages 39-60) had broader distri-butions (mean log standard deviations, 0.76 and 0.44) containing areas with VA/Q ratios in the range 0.01-0.1 in three subjects. As for the young subjects, there was no shunt breathing air, but all five developed a shunt breathing oxygen (mean value 3.2%,) and in one the value was 10.7%. Postural changes were generally those expected from the known effects of gravity, with more ventilation to high VA/Q areas when the subjects were erect than supine. Measurements of the shunt while breathing oxygen, the Bohr CO2 dead space, and the alveolar-arterial oxygen difference were all con-sistent with the observed distributions. Since the method involves only a short infusion of dissolved inert gases, sampling of arterial blood and expired gas, and measurement of cardiac output and minute venti-