Analysis of Factors Affecting Partial Pressures of Oxygen and Carbon Dioxide in Gas and Blood of Lungs: Methods

Abstract

The evaluation of ventilation, circulation, distribution and diffusion requires sampling of inspired gas, expired gas, arterial blood and mixed venous blood, and the subject must be studied at 2 different levels of oxygenation. Analysis of the data ordinarily involves 3 major e steps: (1) P[image]O2 is calculated at both levels of oxygenation, assuming that P[image]CO2 = PaCO2; (2) the gradient, P[image]O2-PaO2, is subdivided into diffusion and venous admixture components e _ by finding, by trial and error, constant values for P[image]O2-[image]CO2 and Qva/Qt such that the associated gradients, P[image] O2 -P[image]O2 and P[image]O2-PaO2, add up to the known values for P[image]O2-PaO2 at both levels of oxygenation; (3) DO2 is calculated from [image]O2 and P[image]O2-[image]CO2, and VO/VE is calculated from PAeCO2, PECO2 and PICO2. When [image]va/[image]t exceeds 20%, 2 additional steps are required: (4) correction of P[image]CO2 by making a 2d approximation; (5) correction of [image]O/[image]E, [image]va/[image]t and DO2. The basic assumptions on which the method is based, namely, that [image]va/[image]t and P[image]O2 - PCO2 remain constant when the subject is studied at 2 different levels of oxygenation, are critically analyzed. Available evidence suggests that [image]va/[image]t and P[image]O2 - [image]CO2 remain nearly constant if SaO2 values of approx. 96 and 82% are chosen for the two levels of oxygenation. The method provides representative values for alveolar gas and for blood leaving the alveolar capillaries, which are prerequisites in the quantitative evaluation of blood-gas distribution and diffusion in pulmonary disease.