Evaluation of uneven distribution of VA/Q ratio from data on exchange of respiratory gases.

Abstract

A new approach has been developed for evaluating uneven distribution of ventilation/perfusion ratio, VA/Q, based on a two-compartment model with a reciprocal VA/Q relation. The VA/Q ratios were expressed by mu . (1-rho)/rho and mu . rho/(1-rho), where mu and rho were referred to as the "ideal" VA/Q ratio and unevenness factor, respectively. During steady state breathing, arterial blood was analyzed for PO2 and PCO2, and end-tidal PCO2 as well as the gas exchange ratio, R, was measured. After steady state breathing a rebreathing experiment was performed, and the arterial-venous O2 content difference, (a-v)CO2, was measured, and then, multiplying it by R, the venous-arterial CO2 content difference, (v-a)CCO2, was obtained. Referring to these values the mixed venous PO2 and PCO2 were estimated from arterial PO2 and PCO2, using the O2 and CO2 dissociation curves. The VA/Q line and iso-R line were drawn to find mu, and further, total arterial, and alveolar PO2 and PCO2 of the two-compartment model were computed by changing both rho and the mixing weight factor, phi. The phi value was determined so as to make the above PO2-PCO2-locus pass through the measured arterial PO2 and PCO2. The rho value was selected so that the computed alveolar PCO2 fitted to the end-tidal one. The experiments were performed on 8 normal subjects in normoxia and hyperoxia with PIO2 245 Torr. The mean phi values in normoxia and hyperoxia were 0.50 and 0.56, respectively. The mean rho values in normoxia and hyperoxia were 0.438 and 0.428, respectively. The VA/Q ratio was decreased in hyperoxia because of a decrease in VA value.