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 twocompartment model with a reciprocal VA/Q relation. The VA/Q ratios were expressed by j • (1-p)/ p and µ • p/(1-p), where p and p were referred to as the "ideal" VA/Q ratio and unevenness factor, respectively. During steady state breathing, arterial blood was analyzed for Pot and P02, 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 02 content difference, (a-v)Co2, was measured, and then, multiplying it by R, the venous-arterial C02 content difference, (v-a) Cco2, was obtained. Referring to these values the mixed venous Pot and Pco2 were estimated from arterial Pot and Pco2, using the 02 and C02 dissociation curves. The VA/Q line and iso-R line were drawn to find p, and further, total arterial, and alveolar Pot and Pco2 of the twocompartment model were computed by changing both p and the mixing weight factor, cIi. The 1i value was determined so as to make the above Pot" P02-locus pass through the measured arterial Pot and Pco2. The p 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 .P values in normoxia and hyperoxia were 0.50 and 0.56, respectively. The mean p 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.Key words : VA/Q unevenness, two-compartment model, 02-C02 gram, normoxia and hyperoxia. dia- WAGNER et al. (1974) elaborated a method for computing the distribution of VA/Q ratio in the lung by using a number of inert gases. However, for clarifying the Pot and Pco 2 distributions in physiological and pathological conditions, it is