Abstract
An assumption of the classical three-compartment lung model is that the physiologic shunt fraction ( Q ̇ s Q ̇ t ) remains constant with changes in cardiac output ( Q ̇ t ). As a result, when Q ̇ t decreases, both the arterial O 2 tension ( P aO 2 ) and the mixed venous O 2 tension ( P vO 2 ) must also decrease. Yet observations in intact dogs with pulmonary edema and humans with the adult respiratory distress syndrome (ARDS) indicate that as Q ̇ t decreases, Q ̇ s Q ̇ t occasionally declines while the P aO 2 increases. The present study was conducted to examine these discordant observations using a computer model of pulmonary gas exchange with a diffusion impairment. A computer model of gas exchange was developed to specifically analyze the effect of changing Q ̇ t upon P aO 2 and Q ̇ s Q ̇ t in the presence of diffusion impairment. Using the classical three-compartment model of gas exchange, a diffusion impairment was inserted by a MIMIC subroutine. The results of this computer simulation indicated that in the presence of a diffusion impairment, lowering Q ̇ t caused P aO 2 to increase while Q ̇ s Q ̇ t and P vO 2 decreased. These changes indicated that a diffusion impairment might account for the experimental observations of the effects of Q ̇ t on P aO 2 in ARDS.
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