Ventilation-perfusion inhomogeneity increases gas uptake in anesthesia: computer modeling of gas exchange

Abstract

Ventilation-perfusion (V˙a/Q˙) inhomogeneity was modeled to measure its effect on overall gas exchange during maintenance-phase N2O anesthesia with an inspired O2 concentration of 30%. A multialveolar compartment computer model was used based on physiological log normal distributions of V˙a/Q˙ inhomogeneity. Increasing the log standard deviation of the distribution of perfusion from 0 to 1.75 paradoxically increased O2 uptake (V˙o 2) where a low mixed venous partial pressure of N2O [high N2O uptake (V˙n 2 o)] was specified. With rising mixed venous partial pressure of N2O, a threshold was observed where V˙o 2 began to fall, whereas V˙n 2 o began to rise with increasing V˙a/Q˙ inhomogeneity. This phenomenon is a magnification of the concentrating effects thatV˙o 2 andV˙n 2 o have on each other in low V˙a/Q˙ compartments. During “steady-state” N2O anesthesia,V˙n 2 o is predicted to paradoxically increase in the presence of worseningV˙a/Q˙ inhomogeneity.