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

Ventilation-perfusion ((V)over dotA/(Q) over dot) inhomogeneity was modeled to measure its effect on overall gas exchange during maintenance-phase N2O anesthesia with an inspired O-2 concentration of 30%. A multialveolar comp artment computer model was used based on physiological log normal distribut ions of (V) over dotA/(Q) over dot inhomogeneity. Increasing the log standa rd deviation of the distribution of perfusion from 0 to 1.75 paradoxically increased O-2 uptake ((V) over dotO(2)) where a low mixed venous partial pr essure of N2O [high N2O uptake ((V) over dotN(2)O)] was specified. With ris ing mixed venous partial pressure of N2O, a threshold was observed where (V ) over dot O-2 began to fall, whereas VN2O began to rise with increasing (V ) over dotA/(Q) over dot inhomogeneity. This phenomenon is a magnification of the concentrating effects that (V) over dotO(2) and (V) over dotN(2)O ha ve on each other in low VA/Q compartments. During "steady"-state" N2O anest hesia, (V) over dotN(2)O is predicted to paradoxically increase in the pres ence of worsening (V) over dotA/(Q) over dot inhomogeneity.