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Departments of 1 Anaesthesia and 3 Respiratory Medicine, Austin and Repatriation Medical Centre, Heidelberg 3084; and 2 Department of Anaesthesia and Pain Medicine, The Alfred, Prahan 3181, Melbourne, Victoria, Australia
Ventilation-perfusion (
A/
)
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
A/
inhomogeneity. Increasing
the log standard deviation of the distribution of perfusion from 0 to 1.75 paradoxically increased O2 uptake
(
O2) where a low mixed venous partial
pressure of N2O [high N2O uptake
(
N2O)] was specified. With
rising mixed venous partial pressure of N2O, a threshold was observed where
O2 began to
fall, whereas
N2O began to rise with increasing
A/
inhomogeneity. This
phenomenon is a magnification of the concentrating effects that
O2 and
N2O have on each other in
low
A/
compartments. During
"steady-state" N2O anesthesia,
N2O is predicted to
paradoxically increase in the presence of worsening
A/
inhomogeneity.
alveolar-arterial difference; oxygen uptake
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