Gas uptake from an unventilated area of lung: computer model of absorption atelectasis.

A computer model of gas uptake from an area of nonventilated lung, such as a pulmonary lobe with an occluded bronchus or an alveolus with an occluded airway, is presented. Previous analyses have assumed that when an inert gas is present, equilibration of O2 and CO2 with mixed venous blood is sufficiently rapid to be treated as instantaneous. This is valid for insoluble gases such as N2 or He when the fractional concentration of inspired O2 (FIO2) is < or = 0.6 but is invalid for a relatively soluble gas such as N2O. When a mixture of O2 and an inert gas is breathed, the time for an area of unventilated lung to collapse depends on the solubility of the inert gas and FIO2. When the solubility is low (N2 or He), collapse takes longer than when 100% O2 is breathed, and the lower the FIO2 the longer the time to collapse. When the gas is more soluble (N2O) and FIO2 is > 0.3, collapse is more rapid than when 100% O2 is breathed.