Figure 36-6 Effect of anesthesia on pulmonary atelectasis and gas exchange. A, Computed tomography of the chest of an awake patient. Adjacent to the scan is the distribution of ventilation (open circles) and perfusion (solid circles) in a 50-compartment lung model, determined by using the multiple inert gas method. A unimodal distribution is shown for the awake individual with a shunt fraction (S/T) of 0.8%. B, After a period of general anesthesia with positive-pressure ventilation, crescent-shaped areas of increased tissue density (arrows) appear in posterior lung regions. Corresponding to these changes are a widening of both V̇A and distributions and an increase in S/T to 7.4%. C, The scan and data are shown for the same individual after application of 10 cm H₂ O of PEEP. Very little change is observed in the areas of atelectasis, and V̇A/ matching is further worsened, with the development of a bimodal ventilation distribution and a further increase in S/T to 11.1%. These changes are accompanied by a drop in PaO₂ from 162 to 137 mm Hg. (Adapted from Tokics L, Hedenstierna G, Strandberg A, et al: Lung collapse and gas exchange during general anesthesia: Effects of spontaneous breathing, muscle paralysis and positive end-expiratory pressure. Anesthesiology 66:157, 1987.)