PREVENTION OF ATELECTASIS DURING GENERAL-ANESTHESIA

Atelectasis is an important cause of impaired gas exchange during gene ral anaesthesia; it causes pulmonary shunting. We studied the effects of gas composition on the formation of atelectasis and on gas exchange during the induction of general anaesthesia. In 12 adult patients, th e lungs were ventilated with 30% oxygen in nitrogen during anaesthesia induction, and in another 12, a conventional technique was used (100% oxygen during induction and 40% oxygen in nitrogen thereafter). Exten t of atelectasis was estimated by computed tomography and the ventilat ion-perfusion relation (VA/Q) by the multiple inert gas elimination te chnique, After anaesthesia induction, there was little atelectasis in the 30% oxygen group (mean 0.2 [SD 0.4] cm(2)) and a significantly gre ater amount (4.2 [5.6] cm(2); p<0.001) in the 100% oxygen group. Patie nts in the 30% oxygen group were observed for another 40 min. 6 contin ued to receive 30% oxygen (subgroup A) and 6 were ventilated with 100% oxygen (subgroup B). During this time, the amount of atelectasis incr eased to 1.6 (1.6) cm(2) in subgroup A and to 4.7 (4.5) cm(2) in subgr oup B (p=0.047 for difference between groups). In subgroup A, the shun t (VA/Q) <0.005) increased from 1.6 (2.0)% of cardiac output to 3.2 (2 .7)%, but the arterial oxygen tension did not change. In subgroup B, t he shunt increased from 2.6 (5.2)% to 9.8 (5.7)% of cardiac output. Th ese results suggest that the composition of inspired gas is important in atelectasis formation during general anaesthesia. Use of a lower ox ygen concentration than is now standard practice might prevent the ear ly formation of atelectasis.