Partitioning of respiratory mechanics in young adults. Effects of duration of anesthesia.

Abstract

General anesthesia increases total respiratory elastance and flow resistance within minutes after induction. We determined if respiratory mechanics changed progressively during anesthesia by measuring total respiratory elastance and resistance and their respective lung and chest wall components in 10 young adults free of cardiorespiratory disease at the start and end of premedicated anesthesia administered for orthopedic surgery (isoflurane, enflurane, or halothane, minimal alveolar concentration approximately 1.5 in 60% N2O-40% O2; interval between measurements was 0.42 to 5.0 h, mean +/- SD = 2.08 +/- 1.56 h). Static lung recoil pressure, static total respiratory and lung elastance, dynamic lung elastance, chest wall elastance, and total respiratory and lung and chest wall resistances were measured during steady-state breathing (greater than 10 min after induction). Resistance of the endotracheal tube, pneumotachygraph and connectors were subtracted from the total flow resistance to obtain total intrinsic resistance. Average values of static lung recoil pressure and all elastances and chest wall resistance did not change significantly from start to end of the study, regardless of the elapsed time. Total respiratory and lung resistance increased by 49% (p less than 0.05) and 45% (p less than 0.02), respectively, but were not time-dependent. We conclude that lung static recoil and total respiratory and lung elastances did not change beyond the first 10 min after induction, regardless of duration of anesthesia. The increases in total respiratory and lung resistance were small, independent of duration of anesthesia, and may have been due to accumulated airway secretions.