Respiratory mechanics and intrinsic PEEP during ketamine and halothane anesthesia in young children.

Static compliance of the respiratory system (Crs) was measured by the interrupter technique in 18 anesthetized children to compare the effects of ketamine on Crs with those of halothane. Crs was the slope of the pressure-volume (P-V) curve obtained by repeated brief airway occlusions throughout relaxed expiration, and the intercept of the P-V curve on the pressure axis was the intrinsic positive end-expiratory airway pressure (PEEPi). Expiratory time (Te) was measured during a period of quiet breathing, and the passive expiratory time constant (tau) was measured during the relaxed expiration after an end-inspiratory occlusion. Nine children were anesthetized with a continuous infusion of ketamine and a matching group of nine children inhaled halothane in oxygen. Crs was significantly greater in the ketamine group (22.8 +/- 6.2 ml/cm H2O) than in the halothane group (15.7 +/- 5.5 ml/cm H2O). The tau value was also significantly greater in the ketamine group. Mean PEEPi in the ketamine group was 2.3 +/- 1.8 cm H2O and in the halothane group was 0.4 +/- 0.8 cm H2O. PEEPi correlated inversely with Te/tau according to a logarithmic function. It was concluded that, in children anesthetized with ketamine, Crs is significantly greater than that in children anesthetized with halothane, and the resultant prolongation of tau and decreased Te/tau with ketamine anesthesia lead to increased PEEPi.