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

Static compliance of the respiratory system (C(rs)) was measured by the interrupter technique in 18 anesthetized children to compare the effects of ketamine on C(rs) with those of halothane. C(rs) 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 (PEEP(i)). Expiratory time (T(e)) was measured during a period of quiet breathing, and the passive expiratory time constant (τ) 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. C(rs) 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 τ value was also significantly greater in the ketamine group. Mean PEEP(i) in the ketamine group was 2.3 ± 1.8 cm H₂O and in the halothane group was 0.4 ± 0.8 cm H₂O. PEEP(i) correlated inversely with T(e)/τ according to a logarithmic function. It was concluded that, in children anesthetized with ketamine, C(rs) is significantly greater than that in children anesthetized with halothane, and the resultant prolongation of τ and decreased T(e)/τ with ketamine anesthesia lead to increased PEEP(i).