THE EFFECT OF NITROUS-OXIDE ON CHEST-WALL FUNCTION IN HUMANS AND DOGS

The purpose of this study was to determine the effects of nitrous oxid e (N2O) on the chest wall of anesthetized humans and dogs. Six human s ubjects and six mongrel dogs were studied during 1 minimum alveolar an esthetic concentration halothane anesthesia before and during the subs titution of 70% N2O for 70% N-2 in the inspired gas mixture. On a sepa rate occasion, measurements also were made in pentobarbital-anesthetiz ed dogs. Respiratory muscle activity was measured using electromyograp hic (EMG) electrodes. Chest wall configuration was determined by using fast three-dimensional computed tomography in dogs and by using respi ratory impedance plethysmography in humans. N2O consistently decreased inspiratory ribcage displacement, a decrease attributable in dogs to decreased inspiratory activation of parasternal intercostal muscles; p arasternal intercostal activity was not present in anesthetized humans . The decrease in ribcage motion decreased the tidal volume in humans, but not in dogs, because displacement of the diaphragm was better pre served in dogs, in association with changes in diaphragm EMG activatio n. N2O significantly increased phasic expiratory muscle activity in ha lothane-anesthetized humans and pentobarbital-anesthetized dogs. Thus, as has been demonstrated for other anesthetics, the actions of N2O ar e caused by alterations in the distribution and timing of neural drive to the respiratory muscles, rather than by a global depression of res piratory motoneuron drive. Implications: In this study, we examined th e effects of nitrous oxide on breathing in halothane-anesthetized dogs and humans. Nitrous oxide affected breathing by changing the distribu tion and timing of neural drive to the respiratory muscles in a specie s-dependent manner, rather than by causing a global depression of thei r activity.