Pulmonary gas exchange and diaphragmatic position. Effect of tonic phrenic stimulation compared with that of increased airway pressure.

The influence of diaphragmatic position on abnormal gas exchange has been examined to investigate the theory that the impairment in gas exchange in anaesthetized man is caused by disturbance of diaphragmatic mechanics, resulting in abnormalities of dependent lung ventilation. A gas exchange abnormality, probably caused by airway closure in the dependent regions of the lung, was induced in anaesthetized rabbits by reducing lung volume to residual volume and allowing passive re-expansion. The effects on gas exchange of increases in lung volume produced by two methods--the application of positive end-expiratory pressure (PEEP) and phrenic nerve stimulation (PNS)--were compared. Both methods were adjusted to give the same increase in lung volume. PNS was found to produce greater caudal movement of the diaphragm than PEEP, particularly in the dependent regions. PNS also improved gas exchange significantly more than PEEP. These findings support the theory that alterations in diaphragmatic mechanics during anaesthesia contribute to the gas exchange impairment in man.