Canadian Journal of Anesthesia, Vol 30, 493-500, Copyright © 1983 by Canadian Anesthesiologists' Society

Diazepam Sedation Reduces Functional Residual Capacity and Alters the Distribution of Ventilation in Man

¹ Departments of Anaesthesia and Nuclear Medicine, University of Western Ontario, London, Canada

Address correspondence to: Dr. R.L. Knill, Department of Anaesthesia, University Hospital, P.O. Box 5339, Postal Station A, London, Ontario, N6A 5A5.

We measured ventilation and static lung volumes in five fit volunteers in the right lateral decubitus position, while they were fully awake and while sedated with diazepam. We also assessed the distributions of ventilation and perfusion in the lungs, using inhalations and intravenous injections of xenon-127. Diazepam, 0.04 mg·kg^-1, was administered every three to five minutes as required to induce and maintain a state of sedation which was moderately heavy. Total doses ranged from 0.16 to 0.38 mg·kg^-1. Sedation did not alter minute ventilation, but reduced tidal volume, increased breathing frequency and reduced functional residual capacity slightly. Sedation also diminished the normal gradient of ventilation from non-dependent to dependent regions of the lungs. Spontaneous episodes of very small tidal volume breathing during sedation were associated with a marked reduction or cessation of ventilation of the most dependent region of the lungs. There were no sedation-related changes in the distribution of perfusion. These effects of moderately heavy sedation may contribute to the hypoxaemia and impairment of pulmonary gas exchange often present during recovery from general anaesthesia.

Key Words: ANAESTHESIA: recovery • SEDATION • LUNG: ventilation, perfusion, static volumes