Background: In patients breathing spontaneously during anaesthesia, expiratory muscle activity can be a prominent feature. This activity is triggered or exaggerated by opioid administration, which causes a prompt increase in intra-abdominal pressure. The effect of this increased expiratory activity on end-expiratory lung volume is not described.
Methods: Nine patients having minor gynaecological procedures were studied during stable anaesthetic conditions, breathing sevoflurane (end tidal 2.6%) through a laryngeal mask airway, in a circle system. The spill valve was closed and the fresh gas flow was temporarily reduced to approximate the oxygen uptake. The volume of the reservoir bag was then measured by placing it in a hinged, wedge-shaped container. Fentanyl (0.5 microg kg(-1) ideal body weight) was given after 1 min of stable recording, and the change in end-expiratory volume measured after 3 min.
Results: End-expiratory lung volume decreased in all patients by 160 (111) ml (mean, SD) (P<0.01). The decrease did not relate to obesity.
Conclusions: During sevoflurane anaesthesia, fentanyl causes a rapid reduction in functional residual capacity. This is caused by increased activity of expiratory muscles and an increase in intra-abdominal pressure.