Background: In patients breathing spontaneously during anaesthesia, expiration is active and opioids enhance this effect. The mechanical consequences are not well characterized.
Methods: We studied 14 patients undergoing laparoscopy for minor gynaecological procedures, anaesthetized with isoflurane in nitrous oxide, and breathing spontaneously through a laryngeal mask airway. We made direct recordings of intra-abdominal pressure and respiratory flow before and after giving fentanyl 25 microg i.v.
Results: Satisfactory records were obtained in 11 patients. Before fentanyl, the abdominal pressure changes were small and had an inconsistent pattern, increasing in inspiration in seven patients and during expiration in five. After fentanyl, an increase in pressure during inspiration was seen in only two patients, and the intra-abdominal pressure during expiration was increased. The mean value of maximum abdominal pressure (which always occurred during expiration) increased from 17 (SD 5) cm H2O before to 25 (9) cm H2O after fentanyl (P<0.01).
Conclusions: Direct measurements support previous findings that opioids stimulate active phasic expiratory activity and can cause large increases in abdominal pressure.