Respiratory response to skin incision during anaesthesia with infusions of propofol and alfentanil

Abstract

Background. The ventilatory response to skin incision during anaesthesia with enflurane is an increase in tidal volume without a change in frequency. As opioids affect respiratory frequency and also affect the processing of pain, we investigated if the breathing response to a painful stimulus could be different during anaesthesia using opioids. Methods. We studied 12 patients during anaesthesia with target-controlled infusions of propofol (plasma target concentration 4-6 μg ml-1) and alfentanil (plasma target concentration 40-60 ng ml-1), having varicose vein surgery. Results. After the initial skin incision, tidal volume increased promptly by 17 (4, 81)% (median, quartile values) (P<0.01). Respiratory frequency changed variably with no significant change overall [median change 2 (-8, +50)%]. The duration of inspiration was virtually unaltered, and the duration of expiration decreased gradually by 5 (-7, 32)%. Patients who showed more response also showed more change in tidal volume, so that there was a significant relationship between increased inspiratory flow rate and reduced expiratory time (P<0.05). Conclusions. During opioid anaesthesia, the mechanism of ventilatory increase after stimulation involves changes in both drive and timing of breathing. This pattern of response does not resemble the changes seen during anaesthesia with potent volatile agents.