Influence of the cortical electrical activity level during general anaesthesia on the severity of immediate postoperative pain in the morbidly obese.
BACKGROUND: The objective of anaesthesia is to provide hypnosis, analgesia and adequate conditions during surgery. It is difficult to establish the appropriate dose of general anaesthetic drugs in the morbidly obese patient. Moreover, there are conflicting data concerning adequate anaesthesia levels and the severity of postoperative pain. The aim of this study was to investigate the relationship between the spectral edge frequency (SEF) during general anaesthesia and the severity of immediate postoperative pain following gastric banding surgery in morbidly obese patients. METHODS: Seventy-one ASA 2 morbidly obese patients (BMI > 35%) undergoing elective laparoscopic gastric banding procedure were recruited for this study. Anaesthesia consisted of midazolam, fentanyl and thiopental for induction, vecuronium for muscle relaxation, N2O and isoflurane with additional fentanyl administrations, according to the clinical judgement of the anaesthesiologist, for maintenance. Continuous SEF monitoring was added to the standard monitors (SpO2, ETCO2, ECG, NIBP, O2 and isoflurane concentration), but the EEG monitor screen was hidden from the anaesthesiologist's sight. SEF postoperative analysis divided the patients into two groups: group 1, SEF-recommended target range of 8-12 Hz, more than 80% of the surgical time; and group 2, SEF-recommended target range of 8-12 Hz, less than 80% of the surgery duration. Pain intensity was assessed in the post anaesthesia care unit using a standard visual analogue scale (VAS) of 10 cm, when patients were awake enough to correct a deliberately given wrong own telephone or ID number. Intravenous morphine was administered for postoperative analgesia in 2-mg increments, every 3-4 min, until the patient felt comfortable. A recovery room nurse unaware of the SEF range recorded during surgery registered pain severity and morphine requirements. RESULTS: The end-tidal isoflurane concentration was significantly higher in group 1 than in group 2 (0.83 vs. 0.7 P = 0.016). The intensity of pain at admission into the recovery room and at discharge was significantly lower in group 1 than in group 2 (VAS 6.1 vs. 6.9-P = 0.0049, and 3.9 vs. 4.2-P = 0.00478, respectively). CONCLUSIONS: Keeping the SEF range between 8 and 12 Hz during anaesthesia for laparoscopic gastric banding for morbid obesity, both the immediate post operative pain intensity and morphine requirement, are significantly reduced.