Effect of tramadol on electroencephalographic and auditory-evoked response variables during light anaesthesia

Tramadol is a centrally acting opioid-like analgesic commonly used for analgesia during surgery. It has been stated that the use of tramadol increases the risk of awareness during anaesthesia. We studied 29 patients under steady state anaesthesia, ventilated via a laryngeal mask airway with 0.6 MAC isoflurane in 50% nitrous oxide, and with no surgical stimulus. The electroencephalogram (EEG) and auditory-evoked response (AER) were recorded throughout the study period, as were pulse and arterial pressure. Patients were given randomly a bolus of either saline (S), tramadol 100 mg (T1), or tramadol 200 mg (T2). Significant increases in systolic arterial pressure and decreases in heart rate were seen in the tramadol groups compared to the saline group. Significant, dose-related activation in all EEG variables (median power frequency, spectral edge, Delta Power and Alpha/Delta ratio) but no significant change in Pa or Nb amplitudes or latencies were noted. The EEG changes were not at levels thought to be associated with awareness. This study indicates that tramadol, whilst causing EEG activation, has no effect on depth of anaesthesia as measured by the AER.