Tetanic stimulus of ulnar nerve as a predictor of heart rate response to skin incision in propofol-remifentanil anaesthesia

Background. To study adequate antinociception during general anaesthesia, tetanic stimulus of 5-10 s duration has been used previously as a standardized nociceptive stimulus. However, such stimuli have been found to correlate poorly with intraoperative nociception. We hypothesized that an electrical tetanic stimulus of the ulnar nerve, lasting 30 s, would provide a reliable experimental pain model. Methods. Thirty-three patients, undergoing open abdominal surgery, were studied. Propofol and remifentanil were used for anaesthesia. Patients were randomized to receive remifentanil at three target-controlled infusion levels (1, 3, or 5 ng ml(-1)) during short (5 s, Tet5) and a long-lasting (30 s, TWO) tetanic (50 mA, 50 Hz) stimulus and skin incision. RR intervals (RRI) were obtained from the ECG and the mean RRI before each stimulus (Tet5, TWO, incision) was compared with that after the stimulus. Results. At remifentanil level 1 ng ml(-1), the RRI responses to tetanic stimuli and skin incision were prominent but with higher concentrations (3 and 5 mg ml(-1)), responses were very small. TWO (r(2)=0.780) was the best predictor of the RRI response to skin incision when compared with Tet5 (r(2) =0.611), remifentanil level (r(2)=0.340), or propofol level (r(2)=0.036). Conclusions. Long-lasting tetanic stimulus of ulnar nerve may provide a better experimental pain model for surgical pain during general anaesthesia than shorter stimuli, which have been applied in earlier studies.