Response index of nociception correlates both with remifentanil concentration and level of painful stimulation during surgery

Background and Goal of Study: No numeric measures of nociception during general anaesthesia exist. For monitoring of nociception a multi-parameter approach may be needed (1). We studied the relation between physiological responses and concentration of remifentanil (Rem) at nociceptive stimuli in anaesthetised and paralysed patients. Materials and Methods: 55 females were anaesthetised with propofol (Pr)-Rem target controlled infusions (TCI). Pr was given to maintain EEG State Entropy (SE) at 35-60. Rem TCI was randomised to 1, 3, or 5 ng/ml. Responses to tetanic stimulation of the ulnar nerve (30 s) were studied before surgery in a subgroup of 23 patients. Responses to skin incision were analysed in all patients. Comparison of responses in photoplethysmographic wave amplitude (PPGA), ECG beat-to-beat interval (RRI), and Response Entropy (RE) at different Rem levels was done using the Kruskal-Wallis H test. Results and Discussion: RRI (p=0.00001) and PPGA (p=0.009) responses to tetanic stimulus differed significantly between Rem levels, while for the incision RRI (p=0.019), RRI SD1 (p=0.019) and RE-SE (p=0.018) responses were different. A Response Index of Nociception (RN), developed by combining RE, RRI and PPGA variability, was significantly higher in the low doses of Rem as compared with the higher doses for both tetanic stimulus (p=0.00005) and skin incision (p=0.00003). Conclusions: In paralysed patients anaesthetised with propofol-remifentanil infusions: 1) Combination of information from RE, PPG, and RRI provides promising quantification of responses to abrupt noxious stimuli; 2) Long-lasting 30 sec tetanic stimulus induces qualitatively similar physiological responses as skin incision. Reference: 1) Seitsonen E. Anesthesiology 2002: 97: A-582 (abstract)