Evaluation of analgesia nociception index as a tool to monitor pain and manage analgesia during supratentorial craniotomies

Introduction: Administration of analgesics is guided by clinical experience and by monitoring for somatic responses, such as movement, sweating and increase in heart rate or blood pressure, during surgery. Analgesia Nociceptive Index (ANI) provides objective information about the degree of intraoperative pain and adequacy of analgesia. We conducted a prospective, randomized, double blinded study to assess ANI changes during induction, periods of nociceptive stimulation and recovery and compare ANI in patients, who receive scalp block or infiltration at pin and incision sites. Methods: Sixty adult patients scheduled for elective supratentorial surgery were randomly allocated to receive scalp block or pin and incision site infiltration after induction of anaesthesia. ANI and haemodynamic parameters were recorded and analysed. Results: Thiopentone caused a significant decrease in instantaneous ANI (ANIi) [59 vs 45 p < 0.001, (median pre vs post)] in the three minutes after administration. Laryngoscopy caused a significant decrease in ANIi (59 vs 45, p < 0.001). ANIi decreased significantly in the three minutes following pin fixation in the skin infiltration group (68 vs 28; p <0.001) but not in the scalp block group (63 vs 50, p = 0.151). There was no significant change in the ANIi values during three minutes of skin incision in both the groups. ANIi decreased significantly following extubation in both the groups (57 to 44 and 44.5 to 39 in scalp block and infiltration group respectively, p<0.001). Conclusion: ANI decreases below 50 following thiopentone administration and laryngoscopy. Scalp block provides better analgesia for pin fixation compared to infiltration as demonstrated by ANI. Scalp block or infiltration at the incision site effectively prevented a decrease in ANI values during skin incision. ANI decreases significantly after extubation.