Lumbar fixation surgery is associated with significant pain, necessitating effective analgesic strategies. Opioids are used routinely for perioperative pain management; however, their use is limited by dose related side effects. Ketamine, a N-methyl-D-aspartate (NMDA) receptor antagonist, possess analgesic properties and reduces opioid requirements, but its analgesic effects have not been assessed objectively in lumbar fixation surgery.

Assessment of pain during intraoperative period is often challenging as patients are unable to communicate pain intensity under general anaesthesia. Hence, surrogate markers are required. Noxious stimulation activates sympathetic system, resulting in tachycardia and vasoconstriction. These changes can be detected through pulse oximetry based plethysmography. Surgical pleth index (SPI) is a validated, non invasive tool that quantifies nociception by analyzing Pulse Plethysmographic amplitude (PPGA) and heart rate, thereby providing an indirect measure of sympathetic activity.

This proposed randomized controlled trial will include 60 patients undergoing single level lumbar fixation surgery, aimed at comparing the analgesic effect of ketamine in terms of  reduction in intraoperative opioid consumption as compared to placebo under general anaesthesia using SPI as an objective tool.

General anaesthesia will be induced and maintained using standard guidelines. Just before induction of anaesthesia, patient will be randomized into two groups. The intervention group will receive 0.5 mg/kg of Ketamine hydrochloride intravenously at induction followed by infusion of 0.2 mg/kg/hr and the placebo group will receive 0.9% normal saline of same volume and colour at same rate. Fentanyl supplementation will be guided by SPI values intraoperatively; whenever SPI rises above 50 for 1 minute, a bolus of fentanyl (0.5 mcg/kg) will be given. Additional fentanyl bolus will be given if SPI persists above 50 for five minutes. Maximum allowed cumulative fentanyl dose will be upto 20 mcg/kg. Intraoperative cumulative fentanyl consumption, recovery time, postoperative pain scores, rescue analgesia requirement and post operative nausea and vomiting (PONV) will be compared between both the groups.

We hypothesise that ketamine has analgesic effect at low dose and therefore, leads to reduction by 10% in intraoperative fentanyl consumption in patients undergoing lumbar fixation surgery.