Introduction: Intrathecal opioids potentiate the analgesic property of local anaesthetics. Hence, this study was conducted to evaluate the efficacy and safety of intrathecal Fentanyl (25mg) with hyperbaric Bupivacaine in lower limb orthopaedic surgery. 

Methods: 100 participants, aged 18 to 60 years, of ASA Physical status I and II, scheduled for lower limb orthopaedic surgery under subarachnoid block, were randomly divided into two groups (n=50 each); Group C received 3.0 ml (15 mg) 0.5% hyperbaric Bupivacaine + 0.5 ml Normal Saline (0.9%) and Group F received 3.0 ml (15 mg) 0.5% hyperbaric Bupivacaine + 0.5ml Fentanyl (25mg). Degree of sensory and motor block, quality of intraoperative anaesthesia, postoperative analgesia (VAS score), time of 1st  rescue analgesia (effective analgesia), hemodynamic variables and side effects were evaluated and compared. At VAS ³ 4, rescue analgesic Inj. Diclofenac Sodium intravenous was given. 

Results: Participants in Group F had faster onset and peak, with prolonged duration of sensory block (p<0.0001). Motor characteristics were comparable in both groups. Duration of analgesia was longer in Group F (179±12.08 min) than Group C (135±12.57 min) (p<0.0001). Effective analgesia was longer in Group F (350.1±34.02 min) than in Group C (292.5±31.94 min) (p<0.0001). Incidence of side effects was less in Group F. 

Conclusion: It is concluded that addition of 25mg Fentanyl with 0.5% hyperbaric Bupivacaine, in subarachnoid block, fastens and prolongs sensory block; also improves postoperative analgesia, with fewer side effects.