Subarachnoid block is the most commonly used anesthesia technique for lower limb surgeries and hyperbaric Bupivacaine is the most commonly used local anesthetics for it. Several adjuvants or additives including various opioids, alpha agonists, neostigmine, ketamine, dexamethasone have been used mixing with the bupivacaine to enhance the quality of subarachnoid block.

Though the Fentanyl has been used extensively premixed with hyperbaric bupivacaine for spinal anesthesia, there is paucity of literature on clinical effects and understanding of mechanism of influence of fentanyl on subarachnoid block produced by hyperbaric bupivacaine when administered separately by another syringe, either before or after bupivacaine.

We propose that cerebrospinal fluid dynamics and pharmacokinetics may be affected differently when fentanyl is co-administered in subarachnoid space premixed with bupivacaine compared to when administered separately before or after hyperbaric Bupivacaine. That is the rationale of proposal for conducting this clinical study where in we wish to evaluate the difference in onset and duration of sensory and motor block and the incidence of hypotension after administering hyperbaric Bupivacaine and Fentanyl either in a single syringe or in different syringes prior to hyperbaric Bupivacaine and vice versa.