The effect of posture and baricity on the spread of intrathecal bupivacaine for elective Cesarean delivery

Posture and baricity during induction of spinal anesthesia with intrathecal drugs are believed to be important in determining spread within the cerebrospinal fluid. In this double-blind prospective study, 150 patients undergoing elective cesarean delivery were randomized to receive a hyperbaric, isobaric, or hypobaric intrathecal solution of 10 mg bupivacaine during spinal anesthesia induced in either the sitting or right lateral position. After an intrathecal injection using a combined-spinal technique patients were placed in the supine wedged position. We determined the densities of the three intrathecal solutions from a previously validated formula and measured using a DMA-450 density meter. Data collection included sensory level, motor block, episodes of hypotension, and ephedrine use. Statistical. analysis included analysis of variance and Cuzick's trend. In the lateral position, baricity had no effect on the spread of sensory levels for bupivacaine compared to the sitting position, where there was a statistically significant difference in spread with the hypobaric solution producing higher levels of analgesia than the hyperbaric solution (P = 0.002). However, the overall differences in maximal spread only differed by one dermatome, with the hyperbaric solution achieving a median maximum sensory level to T3 compared with T2 for the isobaric and hypobaric solutions. Motor block was significantly (P = 0.029) reduced with increasing baricity and this trend was significant (P = 0.033) for the lateral position only. Hypotension incidence and ephedrine use increased with decreasing baricity (P = 0.003 and 0.004 respectively), with the hypobaric sitting group having the most frequent incidence of hypotension (76%) as well as cervical blocks (24%; P = 0.032).