Spinal anesthesia is a form of regional anesthesia involving the injection of a local anesthetic into the subarachnoid space, generally through use of a fine needle. Local anesthetic agents intended for spinal anestheia are either heavier (hyperbaric), lighter (hypobaric) or have the same specific gravity (isobaric) as the cerebrospinal fluid (CSF). Hyperbaric solutions tend to spread down (due to gravity) from the level of the injection, while isobaric solutions don’t. Hypobaric solutions are rarely used anymore.
An ideal local anesthetic agent for intrathecal use must have a rapid onset and reliable duration, with a low incidence of adverse effects. Bupivacaine is a local anesthetic largely used for spinal anesthesia, mainly as hyperbaric or isobaric solution. Controversy exists regarding the predictability of the levels of analgesia achieved with isobaric solution when compared to hyperbaric. Most local anesthetics used for spinal anesthesia are available as hyperbaric solutions, due to higher and more effective control of spread. Baricity of the anesthetic can be increased by the addition of dextrose.
In a study of 60 subjects undergoing lower abdominal, hips, and lower extremity surgeries two groups were randomized. Group H received 20 mg of 0.5% hyperbaric bupivacaine while Group I received 20 mg of 0.5% isobaric bupivacaine. Results obtained were based on comparison of sensory and motor blockade, hemodynamic changes and adverse effects. In the majority of cases, it was found that the onset of analgesic and motor blocks was faster with isobaric when compared to hyperbaric solutions. Also, duration of sensory and motor blocks lasted longer with use of isobaric bupivacaine. On the other hand, prediction of the spread of anesthesia was easier with use of the hyperbaric agent. In both groups, hemodynamic changes were not clinically relevant, and the adverse effects were found comparable. In conclusion, the study found that isobaric bupivacaine produced more rapid onset and longer duration when compared to hyperbaric bupivacaine.