Each year, more than 300 million surgical procedures are performed worldwide, with approximately 5% (15 million) conducted under spinal anesthesia (SA). This technique involves administering local anesthetics or opioids, or both, into the spinal space to induce numbness and weakness in the lower body, enabling pain-free surgery. One major application of spinal anesthesia is for cesarean delivery (C-section). In terms of medication selection, an area of ongoing research and discussion is whether hyperbaric spinal anesthesia is superior.
Bupivacaine, a long-acting local anesthetic, is the most common drug, often supplemented with opioids such as fentanyl, sufentanil, or morphine. It comes in two commercially available formulations: isobaric bupivacaine (IB) and hyperbaric bupivacaine (HB). IB has a density equal to that of cerebrospinal fluid (CSF), while HB has a density heavier than CSF. The denser (hyperbaric) bupivacaine is produced by adding glucose (80 mg/mL) to isobaric or plain bupivacaine. The difference in densities of the two preparations is believed to affect their diffusion patterns and thus determine their effectiveness, spread, and side-effect profile. In general, hyperbaric spinal anesthesia should spread more downwards, in the direction of gravity, because of its greater density.
To be reliably hyperbaric in all patients, an anesthetic solution must have a baricity of at least 1.0015 at 37°C. The addition of dextrose to the anesthetic solution is the most common method used to achieve this. Because dextrose is neurologically benign, the concentrations used are usually far higher than those needed to increase baricity above 1.0015. The distribution of hyperbaric spinal anesthetic solutions in CSF is influenced by the patient’s position, with significant differences observed between horizontal or head-down positions and seated positions.
The uptake of local anesthetics injected into the subarachnoid space determines which neuronal functions are affected during spinal anesthesia, while their elimination from the subarachnoid space determines the duration of these effects. The distribution of local anesthetics within CSF determines the extent of altered neuronal function. The physical characteristics of spinal anesthetic solutions, including weight/density of the solution, amount of anesthetic given, concentration of anesthetic in the injectate, and volume of anesthetic solution injected, are major determinants of their spread in CSF.
Anesthesiologists performing spinal anesthesia must choose between the two most commonly available formulations, hyperbaric or isobaric bupivacaine. Despite more than 30 years of use, there is still disagreement regarding the preferred formulation. This decision is often based on personal experience, training, local institutional practices, and drug availability.
Baricity is a significant factor in maternal hemodynamic changes during elective cesarean section, as demonstrated by a hospital-based prospective cohort study by Heloll et al. However, other studies have shown different findings, such as Uppal et al.’s meta-analysis. These authors show that isobaric bupivacaine produces greater changes in blood pressure, incidence of hypotension, and vasopressor requirement than hyperbaric bupivacaine after SA for elective cesarean section. Hyperbaric bupivacaine allows for a relatively rapid onset of motor block, with a shorter duration of motor and sensory block, while isobaric bupivacaine has a slower onset and provides a longer duration of both sensory and motor block.
A recent Cochrane review found that intrathecal hyperbaric bupivacaine had a more rapid onset of sensory blockade at the 4th thoracic vertebra (T4) level than isobaric bupivacaine. However, the overall quality of evidence for most outcomes is low or very low according to the GRADE method. Various advantages of isobaric and hyperbaric bupivacaine have been described, but no definitive evidence exists to recommend one form over the other.
References
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- Helill SE, Sahile WA, Abdo RA, Wolde GD, Halil HM. The effects of isobaric and hyperbaric bupivacaine on maternal hemodynamic changes post spinal anesthesia for elective cesarean delivery: A prospective cohort study. PLOS ONE 2019;14(12):e0226030.
- Uppal V, Retter S, Shanthanna H, Prabhakar C, McKeen DM. Hyperbaric Versus Isobaric Bupivacaine for Spinal Anesthesia: Systematic Review and Meta-analysis for Adult Patients Undergoing Noncesarean Delivery Surgery. Anesthesia & Analgesia 2017;125(5):1627-1637.
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