Baricity
What is the best local anesthetic to use in a spinal anesthetic for total joint replacement?
Isobaric vs Hyperbaric Local Anesthetic Spinals
Our spinal kits come stocked with hyperbaric bupivacaine. Refer to the dermatome map. Consider that for a hip surgery, the dermatomes needing blockade include L1-L4. For a knee, L3-S2. Our lumbar spinals are performed at L2-3, or L3-4.
When hyperbaric bupivacaine is used, and a patient is then laid supine, due to the natural lordosis and kyphosis of the spine, the bupivacaine will spread sacrally and also up to the thoracic nerve roots resulting in a bimodal distribution. When isobaric bupivacaine is used, it tends to stay in the lumbar region where it is most needed. The result is that at equivalent dosage, an isobaric bupivacaine spinal will give a more reliable and longer duration block at the hip or knee compared to a hyperbaric bupivacaine spinal. Furthermore, an isobaric lumbar spinal virtually eliminates any chance of a high spinal, and results in a more hemodynamically stable patient as the sympathectomy is contained to less levels.
Another extremely important application for these concepts applies to our hip patients. Due to the fast paced nature of these ORs, after a spinal is successfully performed, the expectation is that they are laid supine, quickly sedated, and then positioned laterally. When done in quick suscession, this does not allow for ample time for full local anesthetic binding, and a hyperbaric spinal would subsequently sink to the non-operative “down” hip, possibly leaving the operative site not adequately blocked. This concern is eliminated when using isobaric local anesthetic.
Glass Spine Model Demonstrating Local Anesthetic Baricity
" 111 patients were randomly allocated into 3 equal groups for spinal anesthesia in lower limb surgeries. In group 1 (1B) spinal anesthesia was performed with 3 ml of 0.5% isobaric bupivacaine (n= 37); in group 2 (2L)—3 ml of 0.5% levobupivacaine (n= 37), in group 3 (3H)—3 ml of 0.5% hyperbaric bupivacaine (n= 37). The criterion for assessing the effectiveness of anesthesia was the need to switch to another type of anesthesia, or the need for additional use of narcotic analgesics or local anesthesia during surgery"
"The study found 83.7% efficacy of levobupivacaine and 72.9% efficacy of hyperbaric bupivacaine in comparison with isobaric bupivacaine (100%) when administered intrathecally in equal volumes and amounts (by the criterion of additional intraoperative analgesia)."
How to set up for an isobaric spinal
Obtain PRESERVATIVE FREE 0.5% Bupivacaine from pharmacy (also can be found in omnicell in the core)
The isobaric bupivacaine must be added sterily to the spinal kit – either squirt it into an open pocket in the opened spinal kit (preferred), or have an assistant add it to your spinal syringe
Calculate your dose. Be mindful of the conversion between hyperbaric (0.75%) and the isobaric (0.5%) bupivacaine. Each mL of 0.75% bupivacaine contains 7.5mg, whereas each mL of 0.5% bupivacaine contains only 5mg.
1.6 mL 0.75% bupivacaine = 12mg
2.4 mL 0.5% isobaric bupivacaine = 12 mg
Please note – when aspirating CSF, you will NOT see the traditional “swirl” that you do with the heavy bupivacaine as that is a result of the dextrose added to the bupivacaine.
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WARNING! - STOP AND CHECK
Extreme caution must be taken when administering intrathecal medications. One very possible and very lethal scenario is making a drug-swap error and injecting TXA intrathecally. Both the 0.5% isobaric preservative free bupivacaine and the tranexamic acid come in a 10 cc vial. The consequences are LETHAL.
"In this case report, TXA was injected intrathecally instead of hyperbaric bupivacaine 0.5%, because both ampules were having the same appearance with consequent development of myoclonic seizures and ventricular fibrillation. "
"A decrease in venous return and systemic vascular resistance caused by sympathetic nervous system block are the primary causes of spinal anesthesia-induced hypotension. Additionally, extension of the sensory block beyond the T4 level will lead to blockage of cardioaccelerator fibers, with subsequent decrease in the heart rate and cardiac output. A close correlation between peak block height and the amount of systolic pressure decrease and incidence of hypotension was shown. Sensory blocks at or above T6 increased hypotension risk by 2.4 times, which increased to 3.8 in sensory blocks at or above T5. The baricity of a local anesthetic used in spinal anesthesia influences the block level and, as a result, the severity and frequency of changes in blood pressure. Hyperbaric bupivacaine has a greater tendency for cephalic spread than isobaric bupivacaine; therefore, it has a greater peak sensory block height and, as a result, a greater incidence of hypotension and blood pressure drops in patients undergoing nonobstetric surgery"
But the label specifically says "NOT FOR SPINAL ANESTHESIA" - so whats up?!
Off label preservative free isobaric bupivacaine spinal anesthesia has been around for decades and been safely used in millions of patients
"The use of 2-chloroprocaine, isobaric bupivacaine, ropivacaine, and mepivacaine for spinal anesthesia is off-label. However, this is because no one is prepared to finance a formal FDA application for this clinical application. It is not related to any concern about tissue toxicity"
The FDA itself has stated that “once a product has been approved for marketing, a physician may prescribe it for uses or in treatment regimens or patient populations that are not included in approved labeling. Valid new uses for drugs already on the market are often first discovered through serendipitous observations and therapeutic innovations.” Optimal regulatory practices with respect to off-label uses of medications may be a subject of debate in the literature and in the press. However, it is clear that (1) the risks associated with off-label use represent a broad spectrum, (2) the benefits of certain off-label uses may clearly outweigh the associated risksin certain patients, and (3) off-label use plays a vital role in the practice and evolution of modern medicine.
