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Rev. Bras. Anestesiol. vol.57 no.6 Campinas Nov./Dec. 2007
The state of the art of unilateral spinal block
A study on hypobaric 0.5% 1 bupivacaine published recently confirmed the results of the study on hyperbaric bupivacaine 2, which determined that it is virtually impossible to perform unilateral spinal anesthesia with full doses of the drug. In 1961, Tanasichuk et al. described a special technique of spinal anesthesia in patients undergoing orthopedic surgeries of the lower limbs, which they called spinal hemianalgesia 3. The distance between the spinal roots on the right and left sides is, approximately, 10 to 15 mm in the lumbar or thoracic region, and this reduced distance is enough to produce restricted unilateral block of the spinal roots.
It is necessary to introduce the local anesthetic in the right place, to avoid mixing and diluting it in the cerebrospinal fluid (CSF), administered at a volume and concentration that are enough to produce anesthesia and allow the realization of the surgical intervention without causing cardiovascular changes. Restricted spinal block is performed to decrease the extension of the surgical blockade to the side being operated, as well as to maintain the anesthesia for a period compatible with the procedure.
The most important factors to be considered when performing a hemianalgesia are: type and gauge of the needle, density of the local anesthetic relative to the cerebrospinal fluid, position of the patient, speed of administration of the solution, and dose/concentration/volume of the anesthetic solution.
A high success rate with unilateral spinal block was reported with 20G and 22G 3, and 29G 4 needles. The gauge and type of needle seems to be more specifically related with the incidence of side effects. When a liquid is injected through a needle, the speed of administration is important to determine whether the flow will be laminar or turbulent. A turbulent flow provides a fast mixture of the local anesthetic with the CSF, producing a homogenous mixture that reduces the baric gradient between them, avoiding migration of the anesthetic solution. The slow injection is related with a greater prevalence of unilateral spinal block 5.
The position of the patient during and immediately after the injection of the anesthetic influences the dispersion of drugs injected in the subarachnoid space. Thus, the use of a solution with a density lower or higher than that of the CSF is, in theory, capable to control the distribution of the spinal block. Maintenance of the lateral decubitus for a determined length of time might restrict the surgical block to the side to be operated. However, it is difficult to define the ideal length of time, since it is influenced by the type of anesthetic and the dose administered. In fact, when full doses (high doses) are used, the blockade migrates even when changing position one hour later 6. On the other hand, the slow administration of low doses of hypo- and hyperbaric solutions and maintaining the patient in lateral decubitus for 15 to 30 minutes results in, virtually, a restricted distribution to the side to be operated on, therefore producing surgical block only in one side 5,7-9.
The difference between the density of the CSF and the anesthetic solution is the most important factor to be considered when restricting a spinal block. Both hyper- and hypobaric solutions have been used in unilateral spinal block. However, the difference in density between the anesthetic and the CSF is greater for the hyperbaric than the hypobaric solution and the use of the hyperbaric anesthetic results in a more predictable nerve block. Besides, with the same dose (5 mg) of hyperbaric and hypobaric bupivacaine, the greater volume of the hypobaric solution resulted in greater dispersion of the blockade 9. On the other hand, even in low doses, isobaric solutions move in the CSF after 20 minutes and, therefore, the unilateral block is achieved only in 28% of the patients 9,10.
Hyperbaric tetracaine was used in the initial study of Tanasichuk et al. 3 In Brazil, the first report on unilateral spinal block was by Gouveia & Labrunie in 1985 using 0.1% hypobaric tetracaine 11. After a few months, the same authors published a report on the use of 0.15% hypobaric bupivacaine 12. Due to the large diffusion and safety of bupivacaine in spinal anesthesia, it is not surprising that most of the studies on unilateral spinal block use this agent. When using a hyperbaric solution of bupivacaine, one should choose the 0.5% solution in 7.5% or 8% glucose in water. The 0.18% 6 or 0.15% 9,12 solution has been suggested when using the hypobaric solution.
In my clinical experience, I have systematically used solutions of bupivacaine, hyper- and hypobaric enantiomeric excess (S75-R25) bupivacaine with excellent results. To study the different presentations of lidocaine, such as the 1.5% and 2% hyperbaric 13,14, and 2% isobaric 14 solutions, in procedures with short duration, I use 15 to 20 mg of the hyperbaric solutions, and 18 to 30 mg of the hypobaric solution of 0.6% lidocaine. When using lidocaine, the patient should remain in lateral decubitus for 5 to 10 minutes.
The state of the art in unilateral spinal block suggests that, in order to obtain anesthesia restricted to one limb, one can use pencil point or cutting needles, preferentially 27G, with the opening directed to the side one wishes to anesthetize. Despite the preference for bupivacaine, there is room to use low doses of enantiomeric excess (S75-R25) bupivacaine and lidocaine. The rate of administration should always be standardized. Hypobaric solutions should be administered at a rate of 1 mL.15 sec-1, and twice the time (1 mL.30 sec-1) for hyperbaric solutions. When using bupivacaine the patient should remain in lateral decubitus for 15 to 20 minutes, and for lidocaine, 5 to 10 minutes.
Finally, this technique should be used especially in outpatient surgeries, in patients who need good cardiocirculatory stability, and when one does not want to achieve the annoying bilateral motor blockade.
Luiz Eduardo Imbelloni, TSA/SBA
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