Position plays a vital role in subarachnoid blockade specially while using hyperbaric local anesthetic agents. Sitting or supine position may affect the final height of the block. Is there any modification of positioning while using isobaric local anesthetic agents?
Contrary to hypo- and hyperbaric solutions, a true isobaric solution does not exist. This has to do with the variation in the density of c.s.f. between individuals. Hyperbaric solutions are defined as having a density > 1.0015, and these solutions will be hyperbaric in all individuals. Hypobaric solutions are defined as having a density < 0.9990, and these solutions will be hypobaric in all individuals as well. The traditional definition of an isobaric solution is a solution with a density between 0.9990 and 1.0015, but it is obvious that a solution with a density of, say, 0,9992 will be hypobaric in an individual with a c.s.f. density of, say, 1.0012. Conversely, a solution with a density of 1.0012 will be hyperbaric in an individual with a c.s.f. density of 0.9993. Another complicating factor is that the density of a solution is inversely related to temperature. Since local anesthetics are usually injected at room temperature, the density of that solution will decrease during equilibration to body temperature. With a true hypo- or hypobaric solution this change in density will not have much impact, but with a solution that has a density in the range of 0.9990 - 1.0015 the change in density may also cause a change in baricity. This has been amply demonstrated with plain bupivacaine, a solution considered to be isobaric: When plain bupivacaine is stored in a refrigerator an injected with a temperature of 6 degrees Celsius, the solution shows a hyperbaric distribution pattern. When the solution is warmed to body temperature immediately prior to injection, it shows a clearly hypobaric distribution pattern.
Is there nothing to say then about position and "isobaric" solutions? Yes there is. Plain bupivacaine is slightly hyperbaric at room temperature, but hypobaric at body temperature. During thermal equilibration, which will take an assumed 1-2 minutes, the baricity of the solution changes. This is best demonstrated by the following: When injecting plain bupivacaine in the lateral decubitus position, the maximum level of sensory blockade is unpredictable and may vary from very low to high, making this technique only reliable for lower limb surgery. However, injecting plain bupivacaine in the sitting position and maintaining this position for 2 minutes will usually result in a maximum level of sensory blockade above the tenth thoracic dermatome.
An important thing to remember: Position may be used as a tool to affect the maximum level of sensory block, but it is not suitable for accurate "steering" of the maximum level. Postion mainly affects the initial spread during the first minutes. It usually takes 10-15 min to determine if the maximum level of sensory block is going to be adequate, too high or too low; at that time, changes in position will have a minimal or no additional effect on the maximum level, asuming that a normal dose of local anesthetic has been used.
When you are giving your patient an isobaric local anaesthetic solution, one thing to consider is the levels of dermatomes you would love to achieve, compared to hyperbaric solution, isobaric solution would give the dermatome levels lower than the hyperbaric one if you give the same amount of local anaesthetics. Therefore, to achieve the same dermatome blockade, you do have to give more isobaric local anaesthetics.
Not correct when spinal anesthesia is achieved with the patient in the sitting position and this position is maintained for 1-2 min. Maximum levels of sensory blockade with plain or hyperbaric bupivacaine will be comparable.
Excellent answer Rudolf. I find it interesting that despite this evidence the standard teaching is still, for example from Miller's Basics of Anesthesia (6th Ed.): "Because the distribution of [isobaric] local anesthetic solutions is not affected by gravity, spinal anesthesia can be performed without concern that the resultant block might be influenced by patient position."
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