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*Division of Anesthesiology,
Clinic of Orthopedic Surgery, University Hospital Geneva, Geneva, Switzerland
Address correspondence and reprint requests to Dr. Alexandre Faust, Division d’ Anesthésiologie, Hôpital Cantonal—Hôpitaux Universitaires de Genève, Rue Micheli-du-Crest 24, 1211 Genève 14, Switzerland. Address e-mail to alexandre.faust{at}hcuge.ch
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Abstract |
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 Top Abstract IntroductionPatients and MethodsResultsDiscussionReferences |
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IMPLICATIONS: For total hip arthroplasty in the lateral position, spinal hypobaric bupivacaine compared with isobaric prolonged sensory block at the operative side and delayed the time to first analgesic.
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Introduction |
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TopAbstractIntroductionPatients and MethodsResultsDiscussionReferences |
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The use of hypobaric local anesthetic has been reported for single-shot injection (1,2) and continuous spinal anesthesia (3,4). However, possible advantages of hypobaric over isobaric solutions have not been tested in these specific surgical conditions. The aim of the present study was to compare the anesthetic and hemodynamic effects of isobaric (plain bupivacaine mixed with normal saline) and hypobaric bupivacaine (plain bupivacaine mixed with distilled water) solutions for THA performed with patients in the lateral decubitus position.
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Patients and Methods |
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TopAbstractIntroductionPatients and MethodsResultsDiscussionReferences |
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Using a sealed envelope system, the patients were randomly assigned to receive either hypobaric or isobaric bupivacaine solutions, which were prepared as follows. Isobaric bupivacaine: 3.5 mL (17.5 mg) of plain bupivacaine 0.5% (5-mL vial Carbostesine® 0.5%; Astra Zeneca, Grafenau, Zug, Switzerland) diluted with normal saline to a total of 5 mL; measured density at 37°C was 0.999406 g/mL. Hypobaric bupivacaine: 3.5 mL (17.5 mg) of plain bupivacaine 0.5% diluted with distilled water, to a total of 5 mL; measured density at 37°C was 0.997302 g/mL.
According to the randomization, an anesthesiologist not participating in patient care or data collection prepared 10 mL of distilled water or normal saline. The anesthesiologist in charge of the patient withdrew 3.5 mL of plain bupivacaine 0.5% from a 5-mL vial and added 1.5 mL of the prepared solution. The density measurements of study solutions were performed using an Anton Paar DMA 4500 densitometer (Anton Paar GmbH, Graz, Austria). For each solution, three measurements were performed and the mean value was considered.
With the operating table horizontal, the patients were placed in the lateral decubitus position with the operated hip up. Lumbar puncture was performed at the L2–3 interspace with a 25-gauge Whitacre needle. After observing free cerebrospinal fluid (CSF) reflux, the needle aperture was oriented upward and 5 mL of the study solution injected at a rate of approximately 0.5 mL/s. The patients remained in the lateral position until the end of surgery, at which time they were turned supine.
The following variables were measured throughout the study:
All the above variables were determined during anesthesia by the anesthesiologist in charge of the patient, and in the recovery room by nurses who were trained by the investigators. Discomfort related to the lateral position during surgery was treated with fentanyl 1 µg/kg IV (maximal 2 doses) and anxiety with midazolam 1 mg IV.
Prospective power tests defined the sample size using sensory block level regression time to L2 of 157 ± 37 min using 3 mL of plain bupivacaine 0.5% (6). The sample size was computed to detect a 25% difference in favor of the hypobaric group, i.e., a longer duration of block with a power of 80% and a two-tailed significance level of 5% (ß = 0.2; 
= 0.05). A minimal sample of 14 patients for each group met these criteria. Results are expressed as mean ± SD or median (ranges) for discrete variables. Comparisons between groups or between both sides in the same group were performed using the Student’s t-test for unpaired or paired data, the Mann-Whitney U-test, and the 
2 test as required. A P value < 0.05 was considered statistically significant.
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Results |
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TopAbstractIntroductionPatients and MethodsResultsDiscussionReferences |
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Duration of anesthesia, defined as the time between the spinal injection and the end of the surgery, was comparable between the two groups—170 ± 25 min for isobaric and 168 ± 23 min for hypobaric.
When comparing the sensory regression times to L2 between the nondependent and the dependent sides, patients in both groups showed significantly prolonged sensory regression to L2 on the nondependent (operative) side (242 ± 36 versus 219 ± 30 min, P < 0.005; and 287 ± 51 versus 233 ± 38 min, P < 0.0001, for isobaric and hypobaric group, respectively). When comparing both groups (Fig. 2), regression to L2 on the nondependent side was significantly prolonged in the hypobaric group (287 ± 51 versus 242 ± 36 min, P < 0.004); likewise, time to first analgesic requirement was significantly longer in the hypobaric group (290 ± 46 versus 237 ± 39 min, P < 0.001).
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Hemodynamic changes, observed during the first 45 min after spinal injection, were comparable between the two groups. Maximal decrease in MAP was 32% ± 13% versus 31% ± 16% and for HR 14% ± 11% versus 14% ± 10% for the isobaric and hypobaric group, respectively. Ten patients in the isobaric and nine in the hypobaric group received ephedrine; one patient in the isobaric group received atropine.
Finally, two patients in the isobaric group received fentanyl for discomfort and four received midazolam for anxiety. In the hypobaric group, three patients received fentanyl and five midazolam.
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Discussion |
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TopAbstractIntroductionPatients and MethodsResultsDiscussionReferences |
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Although described as a possible anesthetic technique (7), there are only a few studies reporting the use of hypobaric spinal anesthesia (1–4,8). Before the present study, two other reports compared hypobaric and isobaric bupivacaine. Van Gessel et al. (9) reported fewer failures with isobaric versus hypobaric bupivacaine during continuous spinal anesthesia for hip surgery. However, both injection of local anesthetic and surgery were performed with patients supine. Kuusniemi et al. (10) tested small doses (6 mg) of hypobaric and plain bupivacaine in the lateral position for knee arthroscopy; 20 minutes after spinal injection, the patients were turned supine for surgery. A differential spread for both sensory levels and motor block between nondependent and dependent sides was demonstrated for each solution. However, no difference was found between the two solutions, when comparing the same sides.
The present study is the first comparing isobaric and hypobaric bupivacaine in patients undergoing surgery in the lateral position. During progression of spinal anesthesia, both solutions demonstrate qualities of isobaricity (no difference in upper sensory level and maximal degree of motor block between nondependent and dependent sides). The results of other studies investigating subarachnoid distribution of hypobaric local anesthetic in the lateral position suggest a dose-related effect in favor of the nondependent side. Kuusniemi et al. (10) reported a differential sensory and motor block with hypobaric solution (6 mg of bupivacaine in 3.4 mL). Van Gessel et al. (4) observed a differential spread only for motor block when using hypobaric solutions of tetracaine or bupivacaine (7.5 mg in 3 mL). Atchison et al. (1), studying the effects of speed of injection, documented a differential sensory spread between nondependent and dependent sides with 10 mg of hypobaric tetracaine in 5 mL when injected over 250 seconds with an electrically driven syringe pump through a Whitacre needle with the aperture oriented upward. When the same solution was injected rapidly over 10 seconds, approximating the usual clinical spinal injection speeds, no differential sensory block was found. Using an identical methodology, Horlocker et al. (2) did not show any difference in sensory levels between the nondependent and dependent sides in either slow or fast injection groups, using 15 mg of hypobaric bupivacaine in 5 mL. Consequently, the appearance of a differential block seems to be favored by using small dose hypobaric solution injected very slowly.
In our study, the absence of early clinical signs of preferential distribution in favor of the nondependent side in the hypobaric group can be explained essentially by the following mechanism. Unlike in hyperbaric solutions (11), there is a relatively small difference in density between the hypobaric bupivacaine solution used in the present study (0.997302 g/mL) and CSF using measurements previously made (12) in our institution (1.000529 ± 0.000107 g/mL). Given this slight difference in density and given the relatively large dose (17.5 mg) and volume (5 mL) of hypobaric bupivacaine, injected rapidly over 10 seconds, it is not surprising that a dense initial bilateral subarachnoid block developed initially. Thus, despite injecting the anesthetic solution through a directional Whitacre needle, there was no early evidence of a preferential spread.
However, qualities of hypobaricity appeared during regression of spinal anesthesia in both groups, but were clinically more relevant in the hypobaric group: 1) The regression time to L2 between nondependent and dependent sides was significantly different in the two groups; 2) Unlike in the isobaric group, significantly fewer patients in the hypobaric group had complete motor block recovery on the nondependent compared with the dependent side.
The appearance of this delayed asymmetrical block can be attributed to the differences in densities between anesthetic solutions and CSF, associated with prolonged lateral position of approximately 3 h in the present study. It is presumed that part of a local anesthetic injected intrathecally remains free in CSF for at least 60 minutes, because one hour after administration, variations in upper sensory level were found when changing patient position (13,14). We speculate that in the present study, three hours of lateral decubitus positioning allows more neural fixation on the nondependent roots of hypobaric than isobaric bupivacaine. These arguments could explain the more pronounced differential spread of hypobaric over isobaric bupivacaine observed during regression of the spinal anesthesia.
In the present study, some degree of differential block is documented for isobaric bupivacaine. Similar findings are reported by others for patients receiving plain bupivacaine and tested in the lateral position (10,15), questioning whether plain bupivacaine is isobaric or hypobaric (15). Greene (7) stated that the limit between hypobaric and isobaric local anesthetic solutions is a baricity of 0.9990, which is calculated by dividing density of local anesthetic with that of CSF. In 1954, Davis and King (16) stated that this limit is a density of local anesthetic lower than three standard deviations below mean human CSF density. Using more precise techniques of measurement of CSF density, Richardson and Wissler (17) determined the upper limits of hypobaricity as density of local anesthetic between 1.00016 to 1.00037 g/mL according to the variations of CSF density in a different subgroup of patients and considered the mixture plain bupivacaine-morphine with a density of 0.99941 as hypobaric (18). Recently, we reported the density of plain bupivacaine 0.5% of 0.999343 ± 0.000004 g/mL at 37°C (12). Compared with this value, the density of hypobaric bupivacaine investigated in the present study was less (0.997302 g/mL) and that of the isobaric solution more (0.999406 g/mL). Their baricities calculated with density of CSF of 1.000529 g/mL (12) were 0.996774 and 0.998876 for hypobaric and isobaric solutions, respectively. Thus, hypobaric bupivacaine seems to be hypobaric according to the definition of Greene, Davis, and Richardson, whereas isobaric bupivacaine is at the limit between hypobaric and isobaric for Greene but remains hypobaric for Davis and Richardson. It should be noted that these different limits of baricity are given arbitrarily and that, besides baricity, there are >20 demonstrated or theoretical factors influencing subarachnoid distribution of local anesthetics (7). Nevertheless, it is admitted that plain bupivacaine 0.5% administered at the usual volume of 3 mL in patients placed supine after injection behaves clinically as isobaric (7). However, the results of the present study suggest that local anesthetic solutions considered isobaric, with a density even more than that of plain bupivacaine but less than that of the CSF, can show some signs of hypobaricity in patients kept in prolonged lateral position.
In summary, for patients undergoing THA in the lateral position under spinal anesthesia, 17.5 mg of hypobaric bupivacaine, compared with the identical dose of isobaric bupivacaine, prolonged sensory regression to L2 and delayed the use of first analgesic, without further compromising systemic hemodynamics. We believe that 45 minutes longer duration of spinal block is clinically relevant and increases the reliability of hypobaric spinal anesthesia for this type of surgical procedure.
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Acknowledgments |
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References |
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TopAbstractIntroductionPatients and MethodsResultsDiscussionReferences |
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This article has been cited by other articles:
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  G. Danelli, M. Baciarello, S. Di Cianni, M. Zasa, G. De Marco, S. Adamanti, and G. Fanelli Effects of baricity of 0.5% or 0.75% levobupivacaine on the onset time of spinal anesthesia: a randomized trial: [Effets de la baricite de la levobupivacaine a 0,5 % ou de 0,75 % sur le delai d'installation d'une rachianesthesie : une etude randomisee] Can J Anesth, August 1, 2008; 55(8): 501 - 506. [Abstract] [Full Text] [PDF]
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J. A. Kim and H. J. Ahn Hypobaric spinal anesthesia with 0.2% tetracaine for total joint hip arthroplasy: [La rachianesthesie hypobare avec de la tetracaine a 0,2 % pour l'arthroplastie totale de hanche] Can J Anesth, November 1, 2005; 52(9): 958 - 962. [Abstract] [Full Text] [PDF]
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