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Revista Brasileira de Anestesiologia

Print version ISSN 0034-7094

Rev. Bras. Anestesiol. vol.57 no.3 Campinas May/June 2007 



Low dose isobaric, hyperbaric, or hypobaric bupivacaine for unilateral spinal anesthesia*


Baja dosis de bupivacaína isobara, hiperbara o hipobara para anestesia raquidea unilateral



Luiz Eduardo Imbelloni, TSAI; Lúcia Beato, TSAII; Marildo A Gouveia, TSAIII; José Antônio CordeiroIV

IDiretor do Instituto de Anestesia Regional do Hospital de Base da FAMERP; Anestesiologista da Casa de Saúde Santa Maria e Clínica São Bernardo, Rio de Janeiro, RJ
IIAnestesiologista da Casa de Saúde Santa Maria e Clínica São Bernardo, Rio de Janeiro, RJ
IIIDiretor do Instituto de Anestesia Regional do Hospital de Base da FAMERP
IVProfessor da Faculdade de Medicina São José Rio Preto, SP

Correspondence to




BACKGROUND AND OBJECTIVES: Unilateral spinal anesthesia has its advantages, especially in patients undergoing outpatient basis surgeries. Low dose, slow speed of administration, and the lateral positioning make easier the unilateral distribution in spinal anesthesia. Isobaric, hyperbaric, and hypobaric solutions of bupivacaine were compared in the unilateral spinal anesthesia in patients undergoing outpatient basis orthopedic surgeries.
METHODS: One hundred and fifty patients were randomly divided in three groups to receive 5 mg of 0.5% isobaric bupivacaine (Iso Group), 5 mg of 0.5% hyperbaric bupivacaine (Hyper Group), or 5 mg of 0.15% hypobaric bupivacaine (Hypo Group). The solutions were administered in the L3-L4 space with the patient in the lateral decubitus and remaining in this position for 20 minutes. Sensitive anesthesia was evaluated by the pin prick test. Motor blockade was determined by the modified Bromage scale. Both blockades were compared with the opposite side and among themselves.
RESULTS: There was a significant difference between the side of the surgery and the opposite side in all three groups at 20 minutes, but the frequency of unilateral spinal anesthesia was greater with the hyperbaric and hypobaric solutions. Sensitive and motor blockades were observed in 14 patients in the Iso Group, 38 patients in the Hyper Group, and 40 patients in the Hypo Group. Patients did not develop any hemodynamic changes. Postpuncture headache and transitory neurological symptoms were not observed.
CONCLUSIONS: Spinal anesthesia with hypobaric and hyperbaric solutions present a higher frequency of unilateral anesthesia. After 20 minutes, isobaric bupivacaine mobilized into cerebrospinal fluid (CSF) resulted in unilateral spinal anesthesia in only 28% of the patients.

Key Words: ANESTHETICS: bupivacaine; ANESTHETIC TECHNIQUES, Regional: unilateral spinal block; SURGERY, Orthopedics.


JUSTIFICATIVA Y OBJETIVOS: La anestesia raquidea unilateral puede presentar ventajas principalmente en pacientes en régimen ambulatorial. Baja dosis de la solución anestésica, velocidad lenta de la inyección espinal y la posición lateral facilitan la obtención de la distribución unilateral en la anestesia raquidea. Se compararon las soluciones isobara, hiperbara e hipobara de bupivacaína para obtener la anestesia raquidea unilateral en pacientes sometidos a intervenciones quirúrgicas ortopédicas en régimen ambulatorial.
MÉTODO: Ciento cincuenta pacientes fueron aleatoriamente separados en tres grupos para recibir 5 mg de bupivacaína a 0,5% isobara (Grupo Iso) o 5 mg de bupivacaína a 0,5% hiperbara (Grupo Hiper) o 5 mg de bupivacaína a 0,15% hipobara (Grupo Hipo). Las soluciones se administraron en el interespacio L3-L4 con el paciente en la posición lateral y permaneciendo en esa posición por 20 minutos. La anestesia sensitiva se evaluó por el test de la picada de la aguja. El bloqueo motor evaluado por la escala modificada de Bromage. Los dos bloqueos se compararon con el lado no operado y también entre sí.
RESULTADOS: Se notó una diferencia significativa entre el lado operado y no operado en todos los tres grupos a los 20 minutos, pero se obtuvo una mayor frecuencia de anestesia raquidea unilateral con las soluciones hiperbara e hipobara de bupivacaína. Se observó bloqueo sensitivo y motor en 14 pacientes del Grupo Iso, 38 pacientes en el Grupo Hiper y 40 pacientes en el Grupo Hipo. No ocurrieron alteraciones hemodinámicas en ningún paciente. No se observó cefalea postpunción ni síntomas neurológicos transitorios.
CONCLUSIONES: La anestesia raquidea con soluciones hipobara e hiperbara proporcionaron una mayor frecuencia de unilateralidad. Tras 20 minutos la solución isobara de bupivacaína se movió en el LCR resultando en apenas 28% de anestesia raquidea unilateral.




The difference in density between the cerebrospinal fluid (CSF) and local anesthetics solutions should be considered to restrict their distribution to the subarachnoid space. The solution of 0.5% bupivacaine (without glucose) behaves as a discretely hypobaric solution when used in spinal anesthesia 1. Maintaining the patient in the sitting position for two minutes after the administration of the isobaric anesthetic results in a higher blockade than if the patient were placed in the dorsal decubitus immediately after the injection 2.

In theory, unilateral spinal block can be achieved administering a hypobaric, isobaric, or hyperbaric solution in the subarachnoid space with the patient in the lateral decubitus, so the anesthetic forms a layer above (hypobaric or isobaric) or below (hyperbaric) de midline. Thus, the injection directed to one of the sides, with the distribution of the solution depending on its specific gravity, would result in something else, since there is no loss of anesthetic solution secondary to turbulence, which happens with fast administration. The dose of 1.2 mL (6 mg) of 0.5% hyperbaric bupivacaine has had a greater incidence of unilateral anesthesia than 1.2 mL (6 mg) of the isobaric solution, when it is injected slowly through a 27G Whitacre point needle and the patient remains in the lateral decubitus for 20 minutes 3. When 3.4 mL (6.1 mg) 0.18% hypobaric bupivacaine were injected under the same conditions, it showed no advantage over the 1.2 mL (6 mg) of isobaric bupivacaine 4. The use of 1% hyperbaric bupivacaine has no advantage over the 0.5% solution 5. Patients who received 8 mg of 0.5% bupivacaine presented a greater incidence of unilateral blockade than those who received 8 mg of the 1% solution 5. According to several authors, unilateral spinal anesthesia is a low dose and low flow technique, including the maintenance of the lateral decubitus for 15 to 30 minutes 6-10 as the best way of producing unilateral blockade.

The objective of this random, prospective study was to evaluate the frequency of the unilateral blockade when isobaric, hyperbaric, and hypobaric solutions were used for orthopedic surgeries. The efficacy of the blockade, as well as its recovery and hemodynamic changes, were evaluated.



The study protocol was approved by the Ethics Committee of the institution and the procedures were explained to the patients who agreed to participate. Exclusion criteria included hypovolemia, preexisting neurological disease, coagulation disorders, administration of thrombosis prophylaxis less than eight hours prior to the procedure, infection at the puncture site, agitation and delirium, and the presence of vesical catheter. One hundred and fifty patients, ages between 20 and 60 years, ASA physical state I and II, who did not receive premedication, scheduled for unilateral orthopedic surgery under spinal anesthesia participated in the study. A sphygmomanometer was placed on the thigh of every patient and inflated to 350 mmHg. Patients were randomly divided in three groups of 50 patients. Patients in the Iso Group received 1 mL 0.5% pure bupivacaine with a density of 0.9993 g.mL-1; patients in the Hyper Group received 1 mL of 0.5% bupivacaine (5 mg) with 8% glucose, with a density of 1.021 g.mL-1; and patients in the Hypo Group received 3.4 mL of 0.15% bupivacaine (5 mg), with a density of 0.9964 g.mL-1 (prepared from 7.5 mg = 1.5 mL 0.5% isobaric bupivacaine with 3.5 mL of distilled water). The surgeries included knee video-arthroscopy, correction of tibial and ankle fractures, associated or not with ligament lesion, and removal of synthesis material below the knee. Every patient was submitted to a 6-hour fasting period prior to the procedure. Before the anesthesia, a peripheral venous access was placed for hydration with 0.9% normal saline or Ringer's lactate and administration of medications during the procedure. Patients were monitored with continuous ECG, heart rate, and pulse oxymeter, and non-invasive blood pressure. Before the blockade, patients received a dose of 1 µ of intravenous fentanyl. If systolic blood pressure showed a reduction greater than 30% of baseline values, it would initially be treated increasing the speed of the hydration. If the patient did not respond, 2 mg of etilephrine would be administered and the dose repeated until a satisfactory response was achieved. Bradycardia was defined as a reduction of heart rate below 50 bpm. It would be treated with the administration of atropine up to a dose of 0.75 mg. Every patient received oxygen (1 to 2 L.min-1) by Hudson mask. Midazolam (1 to 2 mg) was administered after the second evaluation, at the 40-minute mark. Supplemental intravenous analgesia would be administered if the patient complained of pain or discomfort. Failure of analgesia was defined as absence of analgesia in the perineal region.

Using a list of random numbers, an Anesthesiology resident assigned the patients in one of the treatment groups. The patients in the Iso and Hypo Groups were placed on the side that was not to be operated. The patients in the Hyper Group were placed on the side to be operated on. After cleansing the skin with alcoholic chlorhexidine, the skin of the puncture site was infiltrated with 1% lidocaine. The puncture was paramedian, at the L3-L4 space, with a discardable 27G Quincke needle (B. Braun Melsungen S.A.) without the introducer. Once there was flow of CSF, 1 mL (5mg) of isobaric, 0.5% hyperbaric, or 3.4 mL (5 mg), 0.15% hypobaric bupivacaine was injected at 1 mL.seg-1. The bevel of the needle was directed to the side to be operated in every patient. Patients remained in the same position for 20 minutes before being placed in the dorsal decubitus for the surgery.

Assessment of sensitive and motor blockades was done by an anesthesiologist who was not aware to which group the patient belonged. The level of the sensitive blockade, defined as a lack of pinprick recognition or the touch with the mandril of the puncture needle, was determined bilaterally at the midclavicular line, while the motor blockade was determined by the modified Bromage scale 11: 0 = moving the inferior limbs freely; 1 = unable to raise extended limbs; 2 = unable to bend the knees; and 3 = unable to move the ankles. Motor and sensitive blockades were evaluated on both lower limbs 20, 40, and 60 minutes after the anesthesia and at the end of the procedure, and the limb to be operated on was compared to the opposite limb. The length of analgesia was determined by the time to regain sensitivity in the dermatome corresponding to the puncture site. Hemodynamic parameters were evaluated at three-minute intervals for the first 15 minutes after the administration of the anesthetic. Data regarding the time of recovery of sensitive and motor blockades, as well the first urine voided, ambulation, and conditions for hospital discharge were recorded. Complications in the recovery room, the need for a vesical catheter, pain, and treatment administered were recorded by an observer. The time to begin ambulation was oriented by the surgeon and the postoperative analgesia consisted of intravenous tenoxicam (40 mg) and dypirone (3 g). The first dose of analgesic was administered at the end of the surgical procedure, in the surgical room.

Upon leaving the surgical room, patients were requested to record their opinion about the technique employed. The choices included good, satisfactory, and bad. Patients were followed-up for three days postoperatively over the phone to gather information regarding the presence of headache, temporary neurological symptoms, or back pain. Headache was classified as dura mater postpuncture if it worsened when the patient was in the sitting position, were located in the occipital or frontal region, and increased with coughing, strain, or sneezing. Back pain was considered a transitory neurological symptom (TNS) if the patient experienced pain and/or decreased sensitivity in the back, buttocks, and legs after recovery, which resolved within 72 hours.

Mean age, weight, height, BMI, and duration of the blockade were compared by Analysis of Variance, while the level of the sensitive and motor blockades was analyzed by the Mood test for medians. The probability vectors of the satisfaction levels of the patients in the Iso Group was compared with the mean of the vectors of the other two groups by the pondered minimal quadrature method, and the unilaterality of the anesthesia was analyzed by the Chi-square test for independent samples. A level of significance of a = 0.05 was adopted.



Table I shows the characteristics of the patients and the duration of the surgeries. There were no differences among the groups regarding age, body mass index (BMI), weight, or duration of the surgical procedure (Table I).

Figure 1 shows the evaluation of the sensitive blockade. The level of the sensitive blockade on the limb operated on was higher in the Hypo Group than in the other two, which was statistically significant (p < 0.05); the level of the sensitive blockade in the opposite limb was higher in the Iso Group, except at the end of the procedure, with a p < 0.05; the level of the sensitive blockade was much higher in the operated limb in all three groups at every moment; and the median of the differences of the segments varied from 2 to 5 (p < 0.05), according to the pinprick evaluation. The mean duration of the blockade was longer in the Iso Group (2.94 ± 0.5 h) than in the Hyper Group (2.48 ± 0.38 h) and Hypo Group (2.3 ± 0.36 h) (p < 0.05).



During the time the patient was kept in the lateral decubitus (20 minutes), the sensitive blockade was exclusively unilateral in 31 patients in the Iso Group (62%), 42 patients in the Hyper Group (84%), and 45 patients in the Hypo Group (90%). After placing the patient in the dorsal decubitus, the sensitive blockade spread to the other side, and after 40 minutes the blockade remained unilateral in 16 patients in the Iso Group (32%), 42 patients in the Hyper Group (84%), and 43 patients in the Hypo Group (86%) (p < 0.05). The highest sensitive blockade (at 20 minutes) in the Hypo Group was significantly higher than in the other two groups, which did not show any differences among them (p < 0.05).

Figure 2 shows the evaluation of the motor blockade. While the patients were in the lateral decubitus, the motor blockade was completely unilateral in 32 patients in the Iso Group (64%), 43 patients in the Hyper Group (86%), and 48 patients in the Hypo Group (96%). After replacing the patients in the dorsal decubitus, the motor blockade spread to the other side, and after 20 minutes in this position, it remained unilateral in 14 patients in the Iso Group (28%), 38 patients in the Hyper Group (76%), and 40 patients in the Hypo Group (80%) (p < 0.05).



At 20 minutes, 62% of the patients in the Iso Group, 84% in the Hyper Group, and 86% in the Hypo Group presented unilateral blockade. At 40 and 60 minutes, 28% of the patients in the Iso Group, 76% in the Hyper Group, and 80% in the Hypo Group remained with unilateral blockade. Patients did not need complementary general anesthesia secondary to failure of the spinal anesthesia. Table II shows patient satisfaction with the techniques. Patients in the Hyper and Hypo Groups presented the highest incidence of satisfaction (p < 0.05).

There were no cases of hypotension or bradycardia. Also, no cases of postpuncture headache or urinary retention were recorded. There were no complaints of back pain, pain in the buttocks or in the legs in the next three days.



In this study, 5 mg of hyperbaric and hypobaric bupivacaine provided adequate unilateral spinal anesthesia for surgeries of the lower limbs. More patients obtained unilateral blockade with low dose of hyperbaric (76%) and hypobaric (80%) bupivacaine than with the isobaric (28%) bupivacaine during the dorsal decubitus. Patients were very satisfied with the technique and there were no hemodynamic changes.

The objective of the unilateral spinal anesthesia was to induce unilateral motor blockade, improving patient comfort, since prolonged bilateral motor blockade can be inconvenient, and to prevent hemodynamic changes. Several factors have been related with poor results of unilateral spinal anesthesia, including the position of the patient, the design of the needle, the speed of administration, the amount of local anesthetic injected, and the density of the anesthetic solution compared to the CSF 12. Although the patient is kept in the lateral decubitus after the injection, the isobaric, hyperbaric, and hypobaric solutions can influence the distribution of the anesthetic in the CSF, and the length of time in the lateral position is difficult to be defined, being influenced by the dose of the local anesthetic. Large doses (12 to 20 mg of bupivacaine) are related with important migration of the blockade, even after one hour in the lateral decubitus 13, while small doses (5 to 8 mg of bupivacaine) resulted in a restricted blockade only after 10 to 15 minutes in the lateral decubitus 14. The best unilateral result was obtained with low dose hyperbaric or hypobaric bupivacaine, keeping the patient in the lateral decubitus for 15 to 20 minutes 15. Low dose of 0.15% hypobaric bupivacaine (5 mg) was used previously in orthopedic surgeries and the results indicated that the best unilateral blockade was obtained when the patient remained in the lateral decubitus for at least 20 minutes 10. In this study, the puncture site and the dose were constant, and every patient remained in the lateral decubitus for 20 minutes.

Bupivacaine is a long-acting drug. The density of the 0.5% isobaric bupivacaine at 37°C is 0.9993 g.mL-1 16. When using the hyperbaric solution, 0.5% bupivacaine with 8% glucose can be used, and its density at room temperature is 1.0247 16. When using the hypobaric solution, 0.15% bupivacaine, with a density of 0.996 g.mL-1, is suggested 10 (1.5 mL 0.5% isobaric bupivacaine and 3.5 mL sterile distilled water to prepare 5 mL of 0.15% bupivacaine). The position of the patient during and immediately after the injection of the local anesthetic influences the distribution of the drug in the leptomeningeal space. If a solution that is more or less dense than the CSF is used, it is possible, at least in theory, to control the distribution of the spinal blockade, because the difference of the specific gravity between the hyperbaric solution and the CSF is greater than the differences between the hyperbaric and isobaric solutions. Therefore, the use of the hyperbaric solution has a more predictable result 15. This study showed that the Hyper (76%) and Hypo (80%) Groups had similar sensitive and motor blockades in every evaluation, while the isobaric group presented only 62% of patients with unilateral blockade at 20 minutes, which decreased to 28% when they were placed in the dorsal decubitus, which agrees with other studies in the literature 15,17.

The dose of the local anesthetic seems to be the most important factor influencing the unilateral distribution of spinal anesthesia. A reduction of the dose is critical to restrict the blockade to one side. However, an excessive reduction could increase the failure index of the spinal anesthesia 18. In this study, we used 5 mg of bupivacaine in all three groups, obtaining satisfactory results with the three solutions for the unilateral blockade, without any failures. Increasing the dose increases the recovery time of the blockade 19.

Large variations in volume and concentration of the local anesthetic have little influence in its dispersion in the leptomeningeal channel 20, while the total amount of molecules injected in the spinal canal is more important. In this study, using a fixed dose of 5 mg in all three groups, the volume of isobaric and hyperbaric bupivacaine corresponded to 1 mL, and the volume of hypobaric bupivacaine corresponded to 3.4 mL. The greater volume resulted in greater dispersion of the anesthesia.

Three types of needles are currently being used: the pencil tip model (Whitacre and Sprotte), the modified non-traumatic pencil tip (AtraucanÒ), and the cutting tip (Quincke type). A high success rate of unilateral spinal anesthesia has been reported using 25G Whitacre 20 and 29G Quincke 21 needles. The caliber of the needle seems to be more specifically related with the incidence of side effects, such as headache and lumbar pain. The success rate in 61% (92/150 patients) of the patients submitted to unilateral spinal anesthesia with a 27G Quincke needle demonstrated that it can be a good indication for this type of spinal block, besides being less expensive than the others. Unilateral block has been reported with low doses of hyperbaric or hypobaric bupivacaine when patients remained in the lateral decubitus for 15 to 30 minutes. The level of the blockade with these low doses does not seem to change after the change in decubitus. This was confirmed in this study using small doses of hyperbaric and hypobaric bupivacaine and keeping the patients in the lateral decubitus for 20 minutes. When low dose of isobaric bupivacaine is used, the level of the unilaterality changes when the decubitus changes, even 40 minutes after the injection. The same happens when higher doses of hyperbaric or isobaric bupivacaine are used 22,23. The result of 76% of unilateral spinal anesthesia with the hyperbaric solution of bupivacaine is similar to the 68% and 83% 15 reported in the literature; the 80% incidence of unilateral spinal anesthesia using 0.15% hypobaric bupivaciane is greater than the 52% 4 and 24% 24 reported in the literature, while the 28% incidence of unilateral spinal anesthesia using the isobaric solution is smaller than the 37% 15 reported in the literature.

Small amounts of local anesthetic injected in the subarachnoid space causes minimal hemodynamic changes, maintaining great cardiovascular stability. Hypotension is a complication of spinal anesthesia, which affects 33% of the patients when higher doses are used 25. The unilateral blockade with either one of the three solutions causes less hypotension. When hemodynamic changes secondary to unilateral and bilateral blockade with the same dose of hyperbaric bupivacaine (8 mg) are compared, the frequency of hypotension are 22.4% and 5%, respectively 26. With 7.5 mg of bupivaciane, hypotension was observed in 1/25 patients in the Hyper Group, and 1/25 patients in the Hypo Group, although prophylactic vasopressor or intravenous infusion were not administered 24. In this study, there were no cases of hypotension using 5 mg of bupivacaine.

To conclude, in this study, 5 mg of isobaric, hyperbaric, and hypobaric bupivacaine promoted adequate anesthesia for orthopedic surgeries and high patient satisfaction. Although the unilateral blockade was more frequent in the groups who received hyperbaric (76%) and hypobaric (80%) bupivacaine, there were no differences between them. Contrary to the hyperbaric and hypobaric solutions, isobaric bupivacaine moves in the CSF even after 20 minutes, maintaining the unilateral blockade in only 28% of the patients.



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Correspondence to:
Dr. Luiz Eduardo Imbelloni
Av. Epitácio Pessoa, 2356/203 Lagoa
22471-000 Rio de Janeiro, RJ

Submitted em 5 de dezembro de 2005
Accepted para publicação em 27 de fevereiro de 2007



* Received from Instituto de Anestesia Regional do Hospital de Base da Faculdade de Medicina de São José do Rio Preto (FAMERP), São José do Rio Preto, SP