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The Influence of Spinal Flexion in the Lateral Decubitus Position on the Unilaterality of Spinal Anesthesia

Kim, Jin-Tae, MD, PhD*; Lee, Jong-Hwan, MD, PhD; Cho, Chan-Woo, MD*; Kim, Hyo-Cheol, MD, PhD; Bahk, Jae-Hyon, MD, PhD*

doi: 10.1213/ANE.0b013e3182a1ee53
Regional Anesthesia: Research Report
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BACKGROUND: For unilateral spinal block, local anesthetics should affect the spinal nerves of 1 side. With full flexion of the spine, the sunken cauda equina becomes tightened and is suspended in the middle of the subarachnoid space. We performed this study to assess whether spinal flexion facilitates unilateral spinal anesthesia.

METHODS: Hyperbaric bupivacaine (8 mg) was administered at the L3-4 interspace through a 25-gauge Quincke needle at a rate of 0.02 mL/s. Patients were randomly allocated to group F (with full spinal flexion) or group N (the hips and back straightened). After maintaining the lateral position for 15 minutes with or without spinal flexion, patients were gently returned to the supine position. Spinal blockade was assessed by loss of pinprick sensation and the modified Bromage motor scale.

RESULTS: While the lateral position was maintained, sensory block was noted on the nondependent side in 14 of 16 patients in group N (87.5%) but only in 1 of 16 patients in group F (6.3%) (P < 0.001). The median level of sensory block in group N was L5 on the nondependent side just before turning to the supine position. When patients were returned to the supine position, sensory blockade on the nondependent side was noted in all group N patients (100%) and 15 group F patients (93.7%). The sensory level on the nondependent side between group N and group F were similar after turning supine.

CONCLUSIONS: Strict unilateral sensory block was not achieved even after lateral decubitus positioning with spinal flexion, when 8 mg hyperbaric bupivacaine was administered manually at a conventionally slow rate through a beveled spinal needle. However, maintaining flexion of the spinal column during lateral decubitus positioning altered the initial onset of sensory block with respect to laterality.

From the *Department of Anesthesiology and Pain Medicine, Seoul National University Hospital; Department of Anesthesiology and Pain Medicine, Samsung Medical Center; and Department of radiology, Seoul National University Hospital, Seoul, Korea.

Accepted for publication June 12, 2013.

Published ahead of print September 10, 2013

Funding: None.

The authors declare no conflicts of interest.

Reprints will not be available from the authors.

Address correspondence to Jae-Hyon Bahk, MD, PhD, Department of Anesthesiology and Pain Medicine, Seoul National University Hospital, Seoul National University College of Medicine, 101 Daehak-ro, Jongno-gu, Seoul 110–744, Korea. Address e-mail to bahkjh@snu.ac.kr.

In ambulatory or geriatric limb surgery, unilateral spinal block may result in a more stable hemodynamic profile.1,2 Unilateral spinal block has been attempted with a low dose of local anesthetics of hyper- or hypobaricity, directional pencil-point needles, slow rate of injection of local anesthetics, and maintaining the lateral decubitus position for varying periods of time.3–8

For unilateral blockade, patients are placed in the lateral decubitus position until blockade is established. Therefore, it is possible that, in the lateral position, the cauda equina on the nondependent side may be less affected with hyperbaric local anesthetics and vice versa. The cauda equina has considerable mobility in the cerebrospinal fluid. Spinal flexion (knee-chest position) in the supine position dynamically repositions the cauda equina from a dorsal-dependent area to a ventral-nondependent area by tightening nerves along the vertebral column.9 With straightening of the vertebral column in the lateral position, the cauda equina sinks by gravity and moves to the dependent side.10 With flexion of the vertebral column in the lateral position, the tightened cauda equina would move toward the nondependent side and rest in the central part of the intrathecal sac. However, there is little information in the literature about the effect of spinal posture during lateral decubitus positioning on the unilaterality of spinal block.

We hypothesized that, in the lateral position, patterns of unilateral spinal block would be affected by the relative position of the cauda equina to the layer of local anesthetic solution in the subarachnoid space, which can be changed by degree of spinal flexion and baricity of local anesthetics. This study assessed whether full flexion of the spinal column would facilitate unilaterality of spinal block.

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METHODS

After IRB approval, written informed consent was obtained from 36 male patients aged 20 to 42 years, ASA physical status I to II, and scheduled for elective knee arthroscopy under spinal anesthesia. Before the dural puncture, approximately 5 mL/kg lactated Ringer’s solution was administered without premedication. Patient monitoring included automated noninvasive arterial blood pressure, electrocardiogram, and a pulse oximeter.

The patients were placed in the lateral decubitus knee-chest position with the operating side dependent. Dural puncture was performed at the L3-4 interspace using a 25-gauge Quincke needle (Hakko Co. Ltd., Chikuma, Japan) with the midline approach. After free flow of cerebrospinal fluid, the bevel of the spinal needle was turned toward the dependent side, and 8 mg of 0.5% heavy bupivacaine (Marcaine®; AstraZeneca, Södertälje, Sweden) was administered over 80 seconds (0.2 mL per 10 seconds) without further aspiration. After intrathecal injection, patients in group F maintained the lateral decubitus position with the hips and back fully flexed for 15 minutes, while patients in group N were allowed to straighten their flexed hips and back to obtain the normal lordotic curvature of the spinal column. In both groups, the spinal column was maintained horizontally by tilting or by adding cushions under the legs or shoulder in the lateral decubitus position. The long axis of the spine as seen by the spinous processes was used to correlate the position completely horizontal. After 15 minutes in lateral position, the patients were gently turned to the horizontal supine position with the legs straightened.

The sensory level was assessed on both dependent and nondependent sides in the midclavicular line by pinprick (20-gauge hypodermic needle). Motor blockade was checked using the modified Bromage scale (0 = free movement of legs and feet, 1 = unable to raise the extended legs but able to flex knees with free movement of feet, 2 = unable to flex knees but with free movement of feet, 3 = unable to move legs or feet). Sensory and motor blockade were evaluated every 5 minutes for 20 minutes after intrathecal injection and at 10-minute intervals until 60 minutes, and then every 15 minutes until 210 minutes. After patients were returned to the supine position, an investigator blinded to patient group recorded the spread of sensory and motor block.

A strict unilateral sensory block was defined as analgesia of only the dependent side, while the nondependent side maintained complete somatic sensibility to superficial pain to pinprick. A strict unilateral motor block was defined as a motor block grade 3 on the dependent side in the absence of motor block on the nondependent side. Time to maximal sensory block level, time to 2-segment regression, duration of motor block, and time to complete resolution of pinprick sensation block on the nondependent side were recorded. Hemodynamic variables were recorded for 30 minutes. Postdural puncture headache, low back pain, and dysesthesia in the legs were also recorded for 2 postoperative days.

One volunteer (age, 33 years; height, 167 cm; weight, 67 kg) was studied using T2-weighted magnetic resonance imaging (GE, Milwaukee, WI) to compare the structural change of the cauda equina in the 2 different study positions. A multiecho sequence with a repetition time of 3600 milliseconds and echo time of 95 milliseconds was used, with a 230 × 384 acquisition matrix. Magnetic resonance images were obtained twice from the same volunteer in the lateral decubitus position with the hips and back straightened and with the hips and back fully flexed.

The calculation of the required sample size was based on the mean and standard deviation (SD) of time for the complete regression of pinprick spinal analgesia on the nondependent legs in our pilot study. The SD was approximately 30 minutes and is similar to previous studies.4,11 Sixteen patients per group were required to detect a 30-minute difference in time for complete regression of spinal block in the nondependent legs with an expected effect size to SD ratio of 1.0, and an α error of 0.05 and a β error of 0.2.

For the purpose of statistical analysis, each dermatomal spinal level was assigned a segmental number such as S5 = 1, S1 = 5, L1 = 10, T8 = 15, T3 = 20, and C6 = 25. Statistical analysis was performed using SPSS version 11.5 (SPSS Inc., Chicago, IL). Data distributions for continuous variables were first evaluated by the Kolmogorov–Smirnov test. If the test result P < 0.05, we considered the data as nonparametric. Patient characteristics data were analyzed by an unpaired t test. Differences in spinal blockade on each side between groups were analyzed according to time using nonparametric rank-based methods. If nonparametric rank-based methods detected any differences in spinal blockade on each side between the 2 groups according to time, the data were analyzed using Mann-Whitney U test with Bonferroni-corrected P-value to find which point was the cause of the difference between the 2 groups. Sensory block on the dependent and the nondependent sides within each group was compared using Wilcoxon signed rank test with Bonferroni-corrected P-value for 13 times multiple comparisons. The changes in mean arterial blood pressure and heart rate were analyzed as secondary end points using analysis of variance for repeated measures with Scheffe test for multiple comparisons. Categorical variables were analyzed using the contingency tables analysis with Fisher exact test. Results are expressed as mean (±SD), number (%), median (range), or 95% or 99.6% confidence interval (CI). The 99.6% CIs for nonparametric test were calculated by Hodges–Lehmann method. Statistical analysis was performed using SPSS version 19 (SPSS Inc.) and R program (nparLD package, R Foundation for Statistical Computing, Vienna, Austria). A value of P < 0.05 was considered significant.

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RESULTS

The enrollment, randomization, and analysis process are summarized in Figure 1. All analyzed patients successfully received operations without further analgesics. No difference in age (23 ± 5 vs 23 ± 4 years), height (175 ± 5 vs 175 ± 5 cm), and weight (71 ± 8 vs 70 ± 8 kg) was observed between groups F and N.

Figure 1

Figure 1

Figure 2 shows the evolution of sensory block level on both dependent and nondependent sides between groups. The sensory level on the nondependent side was lower than those on the dependent side in both groups throughout the study. The sensory levels on the dependent side over time were similar between groups. However, the sensory levels on the nondependent side were different between the groups 5, 10, and 15 minutes after intrathecal injection with the 99.6% CI of the difference in the sensory levels of 0 to 5, 0 to 6, and 4 to 7 dermatomal levels, at these times, respectively. The nondependent side was not blocked in group F, while the patient was maintained in the lateral decubitus position. However, the median level of sensory block in group N was L5 on the nondependent side at 15 minutes. After turning to the supine position, the sensory levels on the nondependent side became similar between the 2 groups. The 99.6% CI of the difference in the sensory block level was −6 to 7 dermatome levels at 20 minutes.

Figure 2

Figure 2

A strict unilaterality of sensory block during the lateral decubitus position was higher in group F (87.5%) than group N (6.3%) (P < 0.001). However, the strict unilaterality of sensory block 50 minutes after intrathecal injection was similarly low in group F (6.3%) and in group N (0%). All patients had complete motor block on the dependent side and no motor block on the nondependent side at 15 minutes. There was no difference in the motor score on both dependent and nondependent sides according to time between groups. The range of 99.6% CI of the difference in motor block on each side was within −1 to 1.

The rates of strict unilateral motor block 15 and 50 minutes after spinal injection were similar between groups. Time to complete resolution of pinprick sensation block in the nondependent side was at least 2 minutes faster in group F than group N (P = 0.046) (Table 1). No difference in mean arterial blood pressure and heart rate was found between groups. None of the patients required treatment for hypotension and bradycardia. There was no postdural puncture headache, low back pain, and dysesthesia in the legs for 2 postoperative days. The cauda equina of the 1 volunteer used to compare structural changes was located on the dependent side of the subarachnoid space during lateral decubitus positioning with the hips and back straightened. Flexion of the hips and back repositioned the cauda equina in the middle of the subarachnoid space (Fig. 3).

Table 1

Table 1

Figure 3

Figure 3

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DISCUSSION

This study shows that, during spinal anesthesia with hyperbaric bupivacaine in the lateral decubitus position, flexion of the spinal column increases the likelihood of a strict unilateral sensory block (87.5% vs 6.3%). However, the difference in sensory block between the 2 groups we studied disappeared when the patient was placed supine.

We hypothesized that flexion of the spinal column would result in a high rate of unilaterality even with a Quincke needle. Although a directional needle such as the Whitacre needle would increase the rate of strict unilateral block, we hypothesized that drug administration through a Quincke needle at a slow rate could achieve unilateral block in some patients. In a previous study using a Quincke needle (injection speed 1 mL/min similar to our study), strict unilateral block was achieved in 25% of patients.12 In another study using a Quincke needle, 8 mg of 0.5% bupivacaine was administered over 30 seconds, which was faster than our study and resulted in 13% of patients with strict unilateral block.8 Therefore, we assumed that the incidence of strict unilateral block would occur in at least 20% of patients and that this rate could be increased by full flexion of the spinal column. This assumption was correct only when patients remained in the lateral decubitus position.

Several factors contribute to a strict unilateral spinal block. The injected dose, baricity, volume of local anesthetic, infusion rate, period of lateral decubitus position, and spinal needle type can affect the unilaterality. Although we used a Quincke needle because of availability in our institution, a directional spinal needle such as a Whitacre needle would be preferred compared with a Quincke needle. When 6 mg of 0.5% hyperbaric bupivacaine was injected through a Whitacre needle over 10 seconds into patients in the lateral position for 20 minutes, unilateral block was observed in 83% of patients.13 In addition, when 8 mg of 0.5% hyperbaric bupivacaine was injected over 30 seconds into patients placed in the lateral position for 15 minutes, the Whitacre needle provided a more marked differential sensory block (66% vs 13%) between dependent and nondependent sides when compared with a Quincke needle.8 In previous studies, only 1 unilateral sensory block of 120 cases was observed when 12 mg of 0.5% hyperbaric tetracaine (2.5 mL) was injected at a rate of 0.25 mL/s through a Quincke needle.14,15 Likewise, a slow infusion of local anesthetic compared with faster administration will often lead to unilateral block.6 When 8 mg of 0.5% hyperbaric bupivacaine was injected at a rate of 1 mL/min using an infusion pump through 29-gauge Quincke needle into patients in the lateral position for 20 minutes, the incidence of sympathetic, motor, and sensory unilateral blockade was 69%, 77%, and 28%, respectively.12

In previous reports, there was no discussion of patient posture with regards to spinal flexion during unilateral spinal anesthesia. According to our results, patient position with the hips and back straightened during lateral decubitus positioning decreased the rate of complete unilateral spinal anesthesia. Unilateral spinal anesthesia may be facilitated by maintaining full flexion of the spinal column during lateral decubitus positioning, and this may be one of the factors which contributes to the unilaterality of spinal anesthesia.

Compared with sensory block, strict unilateral motor block was maintained even with the spinal column straightened during lateral decubitus positioning. We believe this may have been due to the local anesthetic concentration in the cerebrospinal fluid and the characteristics of motor nerve. The concentration of the upper layer of bupivacaine is not so dense as to achieve nondependent side motor block. To maximize the unilaterality of spinal anesthesia, sustaining the lateral decubitus position is advised during the surgical procedure. If hypobaric local anesthetics were used, the lateral decubitus position could be maintained in some orthopedic procedures. However, with hypobaric local anesthetics, the influence of posture of the spinal column on unilaterality and the pattern of spinal anesthesia may be different than that with hyperbaric solutions. Spinal flexion during lateral decubitus positioning would bring the cord and cauda equina closer to the “layer” of hypobaric local anesthetic, perhaps decreasing the likelihood of unilateral block. Further study is needed regarding this theory.

In conclusion, when 8 mg hyperbaric bupivacaine was administered manually at a slow rate through a Quincke spinal needle, clinically significant unilateral spinal sensory block was difficult to achieve. However, maintaining flexion of the spinal column increases the likelihood of unilateral spinal block compared with straightening the spinal column during lateral decubitus positioning.

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DISCLOSURES

Name: Jin-Tae Kim, MD, PhD.

Contribution: This author helped design and conduct the study, analyze the data, and write the manuscript.

Attestation: Jin-Tae Kim has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.

Name: Jong-Hwan Lee, MD, PhD.

Contribution: This author helped analyze the data and write the manuscript.

Attestation: Jong-Hwan Lee has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.

Name: Chan-Woo Cho, MD.

Contribution: This author helped analyze the data and write the manuscript.

Attestation: Chan-Woo Cho has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.

Name: Hyo-Cheol Kim, MD, PhD.

Contribution: This author helped analyze the data and conduct the study.

Attestation: Hyo-Cheol Kim has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.

Name: Jae-Hyon Bahk, MD, PhD.

Contribution: This author helped design and conduct the study, analyze the data, and write the manuscript.

Attestation: Jae-Hyon Bahk has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.

This manuscript was handled by: Terese T. Horlocker, MD.

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ACKNOWLEDGMENTS

Statistical analysis was supported by medical research collaborating center in Seoul National University Hospital.

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REFERENCES

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