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From the Department of Anesthesiology, Feinberg School of Medicine, Northwestern University, Chicago, Illinois, USA.
Address correspondence to: Dr. Cynthia A. Wong, Department of Anesthesiology, Feinberg School of Medicine at Northwestern University, 251 E. Huron St., Feinberg 5-704, Chicago, IL 60611, USA. Phone: 312–926–7632; Fax: 312–926–7633; E-mail: c-wong2{at}northwestern.edu
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Abstract |
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Methods: A prospective observational study in 245 term parturients who received intrathecal fentanyl and bupivacaine plus an epidural test dose to initiate labour analgesia at an academic university hospital. The highest sensory blockade to ice and pinprick was determined at 15 and 30 min after the intrathecal injection. Correlations between sensory blockade and parturient height, weight and body mass index (BMI) were determined.
Results: There was no association between highest sensory blockade and parturients’ height. Increasing weight and BMI were associated with increased cephalad sensory blockade at 15 min, but not at 30 min. The estimated difference in sensory level between women at the extremes of BMI, based on our linear regression model, was less than one dermatome.
Conclusion: Height did not influence the extent of sensory analgesia after initiation of intrathecal labour analgesia using a hypobaric solution injected with the parturient in the sitting position. Weight and BMI were associated with a non-clinically significant increase in the cephalic spread of analgesia, suggesting that dose adjustments based on body habitus in this population are not necessary.
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Introduction |
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The effect of body habitus on the dermatome spread of sensory analgesia after the intrathecal injection of hypobaric analgesic solutions has not been studied. We hypothesized that height, weight and BMI do not influence cephalad sensory spread in parturients who receive an intrathecal injection of a hypobaric solution in the sitting position. The primary purpose of this study was to determine whether the cephalad sensory level after the intrathecal injection of a hypobaric solution of bupivacaine and fentanyl injected in the sitting position is influenced by height, weight, or BMI. A secondary objective was to determine if lateral positioning after the intrathecal injection influenced the sensory level.
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Methods |
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Dermatomal level sensory testing was performed in the left and right midclavicular line using ice and an 18-gauge needle at 15 and 30 min after the intrathecal injection. Testing started at the T12 level and progressed cephalad until the parturient first noted the sensation of cold or pinprick. If there was a difference in sensory level between the right and left sides, the higher level was used for data analysis. Maintenance epidural analgesia infusion was begun following the 30-min assessment.
The density of the fentanyl-bupivacaine solution was measured in 12 randomly selected samples. Density was determined gravimetrically using a 2-mL calibrated pycnometer and a precision balance (Mettle, Inc., Toledo, OH, USA) at 37°C. Baricity was calculated by dividing the measured density of the solution by the density of CSF of a term parturient at 37°C.6
The sample size calculated for this study (n = 255) was determined to achieve 90% power to detect a difference of -0.2 between the null hypothesis correlation of zero and the alternate hypothesis correlation of 0.2 using a two-sided hypothesis test with a significance level of 0.05. This level of association between the primary outcome variable (cephalad sensory spread) and height, weight, and BMI was similar to that observed in a study of the influence of body habitus and sensory spread after the intrathecal injection of isobaric bupivacaine.7 Pearson’s correlations and linear regression were used to determine the association between height, weight, BMI and the cephalad sensory level. The right vs left sensory levels were compared using the Wilcoxon signed ranks test. The sensory level of parturients placed in the left vs right lateral position was compared with the Mann-Whitney U test. A P < 0.05 was required to reject the null hypothesis.
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Results |
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. All parturients had complete analgesia (verbal rating pain score 
1) during the 30-min study period. There was no difference in sensory level to cold and pinprick between the right and left sides. In no study subject was the sensory level difference between the right and left sides more than one dermatome.
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and 2). No correlations or differences were found at 30-min. There were no differences in the sensory level to ice or pinprick between parturients placed in the right or left lateral position at 15 or 30 min.
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Discussion |
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The important findings of this study are the lack of clinically significant association of body habitus and the cephalad sensory level after intrathecal injection of a hypobaric solution for the initiation of labour analgesia in the sitting position. Although increased weight and BMI were associated with a higher sensory level, the estimated difference in sensory level between women at the extremes of BMI, based on our linear regression model, was less than one dermatome. This difference is unlikely to be clinically significant and suggests that dose adjustments are not necessary based on BMI. Similar to the results of the current study, several studies in non-pregnant adults found increased BMI was associated with a higher cephalad sensory level when local anesthetic solutions were administered at doses necessary for surgical anesthesia.7–11
There was a large variability in cephalad sensory levels following intrathecal injection of a hypobaric solution in the current study, similar to studies of isobaric and hyperbaric solutions. There have been several small series of case reports in which parturients had sensory blockade of cranial nerves after initiation of CSE analgesia.4,5 Only one (weight = 104 kg) of five parturients reported by Hamilton5 and none of the two parturients reported by Abu Abdou4 were obese. This supports the conclusion that height and weight are not predictors of parturients at risk for unusually high cephalad spread of sensory blockade and suggests that factors other than body habitus determine the spread of sensory analgesia. For example, Carpenter et al. found that lumbosacral CSF volume predicts the extent of the sensory blockade after hyperbaric lidocaine.12 A possible mechanism for the higher spread of sensory block in parturients with increased BMI’s is that increased abdominal mass causes epidural venous engorgement, leading to impingement of the dura upon the subarachnoid space, thus decreasing the lumbosacral CSF volume.13
A limitation of the current study is that the time spent by the parturient in the sitting position was within a two-minute range, but was not fixed. Although this difference may have influenced the cephalad spread of the hypobaric solution, this time frame mimics the clinical situation, since the epidural catheter is inserted and secured prior to placing the parturient in the lateral position. An additional limitation was the administration of an epidural test dose, since volume in the epidural space may influence spread of intrathecally-injected drugs by mechanically displacing CSF.14
In conclusion, height did not influence the extent of sensory analgesia after initiation of CSE labour analgesia using a hypobaric solution injected with the parturient in the sitting position. Weight and BMI were associated with a non-clinically significant increase in the cephalic spread of analgesia, suggesting that dose adjustments based on body habitus in this population are not necessary, and may not prevent the occasional blockade of cranial nerves.
Revision received April 30, 2003. Accepted for publication February 25, 2003.
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References |
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2 Norris MC. Patient variables and the subarachnoid spread of hyperbaric bupivacaine in the term parturient. Anesthesiology 1990; 72: 478–82.[Medline]
3 Hartwell BL, Aglio LS, Hauch MA, Datta S. Vertebral column length and spread of hyperbaric subarachnoid bupivacaine in the term parturient. Reg Anesth 1991; 16: 17–9.[Medline]
4 Abu Abdou W, Aveline C, Bonnet F. Two additional cases of excessive extension of sensory blockade after intrathecal sufentanil for labor analgesia. Int J Obstet Anesth 2000; 9: 48–50.
5 Hamilton CL, Cohen SE. High sensory block after intrathecal sufentanil for labor analgesia. Anesthesiology 1995; 83: 1118–21.[Medline]
6 Richardson MG, Wissler RN. Density of lumbar cerebrospinal fluid in pregnant and nonpregnant humans. Anesthesiology 1996; 85: 326–30.[Medline]
7 Pargger H, Hampl KF, Aeschbach A, Paganoni R, Schneider MC. Combined effect of patient variables on sensory level after spinal 0.5% plain bupivacaine. Acta Anaesthesiol Scand 1998; 42: 430–4.[Medline]
8 Moore DC. Factors influencing spinal anesthesia. Reg Anesth 1982; 7: 20–5.
9 McCulloch WJ, Littlewood DG. Influence of obesity on spinal analgesia with isobaric 0.5% bupivacaine. Br J Anaesth 1986; 58: 610–4.
10 Taivainen T, Tuominen M, Rosenberg PH. Influence of obesity on the spread of spinal analgesia after injection of plain 0.5% bupivacaine at the L3–4 or L4–5 interspace. Br J Anaesth 1990; 64: 542–6.
11 Pitkanen MT. Body mass and spread of spinal anesthesia with bupivacaine. Anesth Analg 1987; 66: 127–31.[Medline]
12 Carpenter RL, Hogan QH, Liu SS, Crane B. Lumbosacral cerebrospinal fluid volume is the primary determinant of sensory block extent and duration during spinal anesthesia. Anesthesiology 1998; 89: 24–9.[Medline]
13 Bridenbaugh PO, Greene NM, Brull SJ. Spinal (subarachnoid) neural blockade. In: Cousin MJ, Bridenbaugh PO (Eds.). Neural Blockade in Clinical Anesthesia and Management of Pain, 3rd ed. Philadelphia: Lippincott-Raven; 1998: 203–41.
14 Takiguchi T, Okano T, Egawa H, Okubo Y, Saito K, Kitajima T. The effect of epidural saline injection on analgesic level during combined spinal and epidural anesthesia assessed clinically and myelographically. Anesth Analg 1997; 85: 1097–100.[Abstract]
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