The spread of spinal anesthesia is highly unpredictable,1,2 and it is hard to predict the resulting sensory level from a given dose of local anesthetic. It is generally accepted that spinal anesthesia spread is affected by a patient’s general characteristics, cerebrospinal fluid characteristics, local anesthetic characteristics, and clinical technique3 with lumbosacral cerebrospinal fluid volume and pressure being major determinants of spinal anesthesia spread.4–6 Because it is not practical to determine lumbosacral cerebrospinal fluid volume and pressure measurements to predict spinal anesthesia spread, patient characteristics such as height, weight, and body mass index are frequently used to predict spinal anesthesia spread, with unsatisfactory results.7–9
Plain bupivacaine is often used for spinal anesthesia.8,10,11 In clinical practice, a greater cephalad spread is often observed in patients with increased abdominal girth and short stature after 3 mL of 0.5% plain bupivacaine is administered subarachnoidally for spinal anesthesia. Thus, we hypothesized that, among general patient characteristics, abdominal girth and vertebral column length could play a role in cephalad spread after subarachnoidally administered plain bupivacaine and help to improve the predictability of spinal anesthesia on an individual basis.
After receiving approval from the Ethical Committee of the Second Affiliated Hospital of Jiaxing University in February 2011 (CZJE201109) and written informed consent by the participants, 115 patients (age, 19 to 55 years; weight, 45 to 99 kg; height, 145 to 181 cm) were enrolled in this study from March 2011 to December 2012. All ASA physical status I to II patients who were scheduled for lower limb orthopedic surgery amenable to spinal anesthesia and who consented for the study were included. Patients with severe cardiovascular disease, contraindication for spinal anesthesia, spinal canal stenosis, and history of intraspinal anesthesia or spinal surgery were excluded from the study, as were patients older than 55 years and pregnant patients.
Patients fasted for 10 hours and were orally pretreated with 5 mg midazolam 30 minutes before entering the operating room. The standard American Society of Anesthesiologists monitors and IV access were established. Patients were placed supine on a horizontal operating table, and the abdominal girth was measured at the level of umbilicus during the end of expiration. During spinal anesthesia, all patients were placed in the right lateral decubitus position with their legs, back, and neck flexed. Before their backs were flexed, the vertebral column length was measured from C7 vertebra to the sacral hiatus (C7-SH). The C7 vertebra and sacral hiatus were both confirmed by radiographic image. After the L3-L4 interspace was confirmed by radiographic image, a 25-gauge Quincke needle was inserted using a midline approach with the bevel cephalad. When free flow of cerebrospinal fluid was obtained, 3 mL of 0.5% plain bupivacaine was injected into the subarachnoid space over 15 seconds. After the procedure, the patients were placed supine on the operating table.
The cephalad spread of spinal anesthesia was assessed in both midclavicular lines using ice for loss of temperature sensation and an 18-gauge needle for loss of pinprick discrimination 30 minutes after intrathecal injection. The testing proceeded from the anesthetized area to the unanesthetized area. Ice was applied for approximately 2 seconds before a negative determination was reached. Three pin touches were used at each level for loss of pinprick discrimination. If a discrepancy in sensory level was found between the left and right sides, the patient would be excluded from further study. The total number of spinal anesthesia block segments was recorded from the fifth sacral vertebra to the segment of spinal anesthesia spread. Surgery was started after the assessment, and general anesthesia was induced if spinal administration failed to produce surgical anesthesia.
If hypotension (value <90 mm·Hg or a systolic blood pressure decrease >30%) occurred, 5 mg ephedrine was administered IV; if bradycardia (heart rate <55 bpm) occurred, 0.5 mg atropine was administered IV. The same anesthesiologist completed all anesthesia procedures, and the assessment of the spread of spinal anesthesia was completed by another anesthesiologist who was unaware of the previously measured characteristics.
Five spinal anesthesia spread predictors were used in this study. By convention, an anticipated effect size of 0.15 was considered medium and suitable.a , 12 A minimum required sample size of 91 was needed to detect an anticipated effect size of 0.15, and desired statistical power level of 0.8 at a probability level was 0.05.1 All data were analyzed with SPSS 19.0 software (SPSS Inc., Chicago, IL). A linear regression analysis was used to determine the correlation of the spinal anesthesia-induced loss of temperature sensation and pinprick discrimination with age, weight, height, body mass index, abdominal girth, and vertebral column length. The point estimates of the correlation coefficients of abdominal girth and cephalad spread of spinal anesthesia were larger than they were between body mass index and cephalad spread of spinal anesthesia. Consequently, abdominal girth was used in subsequent analyses rather than body mass index. The combined linear contribution of patient age, weight, height, vertebral length, and abdominal girth was tested by multiple regression analysis; R2 is the multiple correlation coefficient of determination. The age, weight, height, vertebral column length, and abdominal girth were initially included in the analysis as potential predictors, and the predictors that did not significantly correlate with the dependent variable were removed from the analysis. A P value <0.05 was considered statistically significant.
One patient was excluded from this study because of our failure to perform a spinal at the level of the L3-L4 interspace. The remaining 114 patients were included in the study, and their characteristics are summarized in Table 1. Linear regression analysis showed that there was a significant univariate correlation between height, weight, body mass index, abdominal girth, and vertebral column length with loss of temperature sensation and pinprick discrimination (all P < 0.017) as well as age and loss of pinprick discrimination (P < 0.039) (Table 2; Figs. 1 and 2).
Multiple linear regression analysis between age, weight, height, vertebral column length, abdominal girth, and cephalad spread of spinal anesthesia showed that abdominal girth and vertebral column length were the key determinants of the cephalad spread of spinal anesthesia (both P < 0.0001). Multiple linear regression analysis to determine the contribution of independent predictors showed that age, weight, and height could be omitted without changing the results (all P > 0.059, all 95% confidence limits < 0.372) (Table 3). The coefficient of determination (R2) was 0.805 for the loss of temperature sensation and 0.792 for loss of pinprick discrimination.
In this study, we investigated the combined effect of patient characteristics on the spread of spinal anesthesia. The 5 patient characteristics we included in our study (age, weight, height, abdominal girth, and vertebral column length) yielded a coefficient of determination of approximately 0.8, indicating that these characteristics could explain the variability of spinal anesthesia spread to a large extent. Among the 5 patient characteristics, abdominal girth and vertebral column length were the 2 main determinants of spinal anesthesia spread. There was a positive correlation between abdominal girth and cephalad spread, and an inverse relationship between vertebral column length and cephalad spread. Thus, these findings suggest that patients with greater abdominal girth and shorter vertebral column length would require less local anesthetic to get an expected block level.
Our study found a strong positive correlation between abdominal girth and the cephalad spread of spinal anesthesia. When patients changed position from right lateral decubitus to supine after the spinal anesthesia procedure, intra-abdominal pressure and epidural space pressure increased.13 Larger abdominal girth results in a more significant increase in intra-abdominal pressure. An increase in intra-abdominal pressure, which would cause the lumbosacral cerebrospinal fluid volume to decrease and cephalad spread of bupivacaine mixed solution, could result in increased spread of spinal anesthesia.5 A probable mechanism of decreased lumbosacral cerebrospinal fluid volume due to increased intra-abdominal pressure could be inward movement of soft tissue in the intervertebral foramen, which would displace lumbosacral cerebrospinal fluid.5,14 Two studies have shown that in pregnant patients, the volume of epidural veins increases, and caval compression causes further epidural vein engorgement.15,16 Based on these studies, another probable mechanism of decreased lumbosacral cerebrospinal fluid volume due to increased intra-abdominal pressure is epidural venous engorgement, which leads to shrinkage of the dura on the subarachnoid space. Thus, abdominal girth is an easily obtainable surrogate measurement of lumbosacral cerebrospinal fluid volume that can usually predict spinal anesthesia spread.
In our study, we found that vertebral column length was the other main relevant factor to the spread of spinal anesthesia. Logic might suggest that there would be less cephalad spread of spinal anesthesia for a fixed amount of local anesthetic in taller patients. Indeed, a study has shown that vertebral column length is significantly correlated with spinal anesthesia spread.17 However, Pargger et al.7 found that vertebral column length does not contribute to peak sensory block level. The difference in our studies may be associated with differences patient age in these studies. In the latter study, the mean patient age was 69.6 years, and age-related vertebral column degenerative changes may have affected the results. In our study, patients older than 55 years were excluded to reduce the potential impact of vertebral degeneration in elderly patients. This might be a limitation of the present study.
In the current study, compared with body mass index, a stronger positive relationship between abdominal girth and cephalad spread of spinal anesthesia was found. This is likely because lumbosacral cerebrospinal fluid volume is the primary determinant of sensory block extent during spinal anesthesia,6 and increased intra-abdominal pressure can decrease lumbosacral cerebrospinal fluid volume.5 Patients with increased body mass index generally, but not always, have a greater abdominal girth, as body shapes differ among individuals. This variability likely explains the disparate results of studies investigating the correlation between body mass index and the level of spinal anesthesia.8,11,18 A subsequent further study using 3-dimensional magnetic resonance imaging also found that cerebrospinal fluid volume and body mass index had only a low inverse relationship.19
Hogan et al.5 found that magnetic resonance examination can estimate the pars lumbalis cerebrospinal fluid volume. 19 Further studies should be performed to ascertain the correlation between pars lumbalis cerebrospinal fluid volume and abdominal girth to better illustrate the ability of abdominal girth to predict spinal anesthesia spread.
In conclusion, the combination of the patient’s age, weight, height, abdominal girth, and vertebral column length have a high predictive value for the spread of spinal anesthesia with plain bupivacaine. Among these general patient characteristics, the predictive value of abdominal girth and vertebral column length are especially accurate. Patients with larger abdominal girth and shorter vertebral column length have greater cephalad spread after a dose of plain bupivacaine.
Name: Qing-he Zhou, MD.
Contribution: This author helped conduct the study and write the manuscript.
Attestation: Qing-he Zhou 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: Wang-pin Xiao, MD.
Contribution: This author helped design the study and write the manuscript.
Attestation: Wang-pin Xiao 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: Ying-yan Shen, MD.
Contribution: This author helped analyze the data.
Attestation: Ying-yan Shen 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.
We would like to thank Er-dan An and Hong-li Zhang for their assistance with the study.
a Available at: http://www.danielsoper.com/statcalc3/calc.aspx?id=1. Accessed September 26, 2013.
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