Evaluation of Effect of Hip/Shoulder-Width Ratio on the Sensory Level of Spinal Anesthesia - A Prospective Observational Study.

Background: Certain anthropometric measurements that are practically obtainable explain the variability in the spread of spinal anesthesia. These are useful for quick assessment of the spread of spinal anesthesia to avoid the risk of high block and also the inadequate level of block. Aims: The study aims to evaluate the effect of hip/shoulder-width ratio (HSR) on the sensory level of spinal anesthesia. Settings and Design: This prospective observational study was undertaken at a tertiary care hospital. Statistical Analysis: Pearson's correlation and multiple linear regression analyses were used to analyze the relationship between study variables with the level of sensory block. Materials and
Methods: One hundred patients undergoing various surgical procedures were enrolled for the study. With a patient in a sitting position, hip-width was measured between the two iliac crests, shoulder-width was measured between two acromion processes, and vertebral column length was noted by measuring the distance from C7 vertebra to sacral hiatus. 3 mL of 0.5% hyperbaric bupivacaine was administered intrathecally at L3-L4 with 25G Quincke's needle in the lateral position. Assessment of block level was done by loss of cold sensation and loss of pinprick sensation every 5 min till 30 min. The numbers of segments blocked were noted from the S5 segment. The relationship between various factors with the level of sensory block was analyzed by the Pearson's correlation coefficient.
Results: HSR and body mass index (BMI) have a significant correlation with the sensory level of spinal anaesthesia, HSR (r = 0.297, P < 0.05) and BMI (r =0.385, P < 0.05).
Conclusion: HSR can help predict the cephalad spread of spinal anesthesia. We can expect a higher level of sensory blockade of spinal anesthesia in females who generally have an HSR more than one. Copyright:

INTRODUCTION

Spinal anesthesia is a simple and reliable technique used in a wide variety of lower body surgeries, which produces adequate surgical conditions by injecting a small amount of local anesthetic into the intrathecal space. The first report on the clinical use of spinal anesthesia was performed in 1899 by Dr. August Bier, who described the intrathecal administration of cocaine; since then, many studies have been done to enhance our clinical practice to achieve the goal of providing a safe and adequate level of sensory blockade.[1] Selecting a correct dose of an intrathecal local anesthesia to obtain the desired block for a particular type of surgery for different patients is an important clinical issue. Certain anthropometric measurements that are practically obtainable may explain the variability in the spread of spinal anesthesia to some extent.[2] These can be useful to clinicians for quick assessment of the spread of sensory level of spinal anesthesia in individual patients to avoid the risk of high block and also an inadequate level of block. Racial and ethnic differences in hip-width which is considered one of the surrogates of body fat distribution are well established.[3] Hence, we aimed to evaluate the effect of the hip/shoulder-width ratio (HSR) on the sensory level of spinal anesthesia as there are no such studies performed on the Indian population.

MATERIALS AND METHODS

After obtaining approval from the institutional review board (EC/AP-05/03-2021) on March 19, 2021, in accordance with the Declaration of Helsinki, 1975 (as revised in 2013), this prospective study was registered in the Clinical Trials Registry of India (CTRI/2021/06/034140) and was conducted between July 2021 and October 2021. Written informed consent was obtained from all participants. 100 patients aged between 20 and 60 years, weighing between 50 and 100 kg, and height between 150 and 180 cm undergoing elective infraumbilical surgeries under subarachnoid block were included in the study. Patients with contraindication to spinal anesthesia, patients with spinal deformity, those who have undergone previous spine surgeries, and pregnant patients were excluded from the study.

The height and weight of the patient were noted during the preoperative checkup. Patients were kept nil orally for 6 h before surgery. Patients were shifted to the operation theater, and baseline vitals were recorded. An intravenous (i.v.) line was secured with 18-gauge cannula, and patients were infused with normal saline 5 mL.kg− 1 during the study period. The patient was made to sit on the horizontal table, and hip-width was measured between the two iliac crests, shoulder-width was measured between two acromion processes, and vertebral column length (VCL) was noted by measuring the distance from C7 vertebra to sacral hiatus. Both C7 and sacral hiatus were palpated and marked. An anesthetist who measured the anthropometric parameters was not involved in the assessment of the level of sensory block. Subarachnoid block was given in the lateral decubitus position; lumbar puncture was done in the midline with 25-gauge Quincke's needle bevel facing cephalad; after obtaining free flow of cerebrospinal fluid (CSF), 3 mL of 0.5% hyperbaric bupivacaine was injected intrathecal over 15 s, following which the patient was positioned supine; and vitals such as heart rate, systolic blood pressure, diastolic blood pressure, mean arterial pressure, and oxygen saturation were recorded every 5 min till 30 min. The level of anesthesia was assessed bilaterally from caudal to cephalad every 5 min till 30 min. The loss of temperature sensation was tested using ice, while the loss of pinprick sensation was tested using a 25-gauge needle. Hence, final sensory level was noted, and the total number of segments blocked was noted from the fifth sacral vertebra. If hypotension (value <90 mmHg or systolic blood pressure decrease >20%) occurred, 6 mg ephedrine was administered i.v.; if bradycardia (heart rate <50) occurred, 0.6 mg atropine was administered i.v.

Sample size calculation

A study by Canturk et al.[4] has reported that the correlation between the spread of spinal anesthesia and HSR to be 0.766 based on the findings of the study with power of 80% and α error of 5%, it was estimated that 100 subjects need to be recruited for the study assuming the population correlation coefficient value to be 0.86. The sample size has been estimated employing n Master version 2.0 developed by CMC, Vellore, Tamil Nadu, India.

Statistical methods

Statistical software SPSS (statistical package for social sciences) 22.0 and R environment version 3.2.2. were used for the analysis of the data. Descriptive and inferential statistical analysis has been carried out in the present study. Results on continuous measurements are presented as mean ± standard deviation (minimum–maximum), while results on categorical measurements are presented in number (%). Significance is assessed at a 5% level of significance. The paired t-test is used to test the null hypothesis that the average of the differences between a series of paired observations is zero. Student's t-test (two-tailed, dependent) has been used to find the significance of study parameters on a continuous scale. Pearson's correlation between the study variables is performed to find the degree of relationship. A stepwise multiple linear regression analysis was used to detect the correlation between the spread of spinal anesthesia and participants' age, weight, height, VCL, and HSR. R2 is the determination coefficient of the multiple regression equation. The P value is determined by referring to a t-distribution with n − 2 degrees of freedom.

RESULTS

All patients enrolled in the study achieved adequate sensory level of spinal anesthesia required for the surgery and were included for statistical analysis. Patient variables are summarized in Table 1. Out of the 100 patients enrolled, 61 were male and 39 were female. 52 patients had HSR >1, out of which 69.2% (36) were female and 30.8% (16) were male. All patients who had HSR >1 had maximum sensory levels greater than T6 except 3 patients who had a maximum sensory level of T8 to cold sensation and T10 to pinprick sensation. The maximum level of sensory blockade achieved to cold sensation ranged from T10 to T1, whereas that due to pinprick sensation was T12 to T1. The mean time required to reach the maximum sensory blockade to pinprick was 12.96 ± 3.98. Tables 2 and 3 show the Pearson's correlation coefficient between patient variables and sensory blockade to cold and pinprick. Tables 4 and 5 summarize the multiple linear regression, which shows that there was a significant correlation between only HSR and the sensory level of anesthesia, whereas height, BMI, and VCL showed a negative correlation, although it was not statistically significant. Figure 1 depicts the scatter plot of HSR versus maximum sensory level to cold and pinprick sensation. Only 11% of the patients required treatment with ephedrine for hypotension, whereas none of the patients needed treatment with atropine.

Table 1

Patient variables

Variables	Minimum	Maximum	Mean±SD
Age (years)	20.00	67.00	44.39±10.74
Height (cm)	143.00	177.00	161.00±5.97
Weight (kg)	44.00	92.00	66.00±10.38
BMI (kg.m−2)	18.30	36.40	25.44±3.76
VCL (cm)	27.00	61.00	54.99±4.59
Hip-width (cm)	29.00	49.50	39.68±4.95
Shoulder-width (cm)	24.00	48.00	39.23±5.88
HSR	0.09	1.76	1.00±0.26

BMI=Body mass index, HSR=Hip/shoulder-width ratio, SD=Standard deviation, VCL=Vertebral column length

Table 2

Pearson’s correlation between patient variables and maximum sensory blockade to cold sensation

Variables	r	P
Height (cm)	0.063	0.534
Weight (kg)	0.397*	<0.001
BMI (kg.m−2)	0.385†	<0.00
VCL (cm)	0.017	0.869
HSR	0.297‡	0.003

*P<0.001 correlation coefficient between weight and maximum sensory blockade to cold sensation, †P<0.001 correlation coefficient between BMI and maximum sensory blockade to cold sensation, ‡P=0.003 correlation coefficient between HSR and maximum sensory blockade to cold sensation. r: Correlation coefficient, P<0.05 is significant. BMI=Body mass index, HSR=Hip/shoulder-width ratio, VCL=Vertebral column length

Table 3

Pearson’s correlation between patient variables and maximum sensory blockade to pinprick sensation

Variables	r	P
Height (cm)	−0.011	0.915
Weight (kg)	0.366*	<0.001
BMI (kg.m−2)	0.389†	<0.001
VCL (cm)	−0.038	0.710
HSR	0.263‡	0.008

*P<0.001 correlation coefficient between weight and maximum sensory blockade to cold sensation, †P<0.001 correlation coefficient between BMI and maximum sensory blockade to cold sensation, ‡P=0.003 correlation coefficient between HSR and maximum sensory blockade to cold sensation. r: correlation coefficient, P<0.05 is significant. BMI=Body mass index, HSR=Hip/shoulder-width ratio, VCL=Vertebral column length

Table 4

Multiple linear regression of patient variables and maximum sensory blockade to cold sensation

Variable	Unstandardized coefficients		Standardized coefficients (Β)	P
	β	SE
Constant	39.974	35.383		0.261
Height	−0.197	0.222	−0.470	0.376
Weight	0.348	0.271	1.442	0.202
BMI	−0.636	0.690	−0.955	0.360
VCL	−0.009	0.059	−0.017	0.877
HSR	2.871	0.901	0.300	0.002

The regression equation between the maximum block level and its predictors was Y (max block level)=39.97–0.197 × height + 0.348 × weight - 0.636× BMI - 0.009 × VCL + 2.87s × HSR, R2: Coefficient of determination=0.26. P<0.05 is significant. β=Regression coefficient, BMI=Body mass index, HSR=Hip/shoulder-width ratio, SE=Standard error, VCL=Vertebral column length

Table 5

Multiple linear regression of patient variables and maximum sensory blockade to pinprick sensation

Variable	Unstandardized coefficients		Standardized coefficients (β)	P
	β	SE
Constant	45.504	36.121		0.211
Height	−0.228	0.227	−0.545	0.317
Weight	0.343	0.277	1.425	0.219
BMI	−0.622	0.705	−0.938	0.380
VCL	−0.017	0.060	−0.031	0.777
HSR	2.289	0.920	0.240	0.015

The regression equation between the maximum block level and its predictors was Y (max block level)=45.5–0.228 × height + 0.343 × weight - 0.622 × BMI - 0.017 × VCL + 2.289S × HSR, R2: Coefficient of determination=0.22. P<0.05 is significant. β=Regression coefficient, BMI=Body mass index, HSR=Hip/shoulder-width ratio, SE=Standard error, VCL=Vertebral column length

Figure 1

Scatter plot of HSR versus maximum sensory level to cold and pinprick. HSR = Hip/shoulder-width ratio

DISCUSSION

Body habitus may influence the magnitude of distribution of drugs within the intrathecal space which is highly inconsistent even after a constant dose of the drug is injected, and one of the explanations for this is that the individuals differ in their physical characteristics, CSF volume, and density.[5] When the local anesthesia is injected into the subarachnoid space, it will spread by displacement of CSF as a result of currents created within the CSF due to injection. In the next stage, the spread mainly depends on the densities of CSF and local anesthesia under the influence of gravity.[67] Gravity will be applied through the patient's position and in a horizontal position, by the influence of the curves of the vertebral canal.[78]

The extent of spread of the drug when an injection is made in the lateral position depends upon the inclination of the lumbar area at the time of injection.[9] On the horizontal table with the patient in a lateral position, those with greater hip-width than shoulder-width with the resultant HSR of more than one assume a relative Trendelenburg position that facilitates cephalad distribution of the drug.[10] The mean HSR in our study was 1.0 ± 0.26 whereas the maximum HSR was 1.76. 52% of patients had HSR >1, and the majority of them were females. All patients who had HSR more than one achieved a sensory level of T6 or more. On multiple regression analysis, we found that only HSR had a statistically significant predictive value on the maximum level of spinal anesthesia.

Males tend to have broader shoulders than hips, resulting in HSR of less than one, so the spinal column has a head-up tilt in the lateral position, whereas the reverse is true in females.[11] The distribution of body fat can be influenced by sex and race which thus alters the hip-width among individuals.[12] Females also tend to have more abdominal girth due to fat deposition and also wider hips than shoulders; this partly explains the higher cephalad spread in females than in males. In our study, we had a greater percentage of males compared to females, and this could explain the moderate correlation obtained in our study, whereas Canturk et al.[4] in their study on nonpregnant patients demonstrated a higher correlation between HSR and cephalad spread of spinal anesthesia. However, in our study, more than 90% of females had HSR >1 and had achieved a highest sensory level greater than T6, and on multiple regression analysis, we found that HSR was a predictor of the maximum sensory level achieved which was statistically significant.

The mean vertebral column length in our study was 54.99 ± 4.59, which showed a negative correlation with the cephalad spread of sensory level of anesthesia, although it was not statistically significant, which is similar to the study conducted by Wan Rahiza et al.[13] where no significant association was found between VCL and cephalad spread of anesthesia. However, many other studies like that of Zhou et al.[14] have shown a negative correlation between VCL and sensory blockade. Other parameters evaluated in our study such as weight and BMI have shown a positive correlation whereas height has a negative correlation with the cephalad spread of spinal anesthesia.

Most important limitation of our study is that we used palpation method to identify the landmarks to measure the anthropometric measurements and to administer intrathecal injection instead of ultrasonography. The intrathecal spread of local anesthetics within the intrathecal space to achieve the final level of sensory blockade after spinal anesthesia is influenced by numerous factors. Hence, using a single method to predict the cephalad spread of spinal anesthesia to select a reasonable dose of local anesthetic for a desired level of block is difficult.

CONCLUSION

HSR is an easily measurable patient variable which can help predict the cephalad spread of spinal anesthesia if the ratio is more than one. We can expect a higher level of sensory blockade of spinal anesthesia in females who generally have a HSR more than one.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.