An Observational Study on the Relationship of Patient’s Body Mass Index and Depth of Spinal Needle Insertion

Abstract

Background: The distance from the skin to the subarachnoid space varies at different levels of the vertebrae and from patient to patient. Knowing the distance from the skin to the subarachnoid space beforehand may help the procedure go more smoothly and ensure that the spinal needle use is of the right length.
 Methods: An observational study was conducted on 100 patients posted for infra umbilical surgeries of various specialties under spinal anaesthesia. Intraoperatively after a successful spinal with midline approach at the L3-L4 level, the length of the needle from the skin to the subarachnoid space was immediately measured and this actual length was compared with the expected length obtained using Bonadio's formula. The relationship between a patient's BMI and depth of spinal needle insertion and the reliability of Bonadio's formula to predict skin-to-subarachnoid space was studied.
 Results: From our study, a positive correlation was obtained between patients' weight, BMI, waist circumference and depth of spinal needle insertion and also between the Skin to Subarachnoid space distance (SSD) measured intraoperatively with the predictive value obtained using Bonadio's formula. The correlation between height and arm circumference was not much significant.
 Conclusion: Skin-to-subarachnoid space distance correlates with weight more than BMI. Bonadio's formula can be used to calculate the skin to subarachnoid space depth before performing spinal anaesthesia. After seeing the results, weight was the variable which had a significant correlation with Spinal Needle Depth (SND), hence we formulated two equations using weight to predict the depth of spinal needle insertion to reduce the incidence of multiple attempts during the procedure and to enhance patient comfort. The regression equation using actual values of needle depth: SND = 2.292 + [0.044 x weight] And regression equation using Bonadio's values of needle depth is: Needle depth= 2.681 + [0.037 x weight]