Correlation between depth of subarachnoid space in the lumbar region estimated by Stocker’s formula, ultrasound, and depth of needle insertion: A prospective observational study

BACKGROUND AND AIMS: The main challenge in administration of spinal anesthesia in patients is the unpredictability of the exact skin to subarachnoid space depth (SSD). Approximation of SSD and needle length comes at the expense of patient comfort and multiple attempts increases complications. Our study aimed to evaluate the validity of Stocker’s formula to estimate SSD in comparison to ultrasonography and depth of needle insertion. We also aimed to determine an equation to describe the relationship between patient weight and SSD in an Indian population. MATERIAL AND METHODS: This was a prospective observational study. A total of 234 adults of American Society of Anesthesiologists (ASA) category 1 and 2 were selected for the study. The patients underwent spinal anaesthesia and SSD data were determined using weight-based Stocker’s formula (DS), ultrasonography (DU), and actual depth of needle insertion (DA). Correlation analysis was performed to determine variables that can predict the SSD. A linear regression was calculated to describe the relationship between patient weight and SSD. SSD data were determined using weight-based Stocker’s formula ultrasonography, and actual depth of needle insertion. RESULTS: Mean SSD using Stocker’s formula, ultrasonography, and actual depth of needle insertion was 4.92 ± 0.6 cm, 4.47 ± 0.6 cm, and 4.81 ± 0.6 cm. Our study showed a significant correlation between Stocker’s formula and SSD measured by actual depth of needle insertion (R(2) = 0.376). This was described by the derived formula: SSD (cm) =2.522+ [0.031 × weight (kg)]. CONCLUSION: SSD correlates well with the weight of adult patients. Measurement with Stocker’s formula and actual depth showed better correlation than Stocker’s formula and ultrasonography. This is valuable in resource-poor areas with a lack of ultrasound machines or expertise in training.