Ultrasound Guided Estimation of Skin to Subarachnoid Space Depth in Patients Scheduled for Elective Surgeries under Subarachnoid Block.

Background Subarachnoid block is one of the commonly used techniques of regional anesthesia and accurate placement of spinal needle is crucial. A conventional spinal needle may be too long for a lean patient or too short in obese patients leading to multiple attempts, inadvertent nerve injuries and patient discomfort. So a pre-procedural estimation of the skin to subarachnoid space depth may be beneficial. Objective To estimate the skin to subarachnoid space depth using ultrasound and correlate it with the length of spinal needle to be inserted during subarachnoid block. Method This was a prospective, observational study conducted at Bir Hospital, Kathmandu in patients undergoing elective surgeries under subarachnoid block. A pre-procedural ultrasound of lumbo-sacral spine using 2-5 Hz curvilinear probe was done to measure skin to subarachnoid space depth (SSD) at the level of L3-L4 interspace. Then under all aseptic precautions, subarachnoid block was performed and the length of spinal needle outside the skin was measured and that length was subtracted from the standard length of needle to get the inserted length of spinal needle. These two measurements were compared. Result In the fifty patients included in the study, ultrasound estimated skin to subarachnoid space depth was found to be 4.24 ± 0.48 cm and the inserted length of spinal needle was 4.24 ± 0.46 cm. A significant correlation r=0.96 (p < 0.05) was found between the two measurements in the study population. Conclusion Ultrasound estimated skin to subarachnoid depth in the study population was found to be 4.24 ± 0.48 cm which correlated with the inserted length of spinal needle. So, use of ultrasound can be very helpful in performing subarachnoid block.