Ultrasound-guided peripheral regional anaesthesia: a feasibility study in obese versus normal-weight women

BACKGROUND AND OBJECTIVES: In particular, obese patients may profit from peripheral regional anaesthesia due to avoidance of general anaesthesia. Currently, ultrasound (US) guidance is described as the golden standard in regional anaesthesia, but no studies have so far evaluated the US behaviour of peripheral nerve structures in obese versus normal-weight patients. To be able to perform such studies, it is necessary to develop new and more objective methods to quantify nerve visibility by US. We therefore designed a prospective, observational, comparative and blinded study to investigate the visibility of peripheral nerves in obese versus normal-weight patients by using a novel method based on histogram grey-scale values. METHODS: We scanned the median and sciatic nerves in 40 obese and normal-weight female patients and calculated differences of histogram grey-scale values between nerves and surrounding tissues. RESULTS: Histogram value analysis showed less US visibility of sciatic nerves in obese versus normal-weight study patients, which is caused by higher surrounding tissue histogram values. No differences could be detected for median nerves. CONCLUSIONS: The novel technique of comparing histogram grey-scale values to determine the visibility of the peripheral nerve in different patient categories was found feasible. Median nerves are appropriately visible by US in both normal and obese subjects, whereas sciatic nerves are less visible in obese as compared with normal-weight women. Our results serve as the rationale behind difficulties in peripheral regional anaesthesia in obese patients.