The Perioperative Effect of Increased Body Mass Index on Peripheral Nerve Blockade: An Analysis of 528 Ultrasound Guided Interscalene Blocks

Background and objectives: Obese patients can pose a unique perioperative anesthetic challenge, making regional anesthetic techniques an intriguing means of providing analgesia for this population. Ultrasound guidance has been touted recently as being beneficial for this population in which surface landmarks can become obscured. In this study, the effect of increased Body Mass Index (BMI) on ultrasound guided interscalene peripheral nerve blockade is investigated. Material and methods: This study is a retrospective review of 528 consecutive patients who received preoperative ultrasound-guided interscalene nerve blocks at the University of Wisconsin Hospital and Clinics. We examined the association between BMI and the following parameters: time required for block placement; presence of Postoperative Nausea and Vomiting (PONV); postoperative Post Anesthesia Care Unit (PACU) pain scores; volume of local anesthetic injected; acute complications; and opioid administration preoperatively, intraoperatively, and postoperatively. Univariate and multivariate least squares and logistic regression models were used. Results: An elevated BMI was associated with an increased: time required for block placement (p-value = 0.025), intraoperative fentanyl administration (p-value < 0.001), peak PACU pain scores (p-value < 0.001), PACU opioid administration (p-value < 0.001), PACU oral opioid administration (p-value < 0.001), total PACU opioid administration (p-value < 0.001) and incidence of PACU nausea (p-value = 0.025). Conclusions: Ultrasound guided interscalene nerve blocks for perioperative analgesia can be safely and effectively performed in the obese patient but they may be more difficult to perform and analgesia may not be as complete.