Evoked Motor Response of the Sartorius Muscle and Femoral Nerve Blockade

To the Editor In a recent paper decribing success of femoral nerve blockade, Anns et al. reported that an injection of ropivacaine 0.5% (20 mL) after either sartorius or quadriceps evoked motor response resulted in similar block quality.1 According to the documentation of the anesthetic spread by ultrasound, all injections seemingly occurred at the point at which the branch(es) to the sartorius and femoral nerves were contiguous. Of note, the Anns et al. study excluded patients with difficult anatomy (obesity). However, 2 factors make it difficult to predict whether a sartorius motor response occurs where the branches to sartorius and femoral nerve are contiguous: the anatomical variability in the departure of the (often multiple) branches to the sartorius muscle, and the unpredictable relationship between surface anatomy and underlying neurovascular anatomy in the obese population (Fig. 1).2,3 An injection of local anesthetic after stimulation of a branch to the sartorius at a sufficient distance from the femoral nerve may preclude successful femoral nerve blockade (Fig. 1). Therefore, when using a nerve stimulator alone to guide femoral nerve block, obtaining quadriceps motor response should result in a more consistent blockade in different practice/skill settings and/or in patients with less than ideal anatomy.Figure 1: Anatomy of the femoral nerve (FN). Proximally, femoral nerve (black line) is seen lateral to the femoral artery (FA) before departure of the branches to the sartorius muscle (SM). More distally, branches (SN) for sartorius muscle are seen lateral to the main trunk of the femoral nerve. Catherine Vandepitte, MD Jerry Vloka, MD Jeff Gadsden, MD, FRCPC, FANZCA Admir Hadzic, MD, PhD Katholieke Universiteit Leuven, Belgium St. Luke's-Roosevelt Hospital Center New York, New York [email protected]