Lumbar plexus block (LPB) produces anesthesia of the major components of the ipsilateral lumbar plexus, the femoral nerve (FN), lateral femoral cutaneous nerve (LFCN), and the obturator nerve (OBN)1 LPB is used as a sole technique2,3,4 or in combination with a sciatic nerve5 block for anesthesia or analgesia in patients having hip2,3,4 or lower extremity surgery.2 It is also referred to as psoas compartment block (PCB)2,6,7 or posterior lumbar plexus block (PLB).3,7 The term PCB was originally coined by Chayen and colleagues.6 They believed that branches of the lumbar plexus and parts of the sacral plexus were located close to each other in a “compartment” between the psoas major and quadratus lumborum muscles at the level of the L4 vertebra6 and could be identified using “loss of resistance.”6 However, the lumbar plexus is located within the substance of the psoas muscle8,9,10,11 and local anesthetic is injected into a fascial plane within the posterior aspect of the psoas muscle during an LPB.9,10
LPB is traditionally performed using surface anatomical landmarks and peripheral nerve stimulation.12 The main challenges with accomplishing LPB with anatomical landmarks and peripheral nerve stimulation relate to the depth at which the lumbar plexus is located.13 Small errors in estimation of landmark or angle of needle insertion can lead to the block needle being directed away from the plexus, resulting in inadvertent deep needle insertion or renal12,14 or vascular injury.12,14,15,16 Therefore, real-time monitoring of the needle and local anesthetic injection during an LPB is desirable and may improve the accuracy and safety of the technique. While fluoroscopy and computed tomography can be used to improve precision during an LPB, they are impractical in a busy operating room environment, costly, and, more importantly, associated with exposure to radiation. Ultrasound (US) is increasingly being used to guide peripheral nerve blocks, and it is only logical that ultrasound-guided (USG) LPB is of interest because of the ever-increasing availability of US machines, which produce high-quality images, in the operating room. US has been used to preview the relevant anatomy,7,9,10,13,17 measure the depth to the transverse process, guide the block needle to the posterior aspect of the psoas muscle or the lumbar plexus in real time,10,13,17,18,19 and monitor needle–nerve contact10 or spread of local anesthetic10,17,19 during an LPB. Understanding of the sonoanatomy of the lumbar paravertebral region is a prerequisite to using US for LPB. This chapter briefly describes the techniques used to perform lumbar paravertebral sonography, the relevant sonoanatomy, and the practical considerations for using US for LPB.