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The extent of anesthesia after selective blocks of the peripheral nerves originating from the lumbar and lumbosacral plexi is often assessed before surgery can begin. This assessment may be deceptively difficult if one has incomplete understanding of lower extremity innervation or if the evaluation is hindered by slow onset of sensory block. Conversely, timely evaluation of the block allows the anesthesiologist to identify inadequately anesthetized nerves, thus providing the opportunity to modify the block before incision.

A simple system for assessing the adequacy of upper extremity nerve block has been described and enjoys acceptance by anesthesiologists worldwide. The four P's (push, pull, pinch, punt) concept was likely developed during World War II as a straightforward method for medics to determine the extent of battlefield injury. The concept was later formalized and popularized by Thompson and Brown.1 Herein is described a methodology for assessing lower extremity anesthesia that is a variation of the four P's concept. For lower extremity evaluation, the P's are modified slightly to become push, pull, pinch, punt.

Several minutes after depositing local anesthetic near a peripheral nerve, the following individual assessments are undertaken to determine blockade of the four major nerves of the lower extremity.

The sciatic nerve is derived from the lumbosacral plexus and provides motor control to the posterior thigh and the entire lower leg and foot. Its sensory distribution includes the posterior thigh, the posterior knee joint, and all the lower leg except for the saphenous nerve distribution (medial lower leg and ankle). Because plantar flexion of the foot is controlled by the sciatic nerve, its function is evaluated by asking the patient to push the foot against the resistance of the examiner's hand or “step on the gas” (Figure 32–1). Anesthesia within the sciatic nerve distribution is indicated by weakness during the performance of this maneuver. Because the evaluation is performed distally, it is applicable to all sciatic nerve block approaches, including the blocks at the popliteal fossa.

Fig. 32-1

Push. Patients with successful sciatic or popliteal fossa block are unable to push the target with their foot. A: Tibial nerve or dorsiflex the foot; B: Common peroneal nerve.

The obturator nerve, a component of the lumbar plexus, provides motor innervation to the adductors of the thigh and variable sensory innervation to the proximal medial thigh. It also has small branches to the knee and hip joints. To assess obturator nerve function, the anesthesiologist abducts the patient's leg and then requests that the patient pull the leg toward the midline against resistance (Figure 32–2). The obturator nerve has been successfully blocked if the patient exhibits adductor weakness during this task.

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