Peripheral nerve blocks
With neuraxial anesthesia, it is difficult to block only the area of interest. Almost by definition, surgical anesthesia at the desired level includes everything “south” as well. Peripheral nerve blocks provide an alternative, interrupting nerve impulses at specific points in their course, rather than the entire spinal cord. Table 4.6 lists some of the blocks we perform.
While local anesthetics can diffuse a small distance, depositing the drug in close proximity to the desired nerve increases the likelihood of a successful block. Therefore, knowledge of anatomy is paramount. Sometimes, anatomic landmarks
for example, we can deposit local anesthetic in the axillary sheath by
traversing its artery (Fig. 4.7). For most
other blocks, in order to ensure the needle tip lies within millimeters of the
intended nerve (and not in the
nerve), we use one of two common techniques:
· Paresthesia technique, in which placement of a needle in close proximity to a nerve causes a “pins and needles” sensation in the nerve’s peripheral distribu-tion. Depending on the area of the intended block, specific paresthesias can be sought with manipulation of the needle. This technique can be uncom-fortable for the patient, yet requires the patient to be sufficiently awake to respond. We need to watch the patient while gauging the pressure we apply to the plunger of the syringe. The patient will let us know if he feels an “electric shock” or pain – signs we associate with the intraneural placement of the needle, at which point we do not proceed to inject drug under high pressure, which would compress the nerve in its sheath, causing nerve ischemia and injury.
· Nerve stimulator technique, in which we apply a small electrical current to an insulated needle, causing motor stimulation when near a nerve. We adjust the needle position to achieve the maximal motor response in the desired distribution. This technique enables us to exploit anatomical cues to direct needle movement. For example, stimulation of the phrenic nerve (the patient will hiccup) when performing an interscalene block tells us the brachial plexus lies just a centimeter lower in the neck.
Peripheral nerve blocks may be performed for operative procedures, as well as for post-operative pain management. Through blockade of nerve impulses, pre-emptive analgesia may be obtained. Furthermore, catheter techniques enable post-operative pain management with continuous infusions of local anesthetic and/or opioids. Such infusions can improve perfusion to the operative extremity, reduce pain with movement, speed recovery, and improve quality of life even weeks after the operation.
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