67: Peripheral Nerve Blocks

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CHAPTER 67 Peripheral Nerve Blocks

Ronald Valdivieso, MD

1 What are the advantages of peripheral nerve blocks?

Peripheral nerve blocks (PNBs) share many of their advantages with neuraxial (spinal and epidural) anesthetic and analgesic techniques, first of which is the lack of need for airway instrumentation. This feature makes PNBs useful in cases in which airway management will be difficult or in which the patient has borderline respiratory function. PNB allows for shorter discharge times in ambulatory settings because of the decreased incidence of nausea, vomiting, and severe pain. PNB may diminish or prevent the development of chronic pain syndromes because of the lack of central nervous system sensitization that occurs after acute injury. Finally, patients with PNB have minimal if any opioid requirements in the immediate postoperative phase.

2 What basic principles should be followed to ensure a safe and successful peripheral nerve block?

Patients should be informed about the potential risks and benefits of PNB and allowed to decide on the anesthetic they prefer. Not all patients are good candidates for regional anesthetics. For example, performing PNB after high-velocity trauma to a lower extremity might obscure the diagnosis of a compartment syndrome. Sedation should be carefully titrated while performing a PNB because the patient should be able to communicate and give feedback to the clinician. This helps in confirming correct needle placement and giving an early warning of local anesthetic toxicity.

The clinician must have knowledge of the anatomy, technique, and equipment necessary to perform the most appropriate block for a given situation. The use of aseptic technique, correct equipment (B-bevel needles, nerve stimulators, ultrasound), and basic physiologic monitoring is mandatory. The area in which the PNB is performed should have immediate access to resuscitative equipment and medications.

3 What are the risks of performing a peripheral nerve block?

image There may be inadvertent damage to anatomic structures by the advancing needle. Examples include direct trauma to the nerve or spinal cord by intraneural injection of local anesthetic, nerve laceration, vascular injury with resulting hematoma formation, or pneumothorax.
image The drugs that are injected may have undesirable local and systemic effects. Allergic reactions to local anesthetics are rare. Ester local anesthetics are derivatives of para-aminobenzoic acid, a known allergen, and therefore more likely to cause allergic reactions than the amide local anesthetics. Any local anesthetic injected intravascularly has the potential for systemic reactions, including seizures and cardiovascular collapse.

4 How can the risks from a peripheral nerve block be minimized?

It is essential to ensure correct needle placement. Knowledge of the anatomy of the target region and the surrounding structures is necessary. Knowledge of the equipment and the pharmacology of local anesthetics is also required. Do not perform a PNB with which you are unfamiliar or not trained to do.

5 Describe a good technique for advancing the needle and injecting the local anesthetic solution

Continuous aspiration is mandatory while advancing the needle; flow of blood or cerebrospinal fluid is an obvious sign that the needle needs to be redirected and the landmarks reevaluated. Be attentive to reports of symptoms experienced by the patient. A paresthesia indicates close proximity to a nerve; and, depending on the technique being used, it might constitute the end point.

Once it is determined that the needle is correctly positioned, a 1-ml test dose is administered. Severe pain during injection might be a sign of intraneural needle placement. Evidence suggests that monitoring the injection pressure is useful in preventing intraneural injection. After the test dose is administered, slow incremental injection of the local anesthetic solution is started. This is a very important safety feature since it allows recognition of signs of local anesthetic toxicity before cardiac collapse occurs.

6 How are peripheral nerves localized?

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