Anesthesia for Lower Extremity Surgery (Peripheral Nerve Block) Case File

Anesthesia for Lower Extremity Surgery (Peripheral Nerve Block) Case File

Lydia Conlay, MD, PhD, MBA, Julia Pollock, MD, Mary Ann Vann, MD, Sheela Pai, MD, Eugene C. Toy, MD

Case 14

A 72-year-old man has had pain and drainage from several non-healing ulcers on the toes and ball of his right foot. He was treated initially with careful dressing changes, antibiotics, and weight-bearing restrictions. However, the drainage became frankly purulent and gangrenous changes developed. His vascular surgeon determined that no additional revascularization of the lower extremity was likely to help heal these wounds, so he scheduled the patient for a transmetatarsal amputation of the right foot today.

The patient’s medical history is significant for coronary artery disease (CAD), evidenced by several myocardial infarctions with a coronary artery bypass grafting for revascularization. On recent catheterization, his cardiologist notes no additional treatment is indicated and that he should be managed medically. He also has type II diabetes mellitus, poorly compensated congestive heart failure (CHF), a 40-pack-year smoking history, chronic obstructive pulmonary disease, and paroxysmal atrial fibrillation. In addition to insulin, he takes carvedilol, simvastatin, lisinopril, warfarin (held for 5 days), a baby aspirin, and an albuterol/ipratropium inhaler. On examination, the patient weighs 190 lb and is 5 ft, 7 in tall. He has poor dentition limited mouth opening of approximately 1.5 cm. Auscultation of his heart reveals a slow, regular heart rate. His lungs are coarse with diminished breath sounds in the bases and a prolonged expiratory phase. His labs are significant for an INR of 1.3 and a hematocrit of 28. The patient states that he does not want a general anesthetic.

➤ What type of anesthetic options could be considered for this procedure?

➤ Which spinal nerves provide the innervation of the lower extremity?

➤ What are the risks of neuraxial anesthesia in an anticoagulated patient?

ANSWERS TO CASE 14:

Anesthesia for Lower Extremity Surgery

(Peripheral Nerve Block)

Summary: A 72-year-old man with a gangrenous forefoot presents for transmetatarsal amputation. His past medical history is remarkable for diabetes mellitus, CAD, CHF, and COPD. His laboratory results are significant for INR of 1.3 and Hct of 28. He desires avoiding general anesthesia.

➤ Anesthetic options: Subarachnoid block (spinal), popliteal (sciatic) and saphenous nerve block, or ankle block.

➤ Lower extremity innervation: The lumbosacral spinal nerves from L2 to S3 provide the source of the innervation of the lower extremity. (Please see Figure 14–1.)

➤ Risks of neuraxial anesthesia in an anticoagulated patient: Local bleeding from the puncture site, and in the case of spinal, an epidural hematoma with or without neurological injury.

ANALYSIS

Objectives

1. Understand the regional anesthetic choices for transmetatarsal amputation.

2. Describe the innervation of the lower extremity.

3. Recognize the risks of neuraxial anesthesia in an anticoagulated patient.

Considerations

In this case, our 72-year-old patient asks to avoid general anesthesia, leaving us with the options of neuraxial (spinal or epidural) anesthesia, or a peripheral nerve block. Both peripheral nerve blocks and central neuraxial anesthetics minimize effects on pulmonary function, and generally have fewer hemodynamic alterations than general anesthesia.

A spinal anesthetic is typically performed by a single injection of a local anesthetic into the subarachnoid space. It provides excellent anesthesia over low thoracic, lumbar, and sacral dermatomes. In this patient’s case, a subarachnoid block would be the preferable choice of neuraxial anesthesia, due to the better coverage of sacral nerves and the relatively brief (< 2 hours) duration for the procedure. A spinal also provides an intense block in the entire lower extremity, allowing the surgeon more flexibility particularly if the extent of the patient’s ischemic tissue is uncertain. This patient had been treated with warfarin for his atrial fibrillation, but the warfarin was stopped 5 days prior to surgery and his coagulation parameters had returned to normal.

A peripheral nerve block such as an ankle block is also an excellent choice for this patient, and is associated with fewer propensities for hemodynamic changes and a less time in the recovery room than a spinal. However, peripheral nerve blocks require several injections, and are less well tolerated by some patients.

APPROACH TO

Anesthesia For Lower Extremity Surgery (Peripheral Nerve Block)

CLINICAL APPROACH

Neuraxial anesthesia refers to an anesthetic placed in the epidural or subarachnoid space, or (in the case of obstetric anesthesia) both. The anesthetic can be injected in a single injection, or by continuous infusion. Subarachnoid anesthesia is typically performed as a single injection, and can provide excellent anesthesia over low thoracic, lumbar, and sacral dermatomes. Its advantages include an arguably easier technical approach, the use of a smaller needle, thus a lower risk of bleeding complications (ASRA 2003 Consensus Statement), good coverage of sacral dermatomes for gynecological, urologic, and podiatric procedures, and dense anesthesia. Disadvantages include inability to easily re-dose the anesthetic and difficulty in controlling block height except by drug choice considering drug dosage, volume, and baricity (the anesthetic’s density relative to CSF).

A profound vasodilatation accompanies the sympathectomy that results from neuraxial blockade. The result is often marked hypotension from reduced cardiac filling, from slowing of heart rate from a decreased stretch of the right atrium and great veins, and in blocks above T2 to T4, blockade of the cardio accelerator fibers. If performing a central neuraxial technique, vasopressors should be prepared in advance to counteract the vasodilation in the event that it is needed.

Epidural anesthesia is typically performed as a continuous technique and can provide good anesthetic coverage of thoracic and lumbar dermatomes. It can be associated with similar hemodynamic changes to spinal, though since the onset of an epidural is somewhat slower, so are the hemodynamic changes somewhat more profound. The advantages of an epidural include the ability to titrate block coverage by intermittent injection of local anesthetic until the desired location is adequately anesthetized, and easy re-dosing of the anesthetic in the event of a very long procedure. Disadvantages include the potential for inadequate density of blockade (not “numb” enough), patchy or incomplete blockade, and poor coverage of sacral dermatomes, even with large volumes of local anesthetic.

Figure 14–1. Innervation of the lower extremity. (Redrawn from:Morgan GE,Mikkail MS, Murray MJ. Clinical Anesthesiology, 4th edition. http://www.accessmedicine.com. Copyright © The McGraw-Hill Companies, Inc. All rights reserved.)

Like neuraxial anesthetics, peripheral nerve blocks may be divided in several ways as well. Anesthetics may be placed selectively at nerve roots, in the distribution of large collections of trunks, such as the lumbar plexus (psoas block), at large conducting nerves such as the sciatic or femoral nerves, or at individual nerves such as the deep peroneal nerve as a portion of an ankle block.

Innervation of the Lower Extremity The innervation of the lower extremity begins with the lumbar and sacral spinal nerve roots from L2 to S3. The upper portion of these nerves, from L2 

Figure 14–1.

to L4, forms the lumbar plexus. The lumbar plexus supplies innervation to the anterior and medial thigh and medial leg via branches of the lateral femoral cutaneous nerve (L2-L3), the femoral nerve (L2-L4), and the obturator nerve (L2-L4). The femoral nerve supplies the saphenous nerve below the knee and is responsible for sensation along the medial portion of the leg ankle, and the very posterior medial portion of the foot.

The lower group of spinal nerves, from L4 to S3, form the tibial and peroneal divisions of the sciatic nerve. In addition, sensation over the posterior portion of the thigh is supplied by the posterior cutaneous nerve of the thigh, which is formed separately from the S1 to S3 roots. The peroneal division provides sensation to the lateral portion of the leg via the lateral sural cutaneous nerve and the superficial peroneal nerve. The posterior portion of the leg and the lateral portion of the heel and foot (lateral calcaneal and lateral dorsal cutaneous nerves) are supplied via the sural nerve which is another branch of the peroneal. The tibial nerve supplies sensation over the medial portion of the heel (medial calcaneal nerve), as well as the remainder of the sole of the foot (not including the areas covered by the saphenous and sural branches) via the medial and lateral plantar nerves.

The popliteal (sciatic) and saphenous nerve blockade option may be performed using several approaches. The popliteal block is usually performed approximately 6 to 7 cm above the popliteal crease, in attempt to deposit local anesthetic around both the tibial and peroneal divisions. The approach may be made either from a lateral location, between the vastus lateralis and biceps femoris muscles, or from a posterior approach, between the semitendinosus and the biceps femoris muscles. The saphenous nerve may be blocked around the saphenous vein at the level of the medial malleolus.

For the ankle block, it is important to remember that 5 separate nerves must be anesthetized (see Figure 14–2.) The posterior tibial nerve may be blocked at the level of the medial malleolus, just posterior to the posterior tibial artery. The sural nerve may be blocked posterior to the lateral malleolus and lateral to the calcaneus. The deep peroneal nerve is approached just lateral to the anterior tibial artery on the anterior portion of the ankle deep to the fascial planes. The superficial peroneal nerve branches can be blocked with a subcutaneous ring of local anesthesia from the lateral malleolus to the anterior tibia. The saphenous nerve should then be blocked as described in the description of the popliteal and saphenous combination block.

Neuraxial Anesthesia in the Anticoagulated Patient

The risks of neuraxial anesthesia are not trivial, and the potential for bleeding, infection, and neurologic injury must all be considered. In particular, the increasingly common use of anticoagulants in outpatient therapy, including warfarin, low-molecular-weight heparin, and antiplatelet medications such as clopidogrel, raises the issue of the risk of spinal hematoma from neuraxial anesthetics. Guidance for the safety of performing these blocks comes from the American Society of Regional Anesthesia. There are, however, significant limitations to the evidence-based nature of their recommendations due to the very low incidence of spinal hematoma. The group’s consensus depended upon anecdotal case reports, and the understanding of individual drug pharmacology.

This patient had been maintained on warfarin because of his history of atrial fibrillation until 5 days prior to surgery, and a PT/INR was checked. It is important to remember that PT/INR primarily reflects factor VII, and may overestimate the adequacy of coagulation after discontinuing warfarin therapy. PT/INR should 

Figure 14–2. Anatomy and approaches to the nerves constituting an ankle block. (Redrawn from: Morgan GE Jr, Mikhail MS, Murray MJ, eds. Clinical Anesthesiology, 4th ed.New York,NY: McGraw-Hill, 2006:352.)

be back in the normal range (< 1.5) prior to any neuraxial anesthetic. In the case of other anticoagulants, the ASRA consensus guidelines should be used for reference regarding a time line for discontinuation of medications and testing.

The suitability of patients on antiplatelet drugs for neuraxial anesthesia is a topic of common discussion. Patients receiving thienopyridine medications for anticoagulation pose obvious risks of bleeding in the epidural or subarachnoid space. And patients who have undergone implantation of drug-eluting coronary stents are recommended to continue thienopyridine therapy uninterrupted for 12 months after implantation. On the other hand, these are often the very patients for whom a regional technique would be desirable, in order to minimize any hemodynamic changes associated with general anesthesia. Thus, the choice of anesthetic technique requires balancing the risks of bleeding against the risks of general anesthesia. Many, if not most, patients who require thienopyridine therapy will therefore not be the candidates for neuraxial anesthesia.

Patients who present for lower extremity amputations typically have significant peripheral vascular disease which impairs the normal healing. They often have significant comorbidities, including diabetes, coronary artery disease, hypertension, and renal insufficiency or failure. These pre-existing medical conditions often determine the choice of anesthetic, whether general, neuraxial, or a peripheral nerve block.

Comprehension Questions

14.1. A morbidly obese 55-year-old woman with chronic foot pain presents for a complex forefoot reconstructive procedure that is estimated to take 2 hours. Her past medical history is significant for reactive airway disease, hypertension, and chronic knee pain. All of these are well-controlled according to the patient and the history and physical from her internist. She plans to stay in the hospital overnight as she lives alone. Which of the following anesthetics may be a poor choice for this patient?

    A. General anesthesia with an LMA

    B. Subarachnoid (spinal) anesthesia

    C. Popliteal and saphenous nerve blocks

    D. Epidural anesthesia

14.2. A 49-year-old man with treatment-refractory coronary artery disease and a non-healing ulcer on the ball of his foot presents for transmetatarsal amputation. You plan an ankle block for this anesthetic.

Which of the following nerves may be omitted in this anesthetic?

    A. Deep and superficial peroneal nerves

    B. Saphenous nerve

    C. Sural and posterior tibial nerve

    D. Greater tarsal nerve

14.3. Which of the following medications may be continued prior to performing a neuraxial anesthetic?

    A. Aspirin

    B. Warfarin

    C. Clopidogrel

    D. Low-molecular-weight heparin

ANSWERS

14.1. D. Epidural anesthetics are often chosen for lower extremity surgery as they provide relatively rapid anesthesia, ease of re-dosing, and the potential use postoperatively for analgesia. However, in this patient, the operation will involve some of the sacral dermatomes, which are often poorly anesthetized compared to the lumbar and low thoracic dermatomes. The other three options would be better choices for this operation.

14.2. D. The “greater tarsal nerve” does not need to be anesthetized for this procedure. All of the other listed nerves must be anesthetized. The ankle block is a fundamental skill of all anesthesiologists to provide regional anesthesia over the foot and minimize the systemic anesthetic effect in patients with complex medical problems. Successful placement of the block requires recollection of all five of the nerves that must be anesthetized for a successful block.

14.3. A. Aspirin may be continued prior to placement of a neuraxial blockade. This is especially true for vascular patients, where the risk of discontinuing the drug is greater than the risk of hematoma formation. The other listed medications should be stopped prior to performing neuraxial anesthesia in order to minimize the risk of spinal (epidural) bleeding and potential neurologic compromise. Each of the anticoagulants has a different set of pharmacokinetics and as such, the time each drug needs to be held varies. The ASRA Consensus Guidelines on regional anesthesia in the anticoagulated patient serves as a ready reference for guiding these decisions.

Clinical Pearls

➤ Knowledge of lower extremity innervation provides the anesthesiologist with several options to obtain excellent anesthesia for a variety of lower extremity operations.

➤ Local anesthetic choices are guided by the expected duration of the operation and the degree of expected pain following the procedure.

➤ Patients who require thienopyridine therapy are not generally candidates for neuraxial anesthesia.

References

American Society of Anesthesiologists Committee on Standards and Practice Parameters. Practice alert for the perioperative management of patients with coronary artery stents: A report by the American Society of Anesthesiologists Committee on Standards and Practice Parameters. Anesthesiology. 2009;110:22-23. 

Horlocker TT, Wedel DJ, Bezon H, et al. Regional anesthesia in the anticoagulated patient: Defining the risks (The Second ASRA Consensus Conference on Neuraxial Anesthesia and Anticoagulation). Reg Anesth Pain Med. 2003;28(3):172-197. 

Peterson CM, Peterson KP, Jovanovic L. Influence of diabetes on vascular disease and its complications. In: Moore WS, ed. Vascular Surgery: Comprehensive Review. 5th ed. Philadelphia, PA: WB Saunders; 1998:146-167.

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