Nerve Blocks of the Thorax and Extremities

Chapter 31


Nerve Blocks of the Thorax and Extremities



Virtually every peripheral nerve can be blocked at some point along its course from the spine to the periphery, but digital nerve blocks (e.g., fingers and toes) are more commonly used than proximal blocks. Other common applications include femoral blocks for fractures of the femur, ankle blocks for foot injuries and infections, intercostal blocks for rib fractures, and wrist blocks for injuries to the palm.


The preparation, technique, choice of anesthetic, precautions, and complications are similar for all nerve blocks and are described in general in the following sections. The clinician is encouraged to use the same basic techniques and precautions for all nerve blocks. Specific precautions unique to a particular nerve block are included with the description of that block. Obvious precautions, such as aspiration before injection when the needle is in close proximity to a vascular structure, are not restated to avoid redundancy.



General Concepts



Indications


For most lacerations and injuries seen in the emergency department (ED), local infiltrative anesthesia is adequate and more efficient than using a nerve block (see Chapter 29). Patients who require extensive repair and anesthesia of the entire extremity are often referred to a specialist, who may prefer to examine an unanesthetized limb. A nerve block is indicated when it will provide advantages over other techniques. Scenarios in which this requirement is met include the following:



• When distortion from local infiltration hampers closure (e.g., facial wounds) or compromises blood flow (e.g., fingertip)


• When anesthesia is required over a large area and multiple injections would be painful or when the large amount of anesthetic needed for local infiltration exceeds the recommended dose


• When a nerve block is the most efficacious form of treatment, such as an intercostal block for treating a rib fracture or a patient with chronic obstructive pulmonary disease


• When local infiltration of the wound would be more painful than a regional nerve block, such as in the plantar surface of the foot or the palm of the hand


• When the block is performed to decrease pain during finger or toe dislocation or reduction


• When extensive limb surgery or manipulation is required (e.g., extensive tendon repair) and other options are not available



Preparation


A brief history, including drug allergies (particularly to local anesthetics), medications, and systemic illnesses, should be taken from the patient. Peripheral vascular, heart, and liver disease may increase the risk for severe complications. Therefore, information about the existence of these diseases should also be sought.





Choice of Anesthetic


Factors influencing the choice of anesthetic agent for nerve blocks are similar to those for local infiltration (see Chapter 29 for extensive discussion). In general, most nerve blocks are done for the repair of painful traumatic injuries that are likely to cause pain long after the repair is completed. In such cases, select the anesthetic with the longest duration of action to maximize the patient’s analgesia. For most of the blocks described in this chapter, 0.25% bupivacaine is suggested as the anesthetic of choice, but equal volumes of 1% lidocaine with epinephrine can be substituted. The use of epinephrine on end-organ areas has traditionally been discouraged (e.g., tip of the nose, peripheral ear pinna, distal end of the penis), although the theoretical risk is unsubstantiated in clinical practice. Recent literature describes the use and confirms the safety of lidocaine with epinephrine (1 : 100,000 concentration) for digital blocks.1 It would be prudent to avoid epinephrine-containing anesthetics in injuries involving vascular compromise or for those with obvious peripheral artery disease. Higher concentrations of lidocaine (≤2%) or bupivacaine (0.5%) are commonly used for large nerves. Ropivacaine is a relatively new amide anesthetic with a rapid onset and a long duration of action (several hours). It has been reported to have fewer cardiotoxic and central nervous system effects than bupivacaine does.2,3 Take care to avoid exceeding the recommended dosages of the anesthetic chosen. Buffering the anesthetic is strongly encouraged to lessen the pain of infiltration (see Chapter 29).






Locating the Nerve


When locating a nerve to be blocked, approach it from a site with easily identifiable anatomic landmarks. The best sites are those with good structural landmarks (e.g., prominent bones or tendons) immediately next to the nerve. For example, the four digital nerves are reliably found at the 2-, 4-, 8-, and 10-o’clock positions around and just superficial to the proximal phalanx, whereas the median nerve lies between the palpable palmaris longus and flexor carpi radialis tendons at the proximal wrist crease. Nerves that course adjacent to easily palpable arteries, such as in the axilla and groin, are also easy to locate and are good sites for performing nerve blocks. Nerves that do not have adjacent structural or vascular landmarks are much more difficult to block.


Blocking nerves with good structural or vascular landmarks is straightforward: palpate the landmarks and follow the course of the nerve in relation to these landmarks. After visualizing the anatomy in the mind’s eye, insert the needle in close proximity to the nerve.


Blocking nerves with poor landmarks, such as the radial nerve at the elbow, requires skill, practice, and some degree of luck. To increase the likelihood of successfully blocking these nerves, consider using a nerve stimulator.





Paresthesia


A common technique to ensure that the tip of the needle is in close proximity to the nerve is to elicit a paresthesia. Touching and mechanically stimulating the nerve with movement of the needle tip produces a tingling sensation or jolt known as a paresthesia, and it is felt along the distribution of the nerve. In practice, the jolt of a true paresthesia is often difficult to distinguish from the “ouch” of a pain-sensitive structure. When blocking proximal nerves at the elbow or axilla, the paresthesia travels far enough away from the injection site that it can be reliably distinguished from locally induced pain. Paresthesias at the level of the hand and wrist are more difficult to distinguish from pain. In both cases, paresthesia is a subjective feeling that requires intelligent and cooperative patients to understand what they are expected to feel and to remain relaxed and attentive so that they can distinguish an “ouch” from a jolt. Before the procedure, a simple explanation of what the patient should or may feel will facilitate cooperation. While eliciting paresthesias is generally reliable in demonstrating that the needle is close to its target, some authors believe that it may theoretically increase the rate of complications as a result of mechanical trauma or intraneural injection.8–10 Once the paresthesia is elicited, it is important to withdraw the needle 1 to 2 mm before injecting the anesthetic. If a paresthesia persists, stop the injection and reposition the needle.



Injecting the Anesthetic


Strive to ensure that the anesthetic agent is not inadvertently injected into a vessel or nerve bundle. In practice, a misplaced intravascular injection is usually of minimal consequence, but small amounts of epinephrine may cause systemic symptoms such as tachycardia or anxiety. Intraarterial injection, theoretically, is more dangerous than intravenous injection. Either way, aspirate the syringe to check for blood before injection. If no blood is aspirated, inject the anesthetic while observing the extremity for blanching, which suggests intravascular injection. If blanching occurs, reposition the needle before further injection.


Nerve bundle injection has the potential to cause nonspecific nerve injury. Severe pain or paresthesia during injection or resistance to depressing the plunger suggests the possibility of intraneural placement of the needle. If any of these problems occur, immediately stop injecting and reposition the needle.


The onset and duration of anesthesia are both greatly influenced by how close the injected anesthetic is to the nerve. Onset occurs within a few minutes if the anesthetic is in immediate proximity to the nerve. Onset takes longer or may not occur at all if the anesthetic must diffuse more than 2 to 3 mm, which underscores the importance of locating the nerve before the injection.


More anesthetic is required if it must diffuse a long distance to the nerve. A range of suggested volumes of anesthetic is given with each nerve block description. For blocks in which a definite paresthesia is elicited or a nerve stimulator or ultrasound is used, the minimal recommended amount of anesthetic suffices. For blocks of smaller nerves, paresthesias are often not easily elicited, and the anesthetic must be injected in the general vicinity of the nerve. For these blocks or when doubt exists about the proximity of the needle to the nerve, larger amounts of anesthetic are recommended. This point cannot be emphasized strongly enough. The difference between a successful and an unsuccessful block may be merely an additional 2 mL of anesthetic. When in doubt, err on the high side of the recommended dosage. For blocks of large nerve, many clinicians also opt for 2% lidocaine rather than the 1% solution.


With most blocks the onset of anesthesia occurs in 2 to 15 minutes, depending on the distance that the anesthetic must diffuse to the nerve and the type of anesthetic used. Wait 30 minutes before deciding that a block is unsuccessful.



Complications and Precautions


Complications may result from peripheral nerve blocks but are rare in clinical practice. Most cannot be prevented by even perfect technique. General precautions include measures to minimize nerve injury, intravascular injection, and systemic toxicity. No actual statistics exist on the complication rate from nerve blocks performed by emergency clinicians, but they are extremely rare in clinical practice. Theoretically, infrequently performed blocks, blocks that require high doses of anesthetic, and blocks close to major vascular structures are more likely to have complications.



Nerve Injury


Nerve injury is rare but can occur secondary to (1) chemical irritation from the anesthetic, (2) direct trauma from the needle, or (3) ischemia as a result of intraneural injection. Overall, the incidence of serious neuronal injury is rare and occurs in 1.9 per 10,000 blocks.11 Given that placement of a nerve block is a blind procedure, nerve injuries do not necessarily represent an error in technique.


Chemical neuritis from the anesthetic is the most common nerve injury.9,10 The patient may complain of pain and varying degrees of nerve dysfunction, including paresthesia or motor or sensory deficit. Most cases are transient and resolve completely. Supportive care and close follow-up are the mainstays of treatment. Emergency clinicians should not exceed the recommended doses and concentrations of anesthetic (Table 31-1). In general, lidocaine 1% or 2% or bupivacaine 0.25% or 0.5% is safe for nerve blocks performed in the ED.



Direct nerve damage can be minimized by proper needle style, positioning, and manipulation. Use a short, beveled needle and keep the bevel parallel to the longitudinal axis of the nerve. Sharp pain or paresthesia indicates that the needle is close to or in the nerve. Avoid excessive needle movement when the tip of the needle is in contact with the nerve. If a 25-gauge needle is used, physical damage to a nerve should be minimal, even when directly touched by the tip of the needle. A 27-gauge needle is theoretically attractive, but its small size may limit aspiration testing and it may bend or break when attempting to block deep nerves.


Intraneural injection may rarely cause nerve ischemia and injury. Elicitation of a paresthesia or severe pain suggests that the needle has made contact with the nerve. When a paresthesia is elicited, withdraw the needle 1 to 2 mm before injecting the anesthetic. If the paresthesia occurs during injection, stop the injection and reposition the needle. Most neurons are surrounded by a strong perineural sheath through which the nutrient arteries run lengthwise. Injection directly into a nerve sheath may increase pressure within the nerve and compress the nutrient artery. Impaired blood flow results in nerve ischemia and subsequent paralysis. Intraneural injection is often heralded by severe pain, which worsens with further injection and may radiate along the course of innervation. The operator may notice difficulty depressing the plunger of the syringe. If the tip of the needle is in proper position, slow injection of the anesthetic should be minimally painful, and the anesthetic should go in without resistance.



Intravascular Injection


Intravascular injection may rarely result in both systemic and limb toxicity. Inadvertent intravascular injection produces high blood levels of the anesthetic. Exercise care when administering large amounts of anesthetic in close proximity to large blood vessels.


Intraarterial injection of anesthetics with epinephrine may cause peripheral vasospasm and further compromise injured tissue. Intravascular anesthetic is not toxic to the limb itself, although it may produce transient blanching of the skin by displacing blood from the vascular tree. Epinephrine, however, can cause prolonged vasospasm and subsequent ischemia if it is injected into an artery. This is especially worrisome when anesthetizing areas with little collateral circulation, such as the toes, fingers, penis, and tip of the nose. Severe epinephrine-induced tissue blanching or vasospasm may be reversed with local or intravascular injection of phentolamine (see extensive discussion in Chapter 29).


Vasospasm associated with the epinephrine in anesthetic solutions is rare, but experience in related clinical situations can help guide therapy. Roberts and Krisanda used a total of 5 mg of phentolamine infused intraarterially to reverse arm ischemia following 3 mg of epinephrine inadvertently administered into the brachial artery during cardiac resuscitation.12 Digital ischemia from inadvertent epinephrine autoinjection (Epi-Pen) has been treated both by proximal “digital block” with 2 mg of phentolamine and by local infiltration at the ischemic site with 1.5 mg of phentolamine.13,14


The route of phentolamine administration should be guided by the clinical situation. Phentolamine must reach the site of vasospasm. Local infiltration may be effective for ischemia in a single toe or finger, whereas arterial injection has the advantage of delivering the medication directly to the arteries exhibiting spasm. For larger areas of involvement or in instances in which local infiltration is ineffective, use intraarterial injection. A dose of 1.5 to 5 mg appears to be effective in most cases,12–14 although a total of 10 mg may be used for local infiltration. Phentolamine, 5 mg, can be mixed with 5 to 10 mL of either normal saline or lidocaine. The small volume of the distal pulp space may limit the volume of the infiltration dose to 0.5 to 1.5 mL in the fingertip. Larger volumes and dosages can be used with proximal infiltrations. For intraarterial infusion of the radial artery at the wrist or the dorsalis pedis at the ankle, dosages of 1.5 to 5 mg of phentolamine are suitable. Slow infusion or graded dosages of 1 mg may provide enough phentolamine to reverse the ischemia without excessive systemic effects such as hypotension.







Specific Nerve Blocks



Intercostal Nerve Block


Blocking the intercostal nerves produces anesthesia over an area of their cutaneous distribution (Fig. 31-1) and provides considerable pain relief for patients with rib contusions or fractures. Rib fractures are typically quite painful and cause the patient to try to splint respirations to avoid excessive movement of the injured site. The resulting hypoventilation, atelectasis, and poor expectoration from splinting respirations may cause hypoxia or lead to pneumonia. This is particularly true in patients with preexisting pulmonary disease and minimal respiratory reserve, in whom further impairment of function may cause significant respiratory compromise.



Theoretically, anesthetizing injured ribs eases pain and facilitates deep breathing and coughing. Unfortunately, no controlled studies have compared intercostal blocks and oral analgesics in patients with the types of rib fractures that are commonly managed on an outpatient basis. However, studies do suggest that intercostal blocks may be superior to analgesics in patients who have undergone thoracotomy.16–18 In these studies, those receiving intercostal nerve blocks had better results on pulmonary function tests, greater oxygenation, and earlier ambulation and discharge than did those receiving opioid analgesics.


There are several arguments against the routine use of intercostal nerve blocks in the ED. First, rib fractures are often tolerated well by young patients, who usually require minimal oral analgesics. Second, these blocks have a relatively short duration of action. The typical duration of action of a long-acting anesthetic with epinephrine is 8 to 12 hours. However, it should be noted that patients often experience partial analgesia for up to 3 days, a period that cannot be attributed to direct action of the anesthetic on the nerve. Perhaps the anesthesia reduces muscle spasm and the associated cycle of pain.


Finally, a wrongly perceived high incidence of pneumothorax and unsuccessful blocks deters many clinicians from performing intercostal nerve blocks in the ED. The true incidence of pneumothorax after intercostal nerve blocks is very low and not significant enough to prohibit the procedure. Moore reported that in more than 10,000 individual rib blocks performed, the incidence of pneumothorax was less than 0.1%.19 However, Shanti and associates reported that the incidence of pneumothorax was 1.4% for each individual intercostal nerve blocked.20 If more than one nerve requires blockade, the incidence of pneumothorax may be greater. The suggested approach to discussing intercostal blocks is to give patients the facts with regard to the duration of analgesia and possible complications and then allow them to decide on the method for themselves. Frequently, they prefer oral analgesics initially but may return for further relief of pain, at which time they are more amenable to a nerve block.



Anatomy


Each thoracic nerve exits the spine through the intervertebral foramen, which lies midway between adjacent ribs (Fig. 31-2A). It immediately gives off the posterior cutaneous branch, which supplies the skin and muscles of the paraspinal area. The intercostal nerve then continues around the chest wall and gives off lateral cutaneous branches at the midaxillary line. These branches are the sensory supply to the anterior and posterior lateral chest wall.



The intercostal nerve runs with the vein and artery in the subcostal groove. The vein and artery lie above the nerve and are somewhat protected by the rib during a nerve block. Posteriorly, the nerve is separated from the pleura and lungs by the thin intercostal fascia. When blocking the nerve in the posterior aspect of the back, particular care must be taken to avoid puncture of the thin fascia and underlying lung. Fortunately, most rib fractures occur in the anterior or lateral portion of the rib and can be blocked in the posterior axillary line, where the internal intercostal muscles lie between the nerve and the lung’s pleura and provide a buffer for minor errors in needle placement. Note that blocking the nerve here will anesthetize the entire course of the intercostal nerve because it is blocked before the cutaneous branches are given off.



Technique


To achieve adequate analgesia for most rib fractures, the lateral cutaneous branch needs to be anesthetized. Therefore, perform blocks between the posterior axillary and midaxillary line at a point proximal to the origin of this branch (see Fig. 31-2A, arrow). Explain the procedure and its benefits and its risks, including potential pneumothorax, systemic toxicity, and ineffective block, before proceeding.


Use a 10-mL syringe with a 3.75-cm, 25-gauge needle. Prepare the area to be injected in the usual aseptic manner. Use the index finger of the nondominant hand to retract the skin at the lower edge of the rib cephalad and pull it up and over the rib (see Fig. 31-2B). With the syringe in the opposite hand, puncture the skin close to the tip of the finger that is retracting the skin over the rib. Keep the syringe at an 80-degree angle to the chest wall with the needle pointing cephalad, and rest the hand holding the syringe on the chest wall for stability. In this position, the depth of needle penetration is well controlled. Slowly advance the needle until it comes to rest on the lower border of the rib. The bone should be felt through the tip of the needle.


At this point, release the skin retracted over the rib. As the skin returns to its natural position, the shaft of the needle will become perpendicular to the chest wall and the tip of the needle will be at the inferior margin of the rib. Shift the syringe from the dominant hand to the index finger and thumb of the nondominant hand. Rest the middle finger of the same hand against the shaft of the needle and exert gentle pressure on the shaft to “walk” the needle off the lower edge of the rib. Again, keep the palm of the hand planted firmly on the chest wall to ensure control of the needle. With the help of the dominant hand, slowly advance the needle 3 mm. Aspirate to be sure that the needle has not penetrated a blood vessel, and then inject 2 to 4 mL of anesthetic while carefully moving the needle in and out 1 mm to ensure that the compartment containing the nerve between the internal and external intercostal muscles is penetrated. This may also serve to minimize intravascular injection. Repeat the procedure on the two ribs above and below to ensure that the overlapping innervation from adjacent nerves is blocked.


Although the procedure just discussed seems extensive, it takes 1 to 2 minutes to perform once the operator is familiar with the technique, and three to five intercostals can be blocked in 10 minutes’ total time.



Precautions


Initially place the needle at the lower edge of the rib. If it contacts the rib above this point, it cannot be walked off the lower edge of the rib at the proper angle. If it is inserted too low, over the intercostal space, it may be advanced through the pleura and into the lung before the operator realizes that it is too deep. Before inserting the needle it is always prudent to estimate the depth of the bone. If the bone is not encountered by this depth of insertion, reevaluate the position of the needle. Even after the needle has been properly walked off the edge of the rib, take care to not puncture the pleura and lung. The depth of the intercostal groove in which the nerve runs is 0.6 cm posteriorly and diminishes to 0.4 cm anteriorly.


Because the incidence of pneumothorax is low, a chest radiograph is not routinely required after this procedure. Observe asymptomatic patients for 15 to 30 minutes and instruct them to return if problems arise. If the patient has symptoms of pneumothorax (e.g., cough, a change in nature of the pleuritic pain, or shortness of breath), obtain a chest film before discharge.


If the clinician inadvertently causes a pneumothorax, treatment depends on its size. Many pneumothoraces from this procedure are small and require no specific intervention. Those smaller than 20% may be observed for 6 hours.21 During this time, administer a high concentration of oxygen to help decrease the size of the pneumothorax. If the pneumothorax does not enlarge, the patient may be released home with arrangements for close follow-up. Needle or catheter aspiration of larger pneumothoraces may be all that is needed. A chest tube is necessary if this method fails (see Chapter 10).



Nerve Blocks of the Upper Extremity


The upper extremity is supplied by the brachial plexus. Its branches—primarily the median, radial, ulnar, and musculocutaneous nerves—can be blocked at the axilla, elbow, wrist, hand, or fingers. Nerve blocks at the axilla and elbow are seldom used in the ED. Nerve blocks of the wrist are performed occasionally before painful procedures or for repair of injuries to the hand. Metacarpal and digital blocks are used frequently to treat fractures, lacerations, and infections of the fingers.



Nerve Blocks at the Elbow


The median, ulnar, and radial nerves can be blocked at the elbow to provide anesthesia to the distal end of the forearm and hand (Fig. 31-3). For most injuries extensive enough to require a nerve block at the elbow, all three nerves must be blocked for successful anesthesia because of the variable and overlapping innervation of the forearm. Furthermore, injuries to the proximal and middle aspects of the forearm may require additional circumferential subcutaneous field blocks of the lateral, medial, and posterior cutaneous nerves.




Ulnar Nerve: Anatomy and Technique (Fig. 31-4A): The ulnar nerve can be palpated in the ulnar groove on the posteromedial aspect of the elbow between the olecranon and the medial condyle of the humerus. This nerve supplies innervation to the small finger, the ulnar half of the ring finger, and the ulnar aspect of the hand.



With the elbow flexed, insert a 3.75-cm, 25-gauge needle 1 to 2 cm proximal to the ulnar groove and advance the needle parallel to the course of the nerve. The tip of the needle should come to rest close to the proximal end of the groove. Do not block the nerve in the groove, where it is prone to damage. For similar reasons, a paresthesia may be elicited but is not vigorously sought. If a paresthesia occurs during injection, slightly reposition the needle to avoid intraneural injection. Although an elbow ulnar nerve block is common, many clinicians prefer to block the ulnar nerve at the wrist to limit the risk for injury. Once the tip of the needle is positioned properly, deposit 5 to 10 mL of anesthetic. If a nerve stimulator is used, flexion of the small and ring fingers signals proximity to the nerve.

Sep 17, 2016 | Posted by in SUGERY, ORTHOPEDICS & ANESTHESIA | Comments Off on Nerve Blocks of the Thorax and Extremities
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