Perineurial environment

The axillary sheath is the connective tissue surrounding the neurovascular structures of the brachial plexus. It originates as a continuation of the prevertebral fascia and joins the fascia of the biceps and brachialis muscles distally. This connective tissue extends inward, forming septa between components of the plexus and creating fascial compartments for each nerve.² Controversy exists regarding the ability of the septae to limit the spread of local anesthetics within the sheath. Some investigators report that these fascial compartments limit the circumferential spread of local anesthetics and that injected solutions spread longitudinally up and down the nerve and remain compartmentalized. This concept provides a rational explanation for the occurrence of a rapid and profound block of one nerve, yet partial or absent block in other nerves during brachial plexus blockade.³ Other investigators propose that these septa are incomplete and form small bubble-like pockets when solution is injected. They found that single injections of dye solutions into the axillary sheath resulted in immediate staining of median, radial, and ulnar nerves, despite the presence of septa. These data demonstrate that there are connections between compartments within the sheath, and may explain why single injection techniques have success rates comparable with multiple injection techniques during blockade of the brachial plexus.⁴

Microneuroanatomy

Peripheral nerves are composed of fascicles of individual nerve fibers surrounded by endoneurium. Groups of fascicles are contained within the epineurium. As the nerve travels away from the spinal cord, fascicle numbers increase, while fascicle size decreases.⁵ The nerve roots contain large fascicles, demonstrating a monofascicular or oligofascicular pattern, while a multifascicular pattern is found more distally.^6,7 While the amount of neural tissue remains constant, the amount of non-neural connective tissue increases from proximal to distal. The ratio of neural to non-neural tissue changes from 1 : 1 in the proximal plexus to 1 : 2 in the more distal plexus.^2,7 The presence of non-neural tissue may explain why injections within the epineurium rarely result in neural injury.⁶

Sonoanatomy

The shape and echogenicity of a nerve determine its ultrasound appearance. Structures that strongly reflect ultrasound waves generate large signal intensities and appear white or hyperechoic. In contrast, hypoechoic structures weakly reflect ultrasound waves and appear darker.⁸ Peripheral nerves show a mixture of hypoechoic and hyperechoic structures constituting a typical “honeycomb” structure.⁹ Hypoechoic structures seen with ultrasound correspond to neural tissue, while hyperechoic areas correlate with connective tissue.¹⁰ Ultrasound imaging of the proximal brachial plexus usually shows hypoechoic structures reflecting an oligofascicular pattern. Distal brachial plexus structures display a more hyperechoic, honeycomb appearance, reflecting a multifascicular pattern.⁷

Pharmacokinetics

Local anesthetics differ from other drugs because they are directly delivered at their site of action. Efficacy depends on the amount of LA that reaches the nerve and proximity to the nerve. Diffusion of local anesthetic is dependent on the amount of connective tissue and adipose tissue that is present in the area of the block.

Toxicity

Local anesthetic toxicity has been a concern since its first use in nerve blockade. Regardless of which local anesthetic is injected, traditional methods have required large volumes for successful regional anesthesia such as with brachial plexus blockade or Bier blocks. Frequent aspiration and incremental dosing are imperative, as is communication with the patient to detect early signs of potential intravenous injection before progression to signs and symptoms of toxicity. Factors such as drug dose, rate of absorption, biotransformation, and elimination of the drug from the circulation are determinants of the plasma concentration of local anesthetics. Fortunately, one of the benefits of ultrasound guidance in the use of regional anesthesia has been the decreased need for large volumes.¹³

High plasma levels may be a consequence of direct intravascular injection, plasma absorption, and/or certain underlying medical conditions of the patient (i.e., hypoproteinemia in renal or hepatic disease). Elevated intravascular levels of LA may result in minor CNS symptoms such as dizziness, ringing in the ears, and may proceed to more intense symptoms of loss of consciousness, and seizures. At even higher levels, cardiac arrhythmias ensue, including complete cardiovascular collapse. Use of LA in regional anesthesia demands an appreciation of these toxicities. Understanding agent specific toxic levels is vital – as much as the preparation for these unintended events.

Adjuncts such as epinephrine affect absorption and elimination. The use of epinephrine as a marker of intravascular injection is warranted in almost all situations. Exceptions include those cases where the vasoconstriction resulting from epinephrine may in fact compromise the blood flow to the area.

Physicians must be prepared with monitors, emergency drugs, and airway supplies to facilitate treatment of LA related toxicity. Toxicity related to LA can include but is not limited to oxygen desaturation, hypotension, bradycardia, and seizures. The extent of toxicity is determined by the specific drug’s intrinsic properties as well as the plasma level of the LA. The safety of LA is an essential aspect of regional anesthesia and is dependent on the skill of the physician, placement of the needle, the drug utilized, and patient health. All of these factors must be considered when determining the appropriate procedure and LA dose.¹⁴

Bupivacaine has been in use for many years and has the highest cardiotoxicity potential due to its intrinsic properties. Although the cardiac system is generally resistant to the effects of LA, bupivacaine is the notable exception. An overdose is more likely to result in cardiovascular collapse compared with other LA. This cardiac collapse is difficult to treat traditionally with ACLS/CPR alone. Recent case studies have demonstrated that IV infusion of Intralipid, an emulsified fat, may be successful in ameliorating cardiotoxicity associated with local anesthetics by acting as a “sink.”

Vasoconstrictors

With the addition of vasoconstrictors such as epinephrine or phenylephrine to the LA anesthetic solution, the systemic absorption rate of an LA can be decreased.¹⁴ The dose of bupivacaine increases from 3.5 to 4.0 mg/kg. This action of epinephrine is more significant with lidocaine as the upper limit increases from 3.0 to 7.0 mg/kg. Vasoconstrictors allow the physician to identify an intravascular injection sooner rather than later, due to the development of tachycardia with an intravascular injection.¹⁵ The block can be halted immediately, possibly preventing a more serious intravascular complication. Vasoconstrictors result in decreased plasma uptake and increased duration of local anesthetic effect.¹⁶

LA selection

The choice of a LA depends on toxicity (as discussed previously), duration of effect, and time to onset (Table 4.1).

Table 4.1 Commonly used agents in upper extremity regional anesthesia

Duration is important when considering surgical times when the block is the primary anesthetic. In such instances, the longer acting agents such as ropivacaine or bupivacaine have the distinct advantage as they often can outlast the surgery and offer the greater benefits of postoperative pain management.

Time to onset is also an important factor. Most of the time, regional anesthesia is done prior to the surgery and a fast time to onset is highly desirable. Each LA has its own time to onset or latency. Various factors can shorten this latency, including the addition of bicarbonate, higher dose, and needle location. The recent use of ultrasound allows for a more precise needle placement, which in turn allows for quicker onset.

The choice of local anesthetic affects the quality of the block, time to onset, and duration of action (Table 4.2).¹⁵ Quicker onset generally leads to quicker clearance. Lidocaine and mepivacaine, agents with intermediate duration, have a short latency period which is further shortened by the addition of bicarbonate as mentioned above. In comparison, bupivacaine and ropivacaine, two long acting agents used commonly in regional anesthesia, have a longer latency period, and are not able to mix with bicarbonate due to precipitation concerns. In order to obtain both the quicker onset characteristic and the longer duration, some may consider mixing two agents together to achieve the quicker onset effect and the longer duration of anesthesia/analgesia. These mixtures can be about 50 : 50, however, it can vary depending on the experience and training of the anesthesiologist. The toxicity of mixtures are additive and the mixture does not lower the overall toxicity.¹⁷

Table 4.2 Comparative pharmacology and current use of local anesthetics

Another consideration is the differential blockade, as nerves are blocked unequally and at different rates. Nerve blockade proceeds in the following order: sympathetic nerves, pin-prick sensation, touch, temperature, and finally motor.¹⁸ This is an important attribute of bupivacaine, as one can provide improved analgesia without much motor blockade in the postoperative period if an infusion is run at analgesic doses. Optimally, an LA with sensory selectivity is desired.

Digital block

Digital nerve blockade is easy to perform and provides useful anesthesia for a variety of surgical procedures or injuries isolated to a digit. Many techniques for performing digital nerve blocks have been described. These techniques rely on anesthesia of the volar common digital nerves derived from the median and ulnar nerves as well as the dorsal digital branches of the radial nerve.

The authors’ preferred technique for digital blockade involves volar and dorsal injections. The hand is placed palm up and the skin is cleansed. With a 25-gauge or 27-gauge needle, 5 mL of local anesthetic, usually 1% lidocaine or 0.25% bupivacaine, is injected into the subdermal space directly overlying the A1 pulley of the involved finger. A wheal is slowly raised. The hand is then turned palm down and an additional 2–3 mL of local anesthetic is injected into the subcutaneous tissue over the dorsum of the finger, just distal to the metacarpophalangeal joint.

The use of epinephrine in digital blocks has been a controversial subject. Despite the admonition against epinephrine use in numerous medical textbooks, no case of digital gangrene has been reported in the literature resulting solely from the use of epinephrine with a local anesthetic. A number of studies have demonstrated epinephrine can be safely used as an adjunct for digital block anesthesia. Lalonde et al. performed a randomized, prospective, blinded study with over 3000 consecutive cases and showed no cases of infarction, necrosis or tissue loss.²⁴ Epinephrine can be added to local anesthetics to lengthen the duration of action, lessen bleeding, reduce the need for a tourniquet, and reduce the risk of adverse systemic effects.²⁵

Wrist block

When the entire hand requires anesthesia, a wrist block is appropriate. A wrist block is the technique of blocking the median, ulnar and radial nerves at the level of the wrist. Similar to the digital block, it is easy to perform, has minimal complications, and is highly effective.

The patient should be supine with the arm abducted and wrist in slight dorsiflexion. The median nerve is located between the tendons of the palmaris longus (PL) and the flexor carpi radialis (FCR). The palmaris longus tendon is usually the more prominent of the two; the median nerve passes just radial to it. The ulnar nerve passes between the ulnar artery and tendon of the flexor carpi ulnaris (FCU). The tendon of the flexor carpi ulnaris is superficial to the ulnar nerve. The superficial branch of the radial nerve runs along the medial aspect of the brachioradialis muscle. It then passes between the tendon of the brachioradialis and radius to pierce the fascia on the dorsal aspect. Just above the radial styloid process, it gives digital branches for the dorsal skin of the thumb, index finger, and lateral half of the middle finger.

The median nerve is blocked by inserting a 25-gauge needle between the tendons of the palmaris longus and flexor carpi radialis at a 30° angle. The needle is inserted until it pierces the deep fascia. Piercing of the deep fascia may be appreciated with a fascial “click.” Local anesthetic, 3–5 mL, is injected. There should be no resistance to the injection as the local anesthetic travels up and down the carpal tunnel.

The ulnar nerve is anesthetized by transversely inserting the needle under the tendon of the FCU muscle close to its distal attachment proximal to the ulnar styloid. The needle is advanced 5–10 mm past the FCU tendon. The syringe is aspirated to confirm that it is not intravascular in the ulnar artery. Local anesthetic solution, 3–5 mL is then injected. A subcutaneous injection of 2–3 mL of local anesthesia just above the tendon of the FCU is also advisable in blocking the cutaneous branches of the ulnar nerve.

The radial nerve is essentially anesthetized with a field block. This blockade requires a more extensive infiltration because of the less predictable anatomic location and division into multiple, smaller, cutaneous branches. Local anesthetic, 5 mL is injected subcutaneously just above the radial styloid, aiming medially. The infiltration is then extended laterally, using an additional 5 mL of local anesthetic.

Intravenous regional anesthesia (Bier block)

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