3. LOCAL ANESTHETICS TABLE 3-1 INTRODUCTION NERVE CLASSIFICATION AND SEQUENCE OF BLOCK WHEN EXPOSED TO LOCAL ANESTHETIC Fiber Type Myelin Diameter Function Conduction Velocity Time to Block Compared to general anesthesia with opioid- (µm) based perioperative pain management, regional anesthesia can provide benefits of superior pain A-a Yes 12–20 Somatic motor and proprioception Fast Slow control, improved patient satisfaction, decreased A-β Yes 5–12 Light touch and pressure stress response to surgery, reduced operative and postoperative blood loss, diminished postopera- A-γ Yes 3–6 Muscle spindle (stretch) tive nausea and vomiting, and decreased logistic requirements. This chapter will review the most A-d Yes 1–4 Pain (fast-localizing), temperature, firm touch common local anesthetics and adjuncts used in the B Yes 1–3 Preganglionic autonomic US military for the application of regional anesthetic techniques, with particular emphasis on medica- C No 0.3–1.3 Pain (nonlocalizing ache), temperature, touch, tions used for peripheral nerve block (PNB) and postganglionic autonomic Slow Fast continuous peripheral nerve block (CPNB). BASIC REVIEW OF LOCAL ANESTHETICS achieved by using certain local anesthetics and de- state increases and the onset of the block is slowed. Local anesthetics are valued for the ability to livering specific concentrations to the nerve. Once the local anesthetic has passed through the prevent membrane depolarization of nerve cells. Local anesthetic structure is characterized by cell membrane, it is exposed to the more acidic axio- Local anesthetics prevent depolarization of nerve having both lipophilic and hydrophilic ends (ie, am- plasmic side of the nerve, favoring the ionized state. cells by binding to cell membrane sodium channels phipathic molecules) connected by a hydrocarbon The ionized form of the molecule binds the sodium and inhibiting the passage of sodium ions. The chain. The linkage between the hydrocarbon chain channel and blocks conduction. sodium channel is most susceptible to local anes- and the lipophilic aromatic ring classifies local an- The potency of local anesthetics is determined thetic binding in the open state, so frequently stimu- esthetics as being either an ester (–CO) local anes- by lipid solubility. As lipid solubility increases, the lated nerves tend to be more easily blocked. The thetic, in which the link is metabolized in the serum ability of the local anesthetic molecule to penetrate ability of a given local anesthetic to block a nerve by plasma cholinesterase, or an amide (–NHC) connective tissue and cell membranes increases, is related to the length of the nerve exposed, the local anesthetic, in which the link is metabolized causing the increase in potency. diameter of the nerve, the presence of myelination, primarily in the liver. The duration of action for local anesthetics is de- and the anesthetic used. Small or myelinated nerves The functional characteristics of local anesthetics termined by protein binding. Local anesthetics with are more easily blocked than large or unmyelinated are determined by the dissociation constant (pKa), high affinity for protein binding remain bound to nerves (Table 3-1). Myelinated nerves need to be lipid solubility, and protein binding. The pKa is the nerve membranes longer, resulting in an increased a blocked only at nodes of Ranvier (approximately pH at which a solution of local anesthetic is in equi- duration of action. Binding to serum 1-acid glyco- three consecutive nodes) for successful preven- librium, with half in the neutral base (salt) and half proteins and other proteins decreases the availabil- tion of further nerve depolarization, requiring a in the ionized state (cation). Most local anesthetics ity of free drug in the blood, reducing the potential significantly smaller portion of these nerves to be have a pKa greater than 7.4. Because the neutral base for toxicity in the primary organs. The free fraction exposed to the anesthetic. Differential blockade to form of the local anesthetic is more lipophilic, it can of local anesthetic in the blood is increased in condi- d achieve pain and temperature block (A- , C fibers) penetrate nerve membranes faster. As the pKa of a tions of acidosis or decreased serum protein, thus while minimizing motor block (A-a fibers) can be local anesthetic rises, the percentage in the ionized heightening the potential for toxicity. 11 3 LOCAL ANESTHETICS LOCAL ANESTHETIC TOXICITY thetic, the greater potential it has for causing cardiac Levorotatory enantiomers of local anesthetics are Shortly after Carl Koller introduced cocaine for depression and arrhythmias. typically less toxic than dextrorotatory enantiomers. regional anesthesia of the eye in 1884 and physi- Local anesthetics have been shown to be Because ropivacaine is less cardiotoxic than bupiva- cians worldwide began injecting cocaine near myotoxic in vivo, although little evidence is caine, it is the preferred long-acting local anesthetic peripheral nerves, reports of “cocaine poisoning” available to determine this phenomenon’s clinical for PNB anesthesia for many providers. The motor- began appearing in the literature. Local anesthet- relevance. Nevertheless, practitioners using local block–sparing properties associated with ropiva- ics are indispensable to the successful practice anesthetic for PNB or CPNB should consider the caine spinal and epidural analgesia may provide an of regional anesthesia, and physicians who use myotoxic potential of these medications in cases advantage over bupivacaine. Ropivacaine is consid- these techniques must be familiar with the signs of unexplained skeletal muscle dysfunction. Local ered the safest long-acting local anesthetic currently and symptoms of local anesthetic toxicity. Initial anesthetics have also been demonstrated to be neu- available, but it is not completely safe (cardiovascu- excitatory symptoms of local anesthetic toxicity rotoxic in vitro, but the clinical significance of these lar collapse has been reported with its use), and all are manifestations of escalating drug concentra- findings remains theoretical. standard precautions should be observed with its tion in the central nervous system, specifically the A variety of anesthesia textbooks publish use. Ropivacaine is the long-acting local anesthetic amygdala. Increasing local anesthetic concentra- maximum recommended dosages for local anesthet- of choice at Walter Reed Army Medical Center tion begins to block inhibitory pathways in the ics in an attempt to prevent high dose injections because of its favorable safety profile and efficacy amygdala, resulting in unopposed excitatory leading to toxicity. Because local anesthetic toxicity when used in a variety of regional anesthetics (Table neuron function. This process is manifested clini- is related more to intravascular injection than to total 3-3). cally as symptoms of muscular twitching, visual dose, some physicians have suggested maximum disturbance, tinnitus, light-headedness, or tongue dose recommendations are irrelevant. It is reasonable Bupivacaine. Bupivacaine (Marcaine, Sensorcaine; and lip numbness. Extreme patient anxiety, to assume that intravascular injections will occur, both made by AstraZeneca, London, United screaming, or concerns about imminent death are and practitioners of regional anesthesia should select Kingdom) has a pKa of 8.1. With an extensive also suggestive of toxicity. As the blood concen- techniques designed to minimize their occurrence, history of successful use, bupivacaine is the tration of local anesthetic increases, these initial while maintaining preparation for appropriate treat- long-acting local anesthetic to which others are symptoms, without intervention, will progress to ments to use when such injections occur. The site compared. Although a bupivacaine block is long generalized tonic-clonic convulsions, coma, respi- of injection also affects the blood concentrations of acting, it also has the longest latency to onset of ratory arrest, and death. local anesthetic. Blood absorption of local anesthetic block. Bupivacaine is noted for having a propensity The cardiovascular system, though significantly varies at different injection sites according to the for sensory block over motor block (differential sen- more resistant to local anesthetic toxicity than the following continuum (from greatest to least absorp- sitivity) at low concentrations. These factors, as well central nervous system, will exhibit arrhythmias tion): intercostal > caudal > epidural > brachial as the low cost of bupivacaine compared to newer and eventual collapse as local anesthetic concentra- plexus > femoral–sciatic > subcutaneous > intraartic- long-acting local anesthetics, have established bupi- tions increase. The relationship between the blood ular > spinal. Taking these factors into consideration, vacaine as the long-acting local anesthetic of choice concentration of a particular local anesthetic that recommended techniques and conditions for local in many institutions. When long-duration analgesia results in circulatory collapse and the concentration anesthetic injection are listed in Table 3-2. is required, the use of bupivacaine for low-volume needed to cause convulsions is called the circula- infiltration or spinal anesthesia is well established. tory collapse ratio. As this ratio becomes smaller, Ropivacaine. Ropivacaine (Naropin, Abraxis In spite of the popularity of bupivacaine for the interval between convulsions and circulatory BioScience Inc, Schaumburg,