In regional anesthesia local anesthetics are typically injected or applied very close to their intended site of action; thus their pharmacokinetic profiles are much more important determinants of elimination and toxicity than of their desired clinical effect.
Most mucous membranes (eg, ocular conjunctiva, tracheal mucosa) provide a minimal barrier to local anesthetic penetration, leading to a rapid onset of action. Intact skin, on the other hand, requires a high concentration of lipid-soluble local anesthetic base to ensure permeation and analgesia. EMLA cream consists of a 1:1 mixture of 5% lidocaine and 5% prilocaine bases in an oil-in-water emulsion. Dermal analgesia sufficient for beginning an intravenous line requires a contact time of at least 1 h under an occlusive dressing. Depth of penetration (usually 3-5 mm), duration of action (usually 1-2 h), and amount of drug absorbed depend on application time, dermal blood flow, keratin thickness, and total dose administered. Typically, 1-2 g of cream is applied per 10-cm2 area of skin, with a maximum application area of 2000 cm2 in an adult (100 cm2 in children weighing less than 10 kg). Split-thickness skin-graft harvesting, laser removal of portwine stains, lithotripsy, and circumcision have been successfully performed with EMLA cream. Side effects include skin blanching, erythema, and edema. EMLA cream should not be used on mucous membranes, broken skin, infants younger than 1 month of age, or patients with a predisposition to methemoglobinemia (see Biotransformation and Excretion, below).
Systemic absorption of injected local anesthetics depends on blood flow, which is determined by the following factors.
The rate of systemic absorption is related to the vascularity of the site of injection: intravenous (or intraarterial) > tracheal > intercostal > paracervical > epidural > brachial plexus > sciatic > subcutaneous.
Presence of Vasoconstrictors
Addition of epinephrine—or less commonly phenylephrine—causes vasoconstriction at the site of administration. The consequent decreased absorption reduces the peak local anesthetic concentration in blood, facilitates neuronal uptake, enhances the quality of analgesia, prolongs the duration of action, and limits toxic side effects. Vasoconstrictors have more pronounced effects on shorter-acting than longer-acting agents. For example, addition of epinephrine to lidocaine usually extends the duration of anesthesia by at least 50%, but epinephrine has little or no effect on the duration of bupivacaine peripheral nerve blocks. Epinephrine and clonidine can also augment analgesia through activation of α2-adrenergic receptors.
More lipid-soluble local anesthetics that are highly tissue bound are also more slowly absorbed. The agents also vary in their intrinsic vasodilator properties.
Distribution depends on organ uptake, which is determined by the following factors.
The highly perfused organs (brain, lung, liver, kidney, and heart) are responsible for the initial rapid uptake (α phase), which is followed by a slower redistribution (β phase) to moderately perfused tissues (muscle and gut). In particular, the lung extracts significant amounts of local anesthetic; consequently, the threshold for systemic toxicity involves much lower doses following arterial injections than venous injections (and children with right-to-left shunts are more susceptible to toxic side effects of lidocaine injected as an antiarrhythmic agent).
Tissue/Blood Partition Coefficient
Increasing lipid solubility is associated with greater plasma protein binding and also greater tissue uptake from an aqueous compartment.
Muscle provides the greatest reservoir for distribution of local anesthetic agents in the bloodstream because of its large mass.
Biotransformation and Excretion
The biotransformation and excretion of local anesthetics is defined by their chemical structure.
Ester local anesthetics are predominantly metabolized by pseudocholinesterase (plasma cholinesterase or butyrylcholinesterase). Ester hydrolysis is very rapid, and the water-soluble metabolites are excreted in the urine. Procaine and benzocaine
are metabolized to p
-aminobenzoic acid (PABA), which has been associated with rare anaphylactic reactions. Patients with genetically abnormal pseudocholinesterase would theoretically be at increased risk for toxic side effects, as metabolism is slower, but clinical evidence for this is lacking. Cerebrospinal fluid lacks esterase enzymes, so the termination of action of intrathecally injected ester local anesthetics, eg, tetracaine
, depends on their redistribution into the bloodstream, as it does for all other nerve blocks. In contrast to other ester anesthetics, cocaine
is partially metabolized (N-methylation and ester hydrolysis) in the liver and partially excreted unchanged in the urine.
Amide local anesthetics are metabolized (N-dealkylation and hydroxylation) by microsomal P-450 enzymes in the liver. The rate of amide metabolism depends on the specific agent (prilocaine > lidocaine > mepivacaine > ropivacaine > bupivacaine) but overall is consistently slower than ester hydrolysis of ester local anesthetics. Decreases in hepatic function (eg, cirrhosis of the liver) or liver blood flow (eg, congestive heart failure, β blockers, or H2-receptor blockers) will reduce the metabolic rate and potentially predispose patients to having greater blood concentrations and a greater risk of systemic toxicity. Very little unmetabolized local anesthetic is excreted by the kidneys, although water-soluble metabolites are dependent on renal clearance.
Prilocaine is the only local anesthetic that is metabolized to o-toluidine, which produces methemoglobinemia in a dose-dependent fashion. Classical teaching was that a defined minimal dose of prilocaine was needed to produce clinically important methemoglobinemia (in the range of 10 mg/kg); however, recent studies have shown that younger, healthier patients develop medically important methemoglobinemia after lower doses of prilocaine (and at lower doses than needed in older, sicker patients). Prilocaine is generally not used for epidural anesthesia during labor or in larger doses in patients with limited cardiopulmonary reserve. Benzocaine, a common ingredient in topical local anesthetic sprays, can also cause dangerous levels of methemoglobinemia. For this reason, many hospitals no longer permit benzocaine spray during endoscopic procedures. Treatment of medically important methemoglobinemia includes intravenous methylene blue (1-2 mg/kg of a 1% solution over 5 min). Methylene blue reduces methemoglobin (Fe3+) to hemoglobin (Fe2+).
Because inhibition of voltage-gated Na channels from circulating local anesthetics might affect action potentials in neurons throughout the body as well as impulse generation and conduction in the heart, it is not surprising that local anesthetics in high circulating concentrations could have the propensity for systemic toxicity. Although organ system effects are discussed for these drugs as a group, individual drugs differ.
Potency at most toxic side effects correlates with potency at nerve blocks. Maximum safe doses are listed in Table 16-3, but it must be recognized that the maximum safe dose depends on the patient, the specific nerve block, the rate of injection, and a long list of other factors. In other words, tables of purported maximal safe doses are nearly nonsensical. Mixtures of local anesthetics should be considered to have additive toxic effects; therefore, a solution containing 50% of the toxic dose of lidocaine and 50% of the toxic dose of bupivacaine if injected by accident intravenously will produce toxic effects.
Table 16-3 Clinical Use of Local Anesthetic Agents. ||Download (.pdf)
Table 16-3 Clinical Use of Local Anesthetic Agents.
|Agent||Techniques||Concentrations Available||Maximum Dose (mg/kg)||Typical Duration of Nerve Blocks1|
|Chloroprocaine||Epidural, infiltration, peripheral nerve block, spinal4||1%, 2%, 3%||12||Short|
|Procaine||Spinal, local infiltration||1%, 2%, 10%||12||Short|
|Tetracaine (amethocaine)||Spinal, topical (eye)||0.2%, 0.3%, 0.5%, 1%, 2%||3||Long|
|Bupivacaine||Epidural, spinal, infiltration, peripheral nerve block||0.25%, 0.5%, 0.75%||3||Long|
|Lidocaine (lignocaine)||Epidural, spinal, infiltration, peripheral nerve block, intravenous regional, topical||0.5%, 1%, 1.5%, 2%, 4%, 5%||Medium|
|Mepivacaine||Epidural, infiltration, peripheral nerve block, spinal||1%, 1.5%, 2%, 3%||Medium|
|Prilocaine||EMLA (topical), epidural, intravenous regional (outside North America)||0.5%, 2%, 3%, 4%||8||Medium|
|Ropivacaine||Epidural, spinal, infiltration, peripheral nerve block||0.2%, 0.5%, 0.75%, 1%||3||Long|
The central nervous system is vulnerable to local anesthetic toxicity and is the site of premonitory signs of rising blood concentrations in awake patients. Early symptoms include circumoral numbness, tongue paresthesia, dizziness, tinnitus, and blurred vision. Excitatory signs include restlessness, agitation, nervousness, garrulousness, and a feeling of “impending doom.” Muscle twitching heralds the onset of tonic-clonic seizures. Still higher blood concentrations may produce central nervous system depression (eg, coma and respiratory arrest). The excitatory reactions are thought to be the result of selective blockade of inhibitory pathways. Potent, highly lipid-soluble local anesthetics produce seizures at lower blood concentrations than less potent agents. Benzodiazepines and hyperventilation raise the threshold of local anesthetic-induced seizures. Both respiratory and metabolic acidosis reduce the seizure threshold. Propofol
(0.5-2 mg/kg) quickly and reliably terminates seizure activity (as do comparable doses of benzodiazepines or barbiturates). Maintaining a clear airway with adequate ventilation and oxygenation is of key importance.
Infused local anesthetics have a variety of actions. Systemically administered local anesthetics such as lidocaine (1.5 mg/kg) can decrease cerebral blood flow and attenuate the rise in intracranial pressure that may accompany intubation in patients with decreased intracranial compliance. Infusions of lidocaine and procaine have been used to supplement general anesthetic techniques, as they are capable of reducing the MAC of volatile anesthetics by up to 40%. Infusions of lidocaine inhibit inflammation and reduce postoperative pain. Infused lidocaine reduces postoperative opioid requirements sufficiently to reduce length of stay after colorectal or open prostate surgery.
Cocaine stimulates the central nervous system and at moderate doses usually causes a sense of euphoria. An overdose is heralded by restlessness, emesis, tremors, convulsions, arrhythmias, respiratory failure, and cardiac arrest.
Local anesthetics temporarily inhibit neuronal function. In the past, unintentional injection of large volumes of chloroprocaine into the subarachnoid space (during attempts at epidural anesthesia), produced total spinal anesthesia and marked hypotension, and caused prolonged neurological deficits. The cause of this neural toxicity may be direct neurotoxicity or a combination of the low pH of chloroprocaine and a preservative, sodium bisulfite. The latter has been replaced in some formulations by an antioxidant, a derivative of disodium ethylenediaminetetraacetic acid (EDTA). Chloroprocaine has also been occasionally associated with severe back pain following epidural administration. The etiology is unclear. Chloroprocaine is available in a preservative-free formulation, which has been used in recent studies safely and successfully for short duration, outpatient spinal anesthetics.
Administration of 5% lidocaine has been associated with neurotoxicity (cauda equina syndrome) following infusion through small-bore catheters used in continuous spinal anesthesia. This may be due to pooling of drug around the cauda equina, resulting in high concentrations and permanent neuronal damage. Animal data suggest that the extent of histological evidence of neurotoxicity following repeat intrathecal injection is lidocaine = tetracaine > bupivacaine > ropivacaine.
Transient neurological symptoms, which consist of dysesthesia, burning pain, and aching in the lower extremities and buttocks, have been reported following spinal anesthesia with a variety of local anesthetic agents, most commonly after use of lidocaine for outpatient spinal anesthesia in men undergoing surgery in the lithotomy position. These symptoms have been attributed to radicular irritation and typically resolve within 1-4 weeks. Many clinicians have substituted 2-chloroprocaine, mepivacaine, or small doses of bupivacaine for lidocaine in spinal anesthesia in the hope of avoiding these transient symptoms.
Lidocaine depresses hypoxic drive (the ventilatory response to low PaO2). Apnea can result from phrenic and intercostal nerve paralysis or depression of the medullary respiratory center following direct exposure to local anesthetic agents (as may occur after retrobulbar blocks; see Chapter 36). Apnea after administration of a “high” spinal or epidural anesthetic is nearly always the result of hypotension, rather than phrenic block. Local anesthetics relax bronchial smooth muscle. Intravenous lidocaine (1.5 mg/kg) may be effective in blocking the reflex bronchoconstriction sometimes associated with intubation. Lidocaine (or any other inhaled agent) administered as an aerosol can lead to bronchospasm in some patients with reactive airway disease.
All local anesthetics depress myocardial automaticity (spontaneous phase IV depolarization). Myocardial contractility and conduction velocity are also depressed at higher concentrations. These effects result from direct cardiac muscle membrane changes (ie, cardiac Na channel blockade) and in intact organisms from inhibition of the autonomic nervous system. All local anesthetics except cocaine produce smooth muscle relaxation at higher concentrations, which may cause some degree of arteriolar vasodilation. At low concentrations all local anesthetics inhibit nitric oxide, causing vasoconstriction. At increased blood concentrations the combination of arrhythmias, heart block, depression of ventricular contractility, and hypotension may culminate in cardiac arrest.
Major cardiovascular toxicity usually requires about three times the local anesthetic concentration in blood as that required to produce seizures. Cardiac arrhythmias or circulatory collapse are the usual presenting signs of local anesthetic overdose during general anesthesia. Particularly in awake subjects, signs of transient cardiovascular stimulation (tachycardia and hypertension) may occur with central nervous system excitation at local anesthetic concentrations producing central nervous system toxic side effects.
Intravenous amiodarone provides effective treatment for some forms of ventricular arrhythmias. Myocardial contractility and arterial blood pressure are generally unaffected by the usual intravenous doses. The hypertension associated with laryngoscopy and intubation is attenuated in some patients by intravenous administration of lidocaine (1.5 mg/kg) 1-3 min prior to instrumentation. On the other hand, overdoses of lidocaine can lead to marked left ventricular contractile dysfunction.
Unintentional intravascular injection of bupivacaine
during regional anesthesia may produce severe cardiovascular toxicity, including left ventricular depression, atrioventricular heart block, and life-threatening arrhythmias such as ventricular tachycardia and fibrillation. Pregnancy, hypoxemia, and respiratory acidosis are predisposing risk factors. Young children may also be at increased risk of toxicity. Multiple studies have demonstrated that bupivacaine
is associated with more pronounced changes in conduction and a greater risk of terminal arrhythmias than comparable doses of lidocaine
, and bupivacaine
have chiral carbons and therefore can exist in either of two optical isomers (enantiomers). The R(+) optical isomer of bupivacaine
blocks more avidly and dissociates more slowly from cardiac Na channels than does the S(−) optical isomer. Resuscitation from bupivacaine-induced cardiac toxicity is often difficult and resistant to standard resuscitation drugs. Recent reports suggest that bolus administration of nutritional lipid solutions at 1.5 mL/kg can resuscitate bupivacaine-intoxicated patients who do not respond to standard therapy. Ropivacaine
shares many physicochemical properties with bupivacaine
. Onset time and duration of action are similar, but ropivacaine
produces less motor block when injected at the same volume and concentration as bupivacaine
(which may reflect an overall lower potency as compared with bupivacaine
appears to have a greater therapeutic index than bupivacaine
. This improved safety profile likely reflects its formulation as a pure S(−) isomer—that is, having no R(+) isomer—as opposed to racemic bupivacaine
. Levobupivacaine, the S(−) isomer of bupivacaine
, which is no longer available in the United States, was reported to have fewer cardiovascular and cerebral side effects than the racemic mixture; studies suggest its cardiovascular effects may approximate those of ropivacaine
Cocaine’s cardiovascular reactions are unlike those of any other local anesthetic. Adrenergic nerve terminals normally reabsorb norepinephrine after its release. Cocaine inhibits this reuptake, thereby potentiating the effects of adrenergic stimulation. Cardiovascular responses to cocaine include hypertension and ventricular ectopy. The latter contraindicated its use in patients anesthetized with halothane. Cocaine-induced arrhythmias have been successfully treated with adrenergic and Ca channel antagonists. Cocaine produces vasoconstriction when applied topically and is a useful agent to reduce pain and epistaxis related to nasal intubation in awake patients.
True hypersensitivity reactions to local anesthetic agents—as distinct from systemic toxicity caused by excessive plasma concentration—are uncommon. Esters appear more likely to induce a true allergic reaction (due to IgG or IgE antibodies) especially if they are derivatives (eg, procaine or benzocaine
) of p
-aminobenzoic acid, a known allergen. Commercial multidose preparations of amides often contain methylparaben
, which has a chemical structure vaguely similar to that of PABA. As a consequence, generations of anesthesiologists have speculated whether this preservative may be responsible for most of the apparent allergic responses to amide agents. The signs and treatment of allergic drug reactions are discussed in Chapter 55.
When directly injected into skeletal muscle (eg, trigger-point injection treatment of myofascial pain), local anesthetics are mildly myotoxic. Regeneration usually occurs 3-4 weeks after local anesthetic injection into muscle. Concomitant steroid or epinephrine injection worsens the myonecrosis.
Lidocaine mildly depresses normal blood coagulation (reduced thrombosis and decreased platelet aggregation) and enhances fibrinolysis of whole blood as measured by thromboelastography. These effects may underlie the reduced efficacy of an epidural autologous blood patch shortly after local anesthetic administration and the lower incidence of embolic events in patients receiving epidural anesthetics (in older studies of patients not receiving prophylaxis against deep vein thrombosis).
Local anesthetics potentiate nondepolarizing muscle relaxant blockade in laboratory experiments, but the clinical importance of this observation is unknown (and probably nil).
Succinylcholine and ester local anesthetics depend on pseudocholinesterase for metabolism. Concurrent administration might conceivably increase the time that both drugs remain unmetabolized in the bloodstream. There is likely no actual clinical importance of this potential interaction.
Dibucaine, an amide local anesthetic, inhibits pseudocholinesterase, and the extent of inhibition by dibucaine defines one family of genetically abnormal pseudocholinesterases (see Chapter 11). Pseudocholinesterase inhibitors (eg, organophosphate poisons) can prolong the metabolism of ester local anesthetics (see Table 11-3).
Histamine (H2) receptor blockers and β blockers (eg, propranolol) decrease hepatic blood flow and lidocaine clearance. Opioids potentiate epidural and spinal analgesia produced by local anesthetics. Similarly α2-adrenergic agonists (eg, clonidine) potentiate local anesthetic analgesia produced after epidural or peripheral nerve block injections. Epidural chloroprocaine may interfere with the analgesic actions of neuraxial morphine, notably after cesarean delivery.