Local Anesthetic Toxicity

Adverse effects of local anesthetics are usually caused by high plasma concentrations of the agent, which may result from one of the following:

• Inadvertent intravascular injection
• Excessive dose or rate of injection
• Delayed drug clearance
• Administration into vascular tissue

Patient factors can also affect toxicity. For example, because lidocaine is hepatically metabolized, liver dysfunction increases the risk of toxicity. Because lidocaine is also protein bound, low protein states may also increase risk. Acidosis increases the risk because it favors dissociation of lidocaine from plasma proteins. Interactions with other drugs (eg, cimetidine, beta-blockers) can also affect lidocaine drug levels.

Large doses of lidocaine (up to 55 mg/kg, versus the conventional maximum of 4.5 mg/kg) are used for tumescent anesthesia, in which a dilute local anesthetic solution is injected into subcutaneous tissue until it becomes firm and tense. This technique was developed to facilitate liposuction but its use has expanded to other dermatologic and plastic surgery procedures, as well as to endocrine and vascular surgeries. ^[3] Larger doses of lidocaine are generally tolerated during tumescent anesthesia for liposuction, probably due to coadministration of epinephrine, the low concentration of lidocaine in tumescent solution, and poor systemic absorption from subcutaneous fat. ^{[4, 5, 6, 7]}

Systemic toxicity of anesthetics most often involves the central nervous system (CNS) or the cardiovascular system. Concurrent administration of other drugs, such as benzodiazepines, may mask the development of CNS symptoms but not cardiovascular symptoms.

CNS toxicity is biphasic. The earlier manifestations are due to CNS excitation, with problems such as seizures. Subsequent manifestations include CNS depression with a cessation of convulsions and the onset of unconsciousness and respiratory depression or arrest.

Cardiovascular effects occur at higher serum concentrations of local anesthetics. These effects may include reentrant arrhythmias. Acceleration of the ventricular rate has been reported in patients with atrial arrhythmias. See Presentation.

Relatively rarely (< 1%), local anesthetic agents can affect the immune system, producing an immunoglobulin E (IgE)–mediated allergic reaction. Most cases are associated with the use of amino esters. Some anesthetics, particularly benzocaine, are associated with hematologic effects, namely methemoglobinemia.

The onset of action, potency, and duration of action of a local anesthetic are determined by the agent's pKa, lipid solubility, protein binding, and vasodilatory effects, along with tissue pH. Increasing the dose by administering a high concentration shortens onset while increasing duration of action, as well as increasing the possibility for adverse/toxic reactions.

The pKa of an agent is the primary factor that determines its onset of action. A lower pKa increases tissue penetration and shortens onset of action, because of increased lipid solubility of nonionized (uncharged) particles. A pKa that is closer to pH optimizes penetration. In addition, inflammation in the extracellular space may decrease pH and may slow onset of action. Site of administration is also a factor; onset is prolonged in areas with increased tissue or nerve sheath size. Addition of sodium bicarbonate speeds onset of action by increasing the fraction of agent in the nonionized state.

The following factors influence potency:

• High partition coefficients, which are reflective of lipophility, promote passage of the anesthetic into the lipid nerve membrane, enhancing potency

• Vasodilation promotes vascular absorption, thereby reducing locally available drug and decreasing potency

• Addition of sodium bicarbonate increases pH, thereby increasing nonionized particles, which are more lipid soluble, and increases apparent potency

• Most local anesthetic solutions that contain premixed epinephrine contain preservatives; in these solutions, the pH is adjusted lower to maintain the stability of epinephrine and antioxidants

The following factors influence duration of action:

• Addition of epinephrine to local anesthetic solutions prolongs duration of action by causing vasoconstriction and decreasing systemic absorption

• Degree of protein binding primarily determines duration of action; high protein binding increases duration

Anesthetic concentration and dilution

Drug concentration is expressed as a percentage (eg, bupivacaine 0.25%, lidocaine 1%). Percentage is measured in grams per 100 mL (ie, 1% is 1 g/100 mL [1000 mg/100 mL], or 10 mg per mL).

Calculate the mg/mL concentration quickly from the percentage by moving the decimal point 1 place to the right, as in the following examples:

• Bupivacaine 0.25% = 2.5 mg/mL
• Lidocaine 1% = 10 mg/mL

When epinephrine is combined in an anesthetic solution, the result is expressed as a dilution (eg, 1:100,000), as follows:

• 1:1000 means 1 mg per 1 mL (ie, 0.1%)
• 1:10,000 means 1 mg per 10 mL (ie, 0.01%)
• 1:2000 means 1 mg per 2 mL (ie, 0.05%)
• 1:20,000 means 1 mg per 20 mL (ie, 0.005%)
• 0.1 mL of 1:1000 epinephrine added to 9.9 mL of anesthetic solution = 1:100,000 dilution or 0.01 mg/mL

For higher dilutions, see Table 1, below.

Table 1. Epinephrine Content Examples (Open Table in a new window)

Toxicity mechanisms

CNS toxicity from local anesthetics manifests initially as CNS excitation, followed by CNS depression. This biphasic effect occurs because local anesthetics first block inhibitory CNS pathways (resulting in stimulation) and then eventually block both inhibitory and excitatory pathways (resulting in overall CNS inhibition).

Cardiovascular effects occur because these agents block sodium channels through a fast-in, slow-out mechanism that affects impulse conduction through the heart and nerve tissue. In the heart, this depresses Vmax (ie, the rate of depolarization during phase 0 of the cardiac action potential) and may lead to reentrant arrhythmias. Additionally, conduction through the sinus and atrioventricular nodes is suppressed.

Local anesthetics can be divided into 2 groups: the esters and the amides. See Table 2, below.

Table 2. Local Anesthetic Agents Used Commonly for Infiltrative Injection (Open Table in a new window)

The occurrence of numerous fatalities associated with the cardiovascular toxicity of bupivacaine prompted a search for less toxic long-acting local anesthetic agents. This search resulted in the development of levobupivacaine and ropivacaine.

Bupivacaine is a 50:50 racemic mixture of a dextrorotatory R-(+)-enantiomer and a levorotatory S-(-)-enantiomer. Clinical studies demonstrated that the S-(-)-enantiomer, levobupivacaine, has less potential for CNS and cardiovascular toxicity. In particular, the intravascular dose required to cause lethality is almost 78% greater for levobupivacaine compared with the R-(+) enantiomer.

Further clinical trials in the 1990s led to the introduction in 1996 of ropivacaine, a pure S-(-) enantiomer. Ropivacaine, like bupivacaine, has the capacity to produce differential blockade but has a better sensorimotor dissociation at lower doses. This long-acting amide is the first local anesthetic drug developed with initial extensive toxicological studies before its clinical release. Although ropivacaine may be associated with acute CNS and cardiovascular toxicity, the incidence appears to be extremely low.

Local anesthetic toxicity can occur because of inadvertent intravascular injection or dosing error. Intravascular injection can cause toxicity even if the anesthetic was administered within the recommended dose range.

The minimum doses of anesthetics in which adverse reactions have occurred are listed in Table 3, below.

Table 3. Minimum Intravenous Toxic Dose of Local Anesthetic in Humans ^[8] (Open Table in a new window)

In addition to high doses, high injection rates also increase the risk of adverse reactions to local anesthetics. Patient factors that increase risk include the following:

• Kidney or liver compromise
• Metabolic or respiratory acidosis
• Preexisting heart block or heart conditions
• Pregnancy
• Extremes of age
• Hypoxia

The frequency of local anesthetic toxicity is difficult to determine because these agents are used widely in a variety of settings, most reactions probably involve only minor symptoms, and most reactions are not reported. Systemic toxicity from local anesthetics has been estimated to occur in 0.03% of peripheral nerve blocks, or 0.27 episodes per 1,000 blocks. ^[9]  

In the United States in 2020, according to the American Association of Poison Control Centers (AAPCC), 1885 single exposures to lidocaine were reported, along with 2665 single exposures to other or unknown local and/or topical anesthetics. Of the lidocaine exposures reported by the AAPCC, 576 occurred in children younger than 6 years. ^[10]

If oxygenation, ventilation, and cardiac output are maintained, patients usually have a full recovery without sequelae. Without treatment, local anesthetic toxicity can result in seizures, respiratory depression or arrest, hypotension, cardiovascular collapse or cardiac arrest, and death. According to the AAPCC National Poison Data System 2019 Annual Report, 319 of the lidocaine exposures had no negative outcome; 244 exposures resulted in a minor outcome;96, in a moderate outcome; and 24, in a major outcome; 3 deaths were reported. ^[10]

Advise patients with adverse reactions to a specific anesthetic agent to avoid that specific anesthetic agent in the future and to alert medical personnel of the reaction. If a patient has experienced an adverse reaction to one class of anesthetic (ester or amide), risk for adverse reactions is higher for all agents in that class. However, if the episode involved seizures, the patient should be reassured that this does not indicate an increased risk for the development of a seizure disorder in the future.