Residency · Residency · Anesthesiology

Local Anesthetic Pharmacology and Systemic Toxicity (LAST)

Structure-Activity Relationships

Basic Chemical Structure

All local anesthetics share a common three-part molecular structure: an aromatic ring (the lipophilic portion), an intermediate linkage, and an amine group (the hydrophilic portion). The intermediate linkage defines the two major classes. Ester-type local anesthetics have an ester bond connecting these components and include procaine, chloroprocaine, tetracaine, and cocaine. Amide-type local anesthetics have an amide bond and include lidocaine, bupivacaine, ropivacaine, mepivacaine, and levobupivacaine. A useful mnemonic is that amide agents have two letter "i"s in their name before "-caine" (lidocaine, bupivacaine).

Key Physicochemical Properties

Three physicochemical properties determine the clinical behavior of local anesthetics. The pKa determines onset speed: agents with a pKa closer to physiologic pH (7.4) have a greater un-ionized fraction at physiologic pH, which crosses nerve membranes more readily and produces a faster onset. Lidocaine (pKa 7.9) has a rapid onset, while bupivacaine (pKa 8.1) has a slower onset. Chloroprocaine has a pKa of 9.1, yet achieves rapid onset because it is used at high concentrations. Lipid solubility determines potency: bupivacaine is approximately 8 times more lipid-soluble than lidocaine and roughly 4 times more potent. Protein binding determines duration of action: bupivacaine is approximately 95% protein-bound and has a long duration, while lidocaine at approximately 65% protein binding has an intermediate duration.

Mechanism of Action

Local anesthetics block voltage-gated sodium channels from the intracellular side, preferentially binding to the inactivated (closed) state of the channel. They exhibit use-dependent (frequency-dependent) blockade, meaning that more active nerves are blocked more effectively. To reach the intracellular binding site, the drug must cross the cell membrane in its un-ionized form.

Differential Nerve Blockade

Fiber Sensitivity

Different nerve fiber types vary in their sensitivity to local anesthetic blockade. Small, myelinated fibers (A-delta and B fibers) are blocked before large myelinated fibers (A-alpha and A-beta). Unmyelinated C fibers are blocked at intermediate concentrations. The clinical order of blockade is: sympathetic (B fibers) first, then pain and temperature sensation (A-delta and C fibers), followed by proprioception (A-beta), and finally motor function (A-alpha).

Clinical Significance

This differential sensitivity explains why epidural analgesia can provide pain relief while preserving motor function at lower concentrations. It also explains why the sympathetic blockade from neuraxial anesthesia extends approximately 2 dermatomes above the sensory level, while motor blockade extends approximately 2 dermatomes below.

<image>Detailed diagram of local anesthetic molecular structure showing the aromatic ring, intermediate linkage (ester vs. amide bond highlighted), and amine group. Below, a comparison table of common local anesthetics (lidocaine, bupivacaine, ropivacaine, chloroprocaine) with their pKa, lipid solubility, protein binding, onset time, duration, and maximum doses with and without epinephrine.</image>

Commonly Used Local Anesthetics

Lidocaine

Lidocaine has a rapid onset (1-3 minutes) and an intermediate duration of 60-120 minutes, which is extended with the addition of epinephrine. The maximum dose is 4.5 mg/kg without epinephrine and 7 mg/kg with epinephrine. Its versatility makes it suitable for infiltration, peripheral nerve blocks, epidural anesthesia, spinal anesthesia (though its use intrathecally is controversial due to the risk of transient neurological symptoms), IV regional anesthesia (Bier block), topical application, and as an antiarrhythmic agent.

Bupivacaine

Bupivacaine has an intermediate onset (5-10 minutes) and a long duration of 4-12 hours for peripheral blocks. The maximum dose is 2.5 mg/kg regardless of whether epinephrine is added. It is the most potent commonly used local anesthetic. Its cardiotoxicity risk is significantly higher than that of other agents, as discussed below. Bupivacaine is available as a racemic mixture; the S-enantiomer, levobupivacaine, has a reduced cardiotoxicity profile.

Ropivacaine

Ropivacaine is a pure S-enantiomer that is slightly less potent than bupivacaine but offers reduced cardiotoxicity and CNS toxicity. It demonstrates preferential sensory over motor blockade, making it particularly useful for labor analgesia where motor preservation is desirable. The maximum dose is 3 mg/kg, and peripheral block duration is 4-10 hours.

Chloroprocaine (2-Chloroprocaine)

Chloroprocaine is an ester-type agent with very rapid onset and ultra-short duration (30-60 minutes) due to rapid hydrolysis by plasma cholinesterase. Its extremely low systemic toxicity risk makes it the ideal agent for epidural top-up when an urgent cesarean section is needed in a laboring patient, using a 3% solution. Earlier formulations were associated with neurotoxicity, but this was related to preservatives and is not a concern with modern preservative-free preparations.

Mepivacaine

Mepivacaine is similar to lidocaine but with a slightly longer duration. It is not used in obstetric practice because of slow fetal metabolism. It is commonly used for dental blocks and peripheral nerve blocks.

Adjuvants to Local Anesthetics

Epinephrine

Epinephrine added to local anesthetic solutions reduces systemic absorption through local vasoconstriction, thereby prolonging the block and reducing peak plasma levels. The typical concentration is 1:200,000 (5 mcg/mL). It also serves as an intravascular injection marker, producing tachycardia and hypertension if the anesthetic is inadvertently injected into a blood vessel. The traditional teaching to avoid epinephrine in end-artery territories (digits, penis, ear) has been revised for digital blocks, where dilute epinephrine is now considered safe.

Dexamethasone

Perineural dexamethasone prolongs block duration by 6-8 hours. Interestingly, intravenous dexamethasone may provide a similar degree of prolongation, suggesting a systemic mechanism. The typical dose is 4-8 mg perineural or IV. The exact mechanism remains debated, with proposed explanations including local anti-inflammatory effects, direct neural effects, and systemic anti-nociceptive actions.

Clonidine

Clonidine is an alpha-2 agonist that prolongs both sensory and motor blockade when added to local anesthetic solutions at doses of 0.5-1 mcg/kg. Side effects include sedation, bradycardia, and hypotension.

Sodium Bicarbonate

Adding sodium bicarbonate raises the pH of the local anesthetic solution, increasing the un-ionized fraction and thereby speeding onset. This is most effective for lidocaine (1 mEq per 10 mL) but less useful for bupivacaine, which may precipitate in alkalinized solution.

Local Anesthetic Systemic Toxicity (LAST)

Epidemiology

LAST occurs at an estimated rate of 1-2 per 1,000 peripheral nerve blocks. The risk is highest with inadvertent intravascular injection, excessive dosing, and injection at highly vascular sites. The ranking of absorption rates by injection site, from highest to lowest, is: intercostal, caudal, epidural, brachial plexus, subcutaneous. Presentation can be immediate (with intravascular injection) or delayed up to 60 minutes (from gradual tissue absorption).

Risk Factors

Patients at increased risk for LAST include those at extremes of age (neonates and the elderly), patients with low muscle mass or low plasma protein levels, those with hepatic dysfunction (which impairs amide metabolism) or cholinesterase deficiency (which impairs ester metabolism), patients with cardiac disease (especially heart failure and conduction abnormalities), and those with acidosis or hypoxia (which lower the seizure threshold and increase ionized fraction trapping). Pregnancy increases cardiac sensitivity to bupivacaine.

CNS Toxicity (Occurs First)

CNS toxicity typically precedes cardiovascular toxicity and progresses from excitation to depression as blood levels rise. The progression follows a characteristic sequence: perioral numbness, tongue paresthesia, metallic taste, and tinnitus appear first, followed by lightheadedness, visual disturbances, and agitation, then muscle twitching and tremors, then generalized tonic-clonic seizures, and finally CNS depression, loss of consciousness, and respiratory arrest. The critical exception to this orderly progression is bupivacaine, which can cause cardiac arrest as the first manifestation of toxicity, without preceding CNS symptoms.

Cardiovascular Toxicity

Cardiovascular toxicity is biphasic, beginning with initial hypertension and tachycardia before progressing to hypotension, bradycardia, and conduction delays. Bupivacaine cardiotoxicity is particularly dangerous because the drug binds sodium channels with a "fast in, slow out" kinetic pattern, meaning it dissociates slowly during diastole. This leads to progressive QRS prolongation, reentrant arrhythmias, and ultimately VT/VF that is extremely difficult to resuscitate with standard ACLS protocols -- hence the rationale for lipid emulsion therapy. The CC/CNS ratio (the ratio of the dose causing cardiovascular collapse to the dose causing seizures) is much lower for bupivacaine (approximately 2-3) than for lidocaine (approximately 7), meaning cardiac arrest can occur with minimal warning. Ropivacaine and levobupivacaine have wider safety margins.

<image>Two-panel illustration of LAST progression. Top panel shows a dose-response curve with escalating CNS symptoms (tinnitus, perioral numbness at low levels, seizures at intermediate levels) and cardiovascular symptoms (arrhythmias, cardiac arrest at high levels). Bottom panel shows a comparison of bupivacaine versus lidocaine sodium channel binding kinetics, illustrating bupivacaine's slow dissociation from the channel during diastole and its narrow CC/CNS ratio.</image>

LAST Management: ASRA Checklist

Immediate Actions

The first steps are to stop the injection of local anesthetic, call for help and obtain the LAST rescue kit (containing lipid emulsion), and manage the airway with 100% oxygen, securing the airway if necessary.

Seizure Management

Benzodiazepines are the first-line treatment (midazolam 2-4 mg IV). Propofol should be avoided if hemodynamic instability is present because of its cardiac depressant effects, though a small dose is acceptable if the patient is hemodynamically stable. Succinylcholine should be avoided because it only treats the muscular manifestation of the seizure, not the ongoing neural seizure activity; the continued neural seizure generates acidosis, which worsens toxicity.

Cardiovascular Support

ACLS protocols are followed with important modifications. Vasopressin should be avoided based on ASRA recommendations from animal data. Epinephrine doses should be reduced to less than 1 mcg/kg because large doses may worsen arrhythmias and impair lipid rescue. Calcium channel blockers, beta-blockers, and lidocaine (as an antiarrhythmic) should all be avoided. If an antiarrhythmic is needed, amiodarone is the preferred agent.

20% Lipid Emulsion (Intralipid)

Lipid emulsion is the cornerstone of LAST rescue. The protocol begins with a bolus of 1.5 mL/kg IV over 1 minute (approximately 100 mL for a 70 kg patient), followed by a continuous infusion at 0.25 mL/kg/min. If cardiovascular instability persists, up to 2 additional boluses may be given, and the infusion rate increased to 0.5 mL/kg/min. The maximum recommended dose is 12 mL/kg over the first 60 minutes. The mechanism is believed to involve a "lipid sink" effect, where the emulsion sequesters lipophilic local anesthetic molecules away from cardiac tissue, combined with providing metabolic substrate to the myocardium. The infusion should be continued for at least 15 minutes after hemodynamic stability is achieved.

Refractory Cases

In cases refractory to lipid emulsion and modified ACLS, cardiopulmonary bypass or ECMO should be considered early if available. Prolonged CPR may be necessary because bupivacaine dissociates slowly from sodium channels. These patients can be resuscitated with aggressive, sustained therapy.

<image>ASRA LAST treatment algorithm presented as an emergency checklist poster format. The flowchart shows: recognition of LAST signs, immediate steps (stop injection, call for help, airway management), branching into seizure management (benzodiazepines) and cardiac arrest management (modified ACLS), with lipid emulsion dosing prominently displayed (1.5 mL/kg bolus, 0.25 mL/kg/min infusion, maximum 12 mL/kg). Medications to avoid are highlighted in red (vasopressin, large-dose epinephrine, calcium channel blockers, lidocaine).</image>

Prevention of LAST

Preventing LAST requires several layered precautions. Aspiration before injection and between incremental doses helps detect intravascular needle placement. The lowest effective dose and concentration should be used. Injection should be incremental, in 3-5 mL aliquots with 30-60 second pauses between each. Ultrasound guidance may reduce intravascular injection but does not eliminate LAST risk. An epinephrine-containing test dose (15 mcg epinephrine in 3 mL of local anesthetic) can help identify intravascular injection. In awake patients, monitoring for early symptoms and maintaining verbal contact during injection provide additional safety. Maximum doses should be known and adjusted for patient weight and comorbidities.

Clinical Pearls

The "maximum dose" of local anesthetics is site-dependent: intercostal injection produces the highest plasma levels and subcutaneous infiltration the lowest for the same dose administered. Bupivacaine can cause cardiac arrest as the first sign of LAST, without preceding seizures, so lipid emulsion must always be immediately available. In pregnancy, the threshold for bupivacaine cardiotoxicity is lower, which is one reason ropivacaine is preferred for epidural analgesia in obstetric patients. Chloroprocaine is the safest local anesthetic for epidural use when rapid onset is needed, such as conversion to surgical anesthesia for emergency cesarean delivery, because of its rapid ester hydrolysis. Lipid emulsion should be available wherever regional anesthesia is performed -- this is now a standard of care. Treating muscle activity with succinylcholine must not be confused with treating the seizure itself; the neural seizure continues and generates progressive acidosis even when the muscles are paralyzed.

References

• Neal JM, Barrington MJ, Fettiplace MR, et al. The Third American Society of Regional Anesthesia and Pain Medicine Practice Advisory on Local Anesthetic Systemic Toxicity. Regional Anesthesia and Pain Medicine. 2018;43(2):113-123.
• Berde CB, Strichartz GR. Local anesthetics. In: Miller's Anesthesia. 9th ed. Elsevier; 2020.
• Weinberg GL. Lipid emulsion infusion: resuscitation for local anesthetic and other drug overdose. Anesthesiology. 2012;117(1):180-187.
• Butterworth JF. Models and mechanisms of local anesthetic cardiac toxicity. Regional Anesthesia and Pain Medicine. 2010;35(2):167-176.
• El-Boghdadly K, Pawa A, Chin KJ. Local anesthetic systemic toxicity: current perspectives. Local and Regional Anesthesia. 2018;11:35-44.

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