Toxicology – Class IB Antidysrhythmic Agents

Core Concept

Class IB antidysrhythmic drugs are fast sodium-channel blockers. Historically important examples include:

• Lidocaine
• Mexiletine
• Tocainide — now largely obsolete/discontinued in many regions

Phenytoin also has Class IB-like electrophysiologic properties but is better considered separately.

The most important modern toxicology distinction is that lidocaine toxicity is also local anesthetic systemic toxicity (LAST).

Major manifestations include:

• Early neurologic excitation
• Circumoral or tongue paresthesia
• Tinnitus
• Dizziness
• Seizures
• CNS depression/coma
• Hypotension
• Bradycardia
• Conduction disturbances
• Ventricular dysrhythmias
• Cardiovascular collapse

Severe local-anesthetic toxicity has an important modern rescue treatment:

Intravenous lipid emulsion (ILE)

Thus, the historical statement that there is “no specific antidote” requires qualification for severe lidocaine-associated LAST.

Mechanism

Class IB drugs block fast voltage-gated sodium channels.

Their effects are particularly prominent in:

• Depolarized tissue
• Ischemic myocardium
• Rapidly firing sodium channels

At therapeutic concentrations they can suppress abnormal ventricular automaticity.

With excessive exposure, sodium-channel blockade extends to normal cardiac and nervous tissue.

CNS Sodium-Channel Toxicity

Local anesthetics inhibit neuronal sodium channels.

As concentrations rise, inhibitory CNS pathways may be affected first, producing an initial excitatory phase.

Early symptoms may include:

• Circumoral numbness
• Tongue paresthesia
• Metallic or abnormal taste
• Tinnitus
• Dizziness
• Lightheadedness
• Visual disturbance
• Restlessness
• Tremor
• Confusion

This can progress to:

Agitation → seizure → CNS depression → coma → respiratory arrest

Cardiac Sodium-Channel Toxicity

At higher concentrations:

Na⁺-channel blockade → impaired conduction + reduced myocardial excitability/contractility

Possible consequences include:

• PR prolongation
• QRS widening
• AV block
• Bradycardia
• Ventricular dysrhythmias
• Hypotension
• Cardiogenic shock
• Asystole

Lidocaine

Lidocaine is used as:

• Local anesthetic
• Regional anesthetic
• Topical anesthetic
• Selected ventricular antiarrhythmic therapy

Toxicity may follow:

• Excessive injection
• Accidental intravascular administration
• Rapid systemic absorption
• Excessive topical exposure
• Impaired metabolism
• Medication error
• Rare oral exposure

Acute intravascular exposure can produce toxicity within minutes.

Local Anesthetic Systemic Toxicity – LAST

The modern syndrome of serious systemic toxicity from lidocaine or another local anesthetic is termed:

Local Anesthetic Systemic Toxicity (LAST)

LAST can produce both:

Neurologic toxicity

• Paresthesia
• Tinnitus
• Agitation
• Seizure
• Coma

Cardiovascular toxicity

• Hypotension
• Bradycardia
• Conduction block
• Ventricular dysrhythmia
• Cardiovascular collapse

The traditional progression from neurologic symptoms to cardiovascular toxicity is useful but not universal.

Some patients can present primarily with cardiovascular collapse.

Why Toxicity May Occur at Therapeutic Use

Risk depends on more than the total administered dose.

Important factors include:

• Injection site vascularity
• Accidental intravascular administration
• Rate of administration
• Patient size
• Age
• Cardiac output
• Hepatic function
• Protein binding
• Acid–base status
• Concomitant drugs

Therefore, a single rigid “toxic dose” cannot reliably predict LAST.

Lidocaine Metabolism

Lidocaine is predominantly metabolized in the liver.

Reduced clearance can occur with:

• Severe hepatic dysfunction
• Reduced hepatic blood flow
• Heart failure
• Shock

Repeated dosing or prolonged infusion can therefore cause accumulation.

Acidemia

Acidemia is particularly undesirable in severe local-anesthetic toxicity.

It may increase the active drug fraction and worsen:

• CNS toxicity
• Sodium-channel blockade
• Myocardial depression

Therefore, adequate oxygenation and ventilation are important during resuscitation.

Mexiletine

Mexiletine is an orally active Class IB sodium-channel blocker structurally and pharmacologically related to lidocaine.

Toxicity may produce:

• Nausea/vomiting
• Tremor
• Dizziness
• Ataxia
• Confusion
• Seizures
• Bradycardia
• Hypotension
• Conduction abnormalities
• Ventricular dysrhythmias

Neurologic toxicity may be prominent.

Tocainide

Tocainide is an older orally active lidocaine-like drug.

It has largely disappeared from contemporary clinical practice because of safety concerns and availability of alternatives.

Older toxicology descriptions remain historically relevant but are less important for current practice.

Neurologic Warning Signs

One of the important characteristics of Class IB toxicity is that neurologic symptoms may precede cardiovascular deterioration.

Early findings can include:

• Restlessness
• Dizziness
• Paresthesias
• Tinnitus
• Tremor
• Confusion
• Visual disturbances

These symptoms should not automatically be attributed to anxiety or hyperventilation when systemic local-anesthetic exposure is possible.

Seizures

Seizures are a major manifestation of severe lidocaine/LAST toxicity.

Seizures worsen toxicity by producing:

• Hypoxemia
• Hypercarbia
• Lactic acidosis
• Increased metabolic demand

These changes can intensify cardiovascular toxicity.

Therefore:

Rapid seizure control + oxygenation + ventilation are critical.

Seizure Treatment

Benzodiazepines are generally first-line therapy.

Persistent seizures may require escalation with appropriate anticonvulsant/anesthetic therapy.

Large doses of medications that significantly depress myocardial function should be used cautiously in a patient already developing cardiovascular collapse.

CNS Depression

As toxicity progresses, initial CNS excitation may transition to:

• Drowsiness
• Respiratory depression
• Coma
• Apnea

Airway and ventilation support may therefore become necessary rapidly.

Cardiovascular Findings

Severe toxicity may produce:

• Hypotension
• Bradycardia
• AV block
• QRS widening
• Ventricular dysrhythmias
• Cardiac arrest

Unlike Class IA agents, therapeutic Class IB drugs do not characteristically produce major QT prolongation.

Marked QT prolongation should therefore prompt consideration of:

• Coingestants
• Electrolyte abnormalities
• Another drug mechanism

ECG

Obtain an ECG in significant systemic toxicity.

Assess:

• Rate
• Rhythm
• PR interval
• QRS duration
• QT/QTc
• AV conduction
• Ventricular ectopy

However:

A normal ECG does not exclude early lidocaine toxicity.

Neurologic symptoms may occur before obvious ECG abnormalities.

Methemoglobinemia – Important Clarification

Methemoglobinemia is classically associated with certain local anesthetics, especially:

• Benzocaine
• Prilocaine

Lidocaine can cause methemoglobinemia, but it is substantially less characteristic than with benzocaine or prilocaine.

Consider it when there is:

• Cyanosis disproportionate to respiratory findings
• Low pulse-oximetry readings that do not normalize as expected with oxygen
• Chocolate-brown blood
• An exposure capable of oxidizing hemoglobin

Confirm with co-oximetry.

Laboratory Evaluation

Significant poisoning may warrant:

• Glucose
• Electrolytes
• Potassium
• Magnesium
• Calcium
• Bicarbonate
• Creatinine

Severe poisoning may additionally require:

• Blood gas
• Lactate
• Serial acid–base assessment

Co-oximetry is appropriate if methemoglobinemia is suspected.

Serum Drug Concentrations

Lidocaine or mexiletine concentrations can occasionally support a diagnosis but should not delay treatment.

Clinical toxicity correlates imperfectly with an isolated serum concentration.

Management should be based primarily on:

• Neurologic findings
• ECG
• Hemodynamics
• Respiratory status
• Exposure history

Routine serial drug levels are generally not required to determine recovery in most poisoning cases.

Initial Management

Priorities are:

Stop exposure → airway/oxygenation → control seizures → ECG/circulation → treat cardiovascular toxicity

For suspected LAST, preparation for lipid-emulsion therapy should occur early when severe neurologic or cardiovascular toxicity develops.

Stop Local Anesthetic Administration

If toxicity develops during local or regional anesthesia:

Immediately stop administration of the local anesthetic.

Preventing further systemic exposure is the first intervention.

Oxygenation and Ventilation

Avoid:

• Hypoxemia
• Hypercarbia
• Severe acidemia

These physiologic abnormalities can worsen local-anesthetic cardiotoxicity.

Early airway support may be necessary with:

• Recurrent seizures
• Coma
• Apnea
• Severe cardiovascular collapse

Intravenous Lipid Emulsion

This is the major modern addition missing from the older source.

IV lipid emulsion is an established rescue therapy for severe local anesthetic systemic toxicity, particularly when cardiovascular instability is present.

Its proposed mechanisms include:

• Sequestration/redistribution of lipophilic local anesthetic
• Improved myocardial substrate availability
• Direct beneficial effects on myocardial function

It is especially relevant to severe toxicity from highly lipophilic local anesthetics, but it is also used for severe lidocaine-associated LAST.

Exact administration should follow a current LAST resuscitation protocol.

Cardiac Arrest in LAST

Resuscitation differs somewhat from ordinary cardiac arrest because the myocardium is profoundly affected by the local anesthetic.

Priorities include:

• High-quality CPR
• Adequate oxygenation and ventilation
• Seizure control
• Correction of acidemia
• Early lipid emulsion for severe LAST
• Appropriate defibrillation when indicated

Resuscitation may need to be prolonged because severe local-anesthetic cardiotoxicity can be reversible as the drug redistributes and is metabolized.

Vasopressors

Vasopressor therapy may be required for severe hypotension.

In LAST, standard resuscitation drugs may need modification because excessive catecholamine exposure can worsen arrhythmogenicity and interfere with successful resuscitation.

Current LAST-specific resuscitation guidance should therefore be followed rather than simply applying an old dopamine-first algorithm.

Trendelenburg – Obsolete

Routine Trendelenburg positioning is not recommended as treatment for hypotension.

It does not provide reliable sustained improvement in perfusion and may worsen respiratory mechanics.

Sodium Bicarbonate

Sodium bicarbonate may be considered when significant sodium-channel-blocker physiology is present, particularly with:

• QRS widening
• Severe conduction slowing
• Acidemia

However, in severe LAST it does not replace lipid emulsion and comprehensive resuscitation.

Bradycardia and AV Block

Management centers on:

• Correcting hypoxemia/acidemia
• Treating systemic local-anesthetic toxicity
• Hemodynamic support

Temporary pacing may be considered for refractory severe bradycardia or high-grade block.

However, electrical pacing may be ineffective when myocardial sodium-channel blockade and contractile depression are profound.

Ventricular Dysrhythmias

Management includes:

• Correction of hypoxemia
• Correction of acidemia
• Treatment of electrolyte abnormalities
• Defibrillation/cardioversion when clinically indicated
• LAST-specific resuscitation

Avoid reflexively adding additional sodium-channel-blocking antiarrhythmics because they may worsen the underlying toxicity.

Antiarrhythmics in LAST

The older recommendation to treat lidocaine-induced dysrhythmia with additional Class I drugs should be approached cautiously.

Agents that further impair myocardial conduction can aggravate toxicity.

Treatment should instead focus on reversing the toxic physiology and following contemporary LAST resuscitation guidance.

Torsades de Pointes

Torsades is not the defining dysrhythmia of typical Class IB poisoning.

If polymorphic VT with prolonged QT occurs:

• Correct potassium
• Correct magnesium
• Remove QT-prolonging agents
• Give IV magnesium
• Electrically treat unstable dysrhythmia

Consider another drug or electrolyte abnormality if major QT prolongation dominates the presentation.

GI Decontamination

GI decontamination applies primarily to oral drugs such as mexiletine.

Do not induce vomiting.

Routine gastric lavage is obsolete.

Activated charcoal may occasionally be considered after a substantial recent oral ingestion when:

• The substance is adsorbable
• The airway is safe
• Aspiration risk is acceptable

It has no role in an already absorbed parenteral lidocaine exposure.

Extracorporeal Removal

Routine:

• Hemodialysis
• Hemoperfusion
• Hemofiltration
• Forced diuresis
• Urinary manipulation

do not represent standard toxin-removal strategies for lidocaine poisoning.

In catastrophic refractory cardiovascular collapse, the relevant extracorporeal intervention is circulatory support, not conventional dialysis.

VA-ECMO

Severe LAST is potentially reversible.

When cardiovascular collapse remains refractory despite appropriate resuscitation and lipid therapy, VA-ECMO may be considered at an experienced center.

This modernizes the older concept of emergency cardiopulmonary bypass.

ECMO supports circulation while the local anesthetic redistributes and is metabolized.

Differential Diagnosis

The combination of neurologic toxicity, seizures, and cardiovascular abnormalities can also occur with:

• Other Class I antiarrhythmics
• Tricyclic antidepressants
• Diphenhydramine
• Cocaine
• Chloroquine
• β-blockers
• Calcium-channel blockers
• Other local anesthetics

Nontoxicologic causes include:

• Hypoglycemia
• Electrolyte abnormalities
• Intracranial pathology
• CNS infection
• Primary seizure disorders

Monitoring

Significant toxicity requires monitoring of:

• Mental status
• Respiratory function
• Oxygenation
• Heart rate
• Blood pressure
• Continuous ECG
• Acid–base status when severe

Monitor severe cases for recurrent:

• Seizures
• Dysrhythmias
• Hypotension
• Respiratory failure

Observation

The historical universal 6-hour observation rule should not be applied mechanically.

Observation depends on:

• Drug
• Route
• Formulation
• Amount
• Timing
• Neurologic findings
• ECG
• Hemodynamics
• Coingestants

Parenteral intravascular lidocaine toxicity generally develops rapidly, whereas oral Class IB agents may have a different time course.

Admission

Monitored admission is appropriate for:

• Seizures
• Significant altered mental status
• Respiratory depression
• Hypotension
• Bradycardia
• AV block
• QRS widening
• Ventricular dysrhythmia
• Any clinically significant LAST

Severe cardiovascular or neurologic toxicity warrants intensive care.

Pregnancy

The historical FDA pregnancy letter categories are obsolete.

Management of severe poisoning during pregnancy prioritizes maternal:

• Airway
• Oxygenation
• Ventilation
• Seizure control
• Hemodynamics

Maternal stabilization is also fundamental to fetal oxygenation and perfusion.

Safeguarding

Rigid historical age cutoffs for neglect, abuse, or intentional poisoning are outdated.

Pediatric exposures should instead be assessed according to:

• Developmental capability
• Access
• Circumstances
• Consistency of history
• Recurrent unexplained exposure
• Broader safeguarding concerns

Prognosis

Early neurologic toxicity is often completely reversible when recognized and treated promptly.

Poor outcomes are associated with:

• Prolonged seizures
• Severe hypoxemia
• Acidemia
• Refractory ventricular dysrhythmia
• Prolonged cardiovascular collapse
• Hypoxic brain injury

Even severe LAST can be reversible with prolonged, aggressive resuscitation.

Important Modernization of the Older Source

• Lidocaine toxicity should be recognized within the modern syndrome of local anesthetic systemic toxicity (LAST).
• Neurologic manifestations often precede cardiovascular toxicity, but this sequence is not universal.
• A normal early ECG does not exclude LAST.
• Severe toxicity can cause seizures, conduction block, ventricular dysrhythmias, and cardiovascular collapse.
• IV lipid emulsion is a major modern rescue treatment for severe LAST and is absent from the historical source.
• Avoid hypoxemia, hypercarbia, and acidemia because they worsen toxicity.
• Benzodiazepines are first-line for seizures.
• Sodium bicarbonate may have a role in significant conduction toxicity but does not replace LAST-specific resuscitation.
• Additional sodium-channel-blocking antiarrhythmics can worsen toxicity.
• Major QT prolongation/torsades is less characteristic of Class IB poisoning than Class IA poisoning.
• Methemoglobinemia is much more strongly associated with benzocaine/prilocaine than with lidocaine.
• Routine gastric lavage is obsolete.
• Dialysis is not a useful primary toxin-removal strategy.
• Trendelenburg and dopamine-first hypotension management are outdated.
• “Cardiac bypass” is better conceptualized today as VA-ECMO for selected refractory, potentially reversible cardiovascular collapse.
• Fixed observation periods should be replaced by agent-, route-, formulation-, and symptom-based monitoring.

Key Points

• Class IB drugs block fast Na⁺ channels.
• Major examples are lidocaine and mexiletine; tocainide is largely historical.
• Lidocaine systemic toxicity = LAST.
• Early LAST may cause circumoral paresthesia, tinnitus, dizziness, agitation, and tremor.
• Severe toxicity progresses to seizures, coma, respiratory depression, bradycardia, conduction block, ventricular dysrhythmias, and cardiovascular collapse.
• Control seizures rapidly and prevent hypoxemia, hypercarbia, and acidemia.
• IV lipid emulsion is an important rescue therapy for severe LAST.
• ECG abnormalities may occur late; a normal ECG does not exclude early neurologic toxicity.
• Avoid adding drugs that worsen sodium-channel blockade.
• Severe refractory cardiovascular collapse may require VA-ECMO.