Sodium channel blocker poisoning on ECG

What it looks like on ECG

The baseline picture is diffuse widening of the QRS complex, increasing as the dose or poison concentration in the blood rises. Global QRS duration is measured from the earliest onset to the latest offset of the complex across all 12 leads, rather than the visual width in a single arbitrary lead; lead II is often used separately for comparison with classical TCA overdose criteria.

A classical yet moderately specific and sensitive ancillary sign is a change in the terminal part of the QRS in lead aVR: an R wave height greater than 3 mm or an R/S ratio greater than 0.7. It is described mainly in tricyclic antidepressant (TCA) overdose and reflects a change in the terminal depolarization vector due to slowed conduction in the His-Purkinje system, rather than selective preservation of conduction in a specific myocardial segment. This sign also occurs in other intraventricular conduction disturbances, so it is not diagnostic on its own.

In lead I and other left leads, the terminal part of the QRS is often widened and notched (the terminal S becomes deeper and wider). Sinus tachycardia is frequently present — especially in TCA poisoning due to the anticholinergic effect and inhibition of catecholamine reuptake; independently of this, alpha-blockade by TCAs contributes to hypotension.

As sodium channel blockade worsens, the QRS can reach 160 ms and more. For TCAs, this is a classical reference point for an increased risk of ventricular arrhythmias in overdose studies, whereas a QRS greater than 100 ms is primarily associated with seizure risk; both thresholds are not universal for all sodium channel blockers and are applied alongside clinical findings and ECG dynamics. The QT interval is visually prolonged due to the wide QRS. TCAs and class IA antiarrhythmics additionally block potassium channels and may truly prolong repolarization with a risk of torsades de pointes. For class IC, primarily flecainide, the leading effect is Na-channel blockade: QT prolongation is usually secondary to QRS widening, and pronounced repolarization blockade and torsades are not typical class effects; JT/JTc evaluation is performed only after accounting for QRS width.

In extremely severe cases, the QRS complex becomes so wide and deformed that the ECG approaches a sinusoidal wave — this is a critical intraventricular conduction disturbance with a high risk of a peri-arrest state and the need for immediate resuscitation measures.

Mechanism

Sodium channel blockers (class IA and IC antiarrhythmics, TCAs, certain antihistamines, cocaine) bind to cardiomyocytes' fast Na⁺ channels in the open or inactivated state and slow down phase 0 of the action potential. The rate of depolarization drops, and impulse conduction through the ventricular myocardium and the His-Purkinje system slows down — hence the QRS widening.

Slowing and directional change of terminal depolarization forces shift the terminal QRS vector, which may cause a terminal R wave to appear in lead aVR; this is a consequence of toxic conduction slowing, not proof of preserved conduction in a specific anatomical zone. Conduction slowing creates a substrate for re-entry and ventricular tachyarrhythmias. Additionally, some of these drugs (TCAs, class IA) block potassium channels, which can truly prolong repolarization, and alpha-blockade (especially with TCAs) worsens hypotension.

Causes

• Tricyclic antidepressants (amitriptyline, imipramine, clomipramine) are a classical cause
• Class IA antiarrhythmics (quinidine, procainamide, disopyramide)
• Class IC antiarrhythmics (flecainide, propafenone)
• Cocaine (local anesthetic effect on Na channels)
• Diphenhydramine and other first-generation antihistamines in high doses
• Carbamazepine, certain antipsychotics (thioridazine, chlorpromazine) at toxic concentrations
• Local anesthetics (bupivacaine) in systemic toxicity

Differential diagnosis

• Right bundle branch block — also wide QRS and R wave in right leads, but typical rSR' morphology in V1 and wide notched S in I, V6; an isolated terminal R wave in aVR is not characteristic of RBBB
• Hyperkalemia — QRS widening combined with tall peaked T waves and P flattening; blood potassium confirms the diagnosis
• Brugada syndrome — coved-type ST elevation in V1–V2, QRS may not be as sharply widened, clinical presentation and history differ
• Ventricular tachycardia — wide QRS, but this is a rhythm rather than sinus complex morphology; in toxic QRS widening, the rhythm is usually sinus tachycardia until true VT develops
• Hypothermia — wide QRS is possible, but Osborne wave and bradycardia are typical rather than tachycardia

Clinical significance and management

QRS widening in clinically significant TCA overdose or mechanism-equivalent toxicity (wide QRS combined with hypotension, seizures, or ventricular arrhythmia) is a specific indication for sodium bicarbonate administration: plasma alkalinization and an increase in extracellular Na⁺ decrease drug binding to the channel, narrow the QRS, and reduce arrhythmia risk. As a guide, repeat boluses are used with monitoring of QRS width and hemodynamics after each administration, targeting a plasma pH of approximately 7.50–7.55; during therapy, pH, sodium, and potassium are monitored, avoiding severe alkalemia, hypernatremia, hypokalemia, and volume overload. The specific regimen and repeat doses are determined by clinical protocol and toxicologist consultation, rather than a fixed universal dose.

In unstable ventricular tachycardia or ventricular fibrillation, immediate electrical therapy (defibrillation or synchronized cardioversion) according to resuscitation algorithms is primary; bicarbonate administration is performed in parallel as a specific counter-toxin measure rather than instead of electrical therapy. Depending on the toxin and arrhythmia type, lidocaine or magnesium for torsades de pointes are additionally considered. Lipid emulsion therapy and extracorporeal support methods (e.g., VA-ECMO) are rescue measures for refractory hemodynamic instability or cardiac arrest after standard therapy, used in conjunction with a toxicologist; for bupivacaine, the role of lipid emulsion is significantly more justified than for TCAs, where the evidence base is limited.

Continuous ECG monitoring: dynamic changes in QRS width are assessed as the main marker of severity and response to treatment. Class IA and class IC antiarrhythmics are contraindicated (they will worsen Na-channel blockade). Seizures and acidosis exacerbate toxicity and are therefore actively corrected.

Common interpretation errors

• Mistaking QRS widening for a bundle branch block and failing to suspect poisoning
• Failing to specifically check lead aVR — the terminal R wave is easily missed during a quick reading
• Interpreting any QT prolongation as a wide QRS artifact and failing to account for the risk of true repolarization prolongation with TCAs and class IA
• Do not skip serial ECG monitoring — the QRS duration can change rapidly and requires continuous tracking.
• Ignoring sinus tachycardia as a "nonspecific" finding, even though it is clinically significant in the context of anticholinergic syndrome.

Frequently asked questions

Related terms

More in "Pharmacology and Toxicology"

Entire section

Sources

• Goldberger's Clinical Electrocardiography, current edition
• Surawicz, Knilans «Chou's Electrocardiography in Clinical Practice»
• Orlov, V.N. «Electrocardiography Guide»

Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.