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Toxicology – Widened QRS on ECG
Overview
The QRS complex represents ventricular depolarization and is normally less than 120 milliseconds. Because this process depends on sodium influx, any drug that blocks or delays sodium channels can prolong the QRS duration and increase the risk of dangerous arrhythmias.
Drugs That Cause QRS Widening
Tricyclic Antidepressants (TCAs)
Agents such as amitriptyline, imipramine, nortriptyline, clomipramine, and doxepin commonly cause sodium channel blockade leading to widened QRS complexes.
Class Ia Antiarrhythmics
Medications like quinidine, procainamide, and disopyramide prolong the action potential and slow conduction, resulting in QRS widening.
Class Ic Antiarrhythmics
Flecainide, propafenone, and moricizine significantly block sodium channels and can markedly widen the QRS complex.
Phenothiazines and Related Agents
Drugs such as chlorpromazine, fluphenazine, prochlorperazine, promethazine, and thioridazine, along with other agents like diphenhydramine, carbamazepine, amantadine, cocaine, and propoxyphene, may contribute to sodium channel blockade and QRS prolongation.
Other Medications
Additional drugs that may widen the QRS include venlafaxine, bupropion, citalopram, escitalopram, mesoridazine, local anesthetics (e.g., dibucaine, bupivacaine), and propranolol.
Management
Intravenous sodium bicarbonate is the primary treatment for toxin-induced QRS widening. It is especially indicated in suspected tricyclic antidepressant toxicity when the QRS duration exceeds 100 ms, regardless of blood pH. An initial dose of 1–2 mEq/kg is given as a rapid bolus and may be repeated until the QRS narrows. Continuous infusion is often required afterward, typically by adding sodium bicarbonate to intravenous fluids. Care must be taken to avoid excessive alkalemia (pH >7.55) and hypernatremia, with a target pH of approximately 7.45–7.55.
Mechanism of Antidote
Sodium bicarbonate works by increasing serum sodium concentration, helping to overcome sodium channel blockade and restore normal cardiac conduction.
Key Points
Overview
The QRS complex represents ventricular depolarization and is normally less than 120 milliseconds. Because this process depends on sodium influx, any drug that blocks or delays sodium channels can prolong the QRS duration and increase the risk of dangerous arrhythmias.
Drugs That Cause QRS Widening
Tricyclic Antidepressants (TCAs)
Agents such as amitriptyline, imipramine, nortriptyline, clomipramine, and doxepin commonly cause sodium channel blockade leading to widened QRS complexes.
Class Ia Antiarrhythmics
Medications like quinidine, procainamide, and disopyramide prolong the action potential and slow conduction, resulting in QRS widening.
Class Ic Antiarrhythmics
Flecainide, propafenone, and moricizine significantly block sodium channels and can markedly widen the QRS complex.
Phenothiazines and Related Agents
Drugs such as chlorpromazine, fluphenazine, prochlorperazine, promethazine, and thioridazine, along with other agents like diphenhydramine, carbamazepine, amantadine, cocaine, and propoxyphene, may contribute to sodium channel blockade and QRS prolongation.
Other Medications
Additional drugs that may widen the QRS include venlafaxine, bupropion, citalopram, escitalopram, mesoridazine, local anesthetics (e.g., dibucaine, bupivacaine), and propranolol.
Management
Intravenous sodium bicarbonate is the primary treatment for toxin-induced QRS widening. It is especially indicated in suspected tricyclic antidepressant toxicity when the QRS duration exceeds 100 ms, regardless of blood pH. An initial dose of 1–2 mEq/kg is given as a rapid bolus and may be repeated until the QRS narrows. Continuous infusion is often required afterward, typically by adding sodium bicarbonate to intravenous fluids. Care must be taken to avoid excessive alkalemia (pH >7.55) and hypernatremia, with a target pH of approximately 7.45–7.55.
Mechanism of Antidote
Sodium bicarbonate works by increasing serum sodium concentration, helping to overcome sodium channel blockade and restore normal cardiac conduction.
Key Points
- Sodium bicarbonate is also useful for enhancing elimination of certain toxins such as salicylates and phenobarbital.
- It is an important therapy for correcting severe metabolic acidosis in toxicologic emergencies.
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