Cardiac Rhythm in Amitriptyline Overdose with Cardiac Arrest
In a 59-year-old male with cardiac arrest due to amitriptyline overdose, the rhythm will most commonly present as ventricular tachycardia or ventricular fibrillation, preceded by characteristic ECG changes including QRS widening (>100 ms), QT prolongation, and a rightward shift of the terminal QRS axis. 1, 2
Initial Rhythm Presentation
The cardiac arrest rhythm in tricyclic antidepressant overdose typically manifests as:
- Ventricular tachycardia (VT) or pulseless ventricular tachycardia as the most common arrest rhythms 3, 4
- Ventricular fibrillation (VF) may occur, either as the initial rhythm or following degeneration from VT 1, 3
- The rhythm often alternates between VT and pulseless VT/VF, which can persist for prolonged periods (up to 17 hours in documented cases) 3, 4
Pre-Arrest ECG Warning Signs
Before cardiac arrest occurs, the ECG will demonstrate specific toxic changes:
- QRS duration >100 ms (often >110 ms) is the most clinically significant predictor of life-threatening arrhythmias and indicates severe sodium channel blockade 1, 2, 5
- Rightward axis deviation of the terminal 40 ms of the QRS complex (T40 ms axis) in the frontal plane, best seen in lead aVR, is a specific and sensitive indicator of tricyclic toxicity 2, 5
- QT interval prolongation with QTc often exceeding 500 ms 1, 6, 5
- Sinus tachycardia is the most common initial rhythm due to anticholinergic effects 5, 7
- PR interval prolongation indicating delayed atrioventricular conduction 2, 5
Mechanism of Arrhythmogenesis
The lethal arrhythmias result from:
- Cardiac sodium channel blockade (similar to Class Ia antiarrhythmics), which prolongs the cardiac action potential, increases the refractory period, and delays atrioventricular conduction 1, 5, 8
- This sodium channel blockade is the principal mechanism causing QRS widening and ventricular arrhythmias 1, 5
- Additional effects include alpha-adrenergic blockade causing hypotension and reduced myocardial contractility 5
Less Common Rhythm Presentations
While VT/VF predominate, other rhythms may occur:
- Bradyarrhythmias including atrioventricular block can occur, though less commonly than tachyarrhythmias 1, 5
- Torsades de pointes occurs uncommonly despite QT prolongation 5
- Supraventricular tachycardia may be seen 1
- Sinusoidal ventricular tachycardia can occur in severe overdose due to extreme sodium channel blockade 6
Critical Timing Considerations
Life-threatening arrhythmias and death typically occur:
- Within the first 24 hours of ingestion, with most occurring within 12 hours 5
- Rapid deterioration is common, with toxicity developing within 1 hour of ingestion in severe cases 3
- Maximal ECG changes (QRS widening, T40 ms axis deviation) are usually present within 12 hours but may take up to a week to resolve 5
Important Clinical Pitfall
A critical caveat: The level of consciousness at presentation is actually the most sensitive clinical predictor of serious complications, more so than any single ECG parameter 5. However, QRS duration >100 ms and rightward T40 ms axis are better predictors of cardiovascular toxicity than plasma drug levels 5. The absence of these ECG findings does not exclude toxicity, as patients can succumb to fatal arrhythmias late after overdose 2.