What is the role of ECG in the workup of tricyclic antidepressant (TCA) toxicity in pediatric patients?

Updated: Mar 18, 2020
  • Author: Derrick Lung, MD, MPH; Chief Editor: Stephen L Thornton, MD  more...
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Answer

An electrocardiogram (ECG) is useful as both a screening tool for cyclic antidepressant exposure and as a prognostic indicator in cyclic antidepressant poisoning. [12] See the image below.

Toxicity, antidepressant. ECG shows the terminal R Toxicity, antidepressant. ECG shows the terminal R wave in aVR and the widened QRS complex associated with tricyclic antidepressant (TCA) toxicity.

The most common ECG finding in cyclic antidepressant poisoning is sinus tachycardia, usually due to peripheral antimuscarinic effects. Early ECG changes that suggest significant, evolving toxicity include prolongation of the QRS complex and QT interval; terminal 40-millisecond (msec) right-axis deviation of the QRS in aVR; and the Brugada pattern, including right bundle branch block (RBBB) and a downsloping ST segment elevation in V1-V3. Later ECG changes can include atrioventricular (AV) blocks, ectopy, and ventricular dysrhythmias.

Cyclic antidepressants block fast sodium channels in the myocardium and slow phase zero depolarization of the action potential. Ventricular depolarization is delayed, which leads to a prolonged QRS interval. QRS interval is evaluated best using the limb leads.

Widening of the QRS complex is associated with the development of seizures and dysrhythmias, and QRS duration in the limb leads can be used to assess the severity of cyclic antidepressant toxicity. Patients with a QRS of less than 100 msec are unlikely to develop seizures and dysrhythmias. When the QRS is more than 100 msec, patients have a 34% chance of seizure and a 14% chance of serious dysrhythmia. Patients with QRS complexes of more than 160 msec have a 50% chance of developing ventricular dysrhythmias.

Cyclic antidepressants affect the right fascicle of the heart. The reason is unknown, but the effect can be observed as an exaggerated height of the R wave in aVR. A large R wave in aVR is a highly sensitive screening tool for cyclic antidepressant exposure. Liebelt et al found that the finding of a large R wave in aVR had better test characteristics than any particular QRS length. In this study, an R wave of more than 3 mm in aVR was 81% sensitive and 73% specific for the development of seizures and dysrhythmias. [13]


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