Thu18November0317AM 2

A 23-year-old woman presents to the Emergency department following the deliberate ingestion of 2 g of amitriptyline.

The initial clinical findings are a Glasgow coma score of 6, a pulse rate of 140 beats per minute and a blood pressure of 80/50 mmHg.

Which one of the following ECG changes is the most likely predictor of the development of life-threatening arrhythmias?

(Please select 1 option)

Right axis deviation

Prolongation of the QRS complex Correct

Sinus tachycardia

Right bundle branch block

Widespread ST segment depression

Explanation

Deaths from tricyclic antidepressant (TCA) overdose are usually due to arrhythmias and/or hypotension.

Tricyclic antidepressant toxicity is due mainly to the quinidine-like actions of these drugs on cardiac tissues. Slowing of phase 0 depolarisation of the action potential results in slowing of conduction through the His-Purkinje system and myocardium. Slowed impulse conduction is responsible for QRS prolongation and atrioventricular block, and contributes to ventricular arrhythmias and hypotension.

Other mechanisms contributing to arrhythmias include abnormal repolarisation, impaired automaticity, cholinergic blockade and inhibition of neuronal catecholamine uptake.

Toxicity may be worsened by acidaemia, hypotension or hyperthermia.

Sinus tachycardia is due to the anticholinergic effects of the tricyclic antidepressants as well as blockade of neuronal catecholamine reuptake. Sinus tachycardia is usually tolerated well and requires no therapy. Sinus tachycardia with QRS prolongation may be difficult to distinguish from ventricular tachycardia.

Prolongation of the QRS duration >100 ms predicts a higher risk of arrhythmia and is an indication for systemic sodium bicarbonate administration.

Serum alkalinisation favours dissociation of the tricyclic away from myocardial sodium channels, and the extracellular sodium load improves sodium channel function.

Answer Statistics

1

1%

2

97%

3

1%

4

1%

5

3%

Times answered: 268