ECG Case 055

Main Abnormalities

This is a classic ECG of tricyclic antidepressant toxicity, demonstrating multiple characteristic abnormalities.

Signs of sodium-channel blockade

• Broad QRS complexes (120 ms, or 3 small squares).
• Positive R’ wave in lead aVR > 3 mm.
• Prolonged PR interval (240 ms).
• Long QT interval (> 1/2 the RR interval).
• Brugada-like pattern in V1.

Signs of anticholinergic toxidrome

Sinus tachycardia (~ 110 bpm), with P waves embedded in each T wave

This patient had taken a life-threatening overdose of dosulepin (a Tricyclic antidepressant TCA).

How to Spot Sodium-Channel Blockade

• QRS prolongation (> 100ms or 2.5 small squares), typically measured in lead II.
• A terminal or secondary R wave (R’ wave) in aVR > 3 mm.
• An R’/S ratio in aVR > 0.7.

Prognostic Value of the ECG

In patients with TCA overdose, the degree of QRS prolongation correlates with the degree of clinical toxicity:

• QRS width > 100 ms is predictive of seizures.
• QRS width > 160 ms is predictive of cardiotoxicity (e.g. broad-complex dysrhythmias, hypotension).

The combination of PR prolongation and sinus tachycardia with TCA overdose often makes the P waves difficult to see, and may lead the rhythm to be incorrectly identified as VT. This patient needs bicarbonate and hyperventilation, not electricity and amiodarone!

The clinical significance of a TCA-induced Brugada ECG pattern remains controversial — i.e. is it purely a manifestation of severe sodium-channel blockade, or does it represent “unmasking” of underlying Brugada syndrome? These issues are discussed here.