Beta-blocker and Calcium-channel blocker toxicity

Relevant Agents

Beta-blockers: Atenolol, metoprolol, propranolol, sotalol.
Cardioselective calcium-channel blockers: Verapamil and diltiazem.

Effects on the ECG

Sinus bradycardia.
1st degree, 2nd degree and 3rd degree AV block.
Junctional bradycardia.
Ventricular bradycardia.

A prolonged PR interval is an early sign of beta-blocker or calcium-channel blocker toxicity — even in the absence of significant bradycardia.

Specific Agents

Two beta-blockers have additional important toxic effects:

Propanolol:

Propranolol behaves more like a tricyclic antidepressant in overdose than a beta-blocker, due to its blockade of myocardial and CNS fast sodium channels.
Propranolol toxicity is associated with QRS widening and a positive R’ wave in aVR (signs of sodium channel blockade), which portend the onset of coma, seizures, hypotension and ventricular arrhythmias.

Sotalol

Sotalol blocks myocardial potassium channels, causing QT prolongation and Torsades de Pointes in overdose.

ECG Examples

Example 1

ECG Sinus bradycardia 1st degree AV block

Sinus bradycardia with 1st-degree AV block:

Heart rate 45 bpm.
PR interval 240 ms.

This type of ECG pattern is commonly seen in the early stages of beta-blocker / calcium-channel blocker poisoning.

Example 2

Slow junctional rhythm:

Regular rhythm at 30 bpm.
Narrow QRS complexes
No visible P waves

Example 3

Complete heart block:

Sinus rhythm – P waves occur at a rate of around 90 bpm.
3rd degree AV block – there is no relationship between the P waves and QRS complexes.
Slow escape rhythm (30 bpm) with a RBBB morphology – this could be either a ventricular escape rhythm or a slow junctional rhythm with RBBB.

Example 4

ECG Sinus braycardia prolonged QT sotalol OD

This ECG demonstrates the key features of sotalol overdose:

Sinus bradycardia.
Very long QT interval (~600ms).

NB. This patient is at significant risk of Torsades de Pointes.

Example 5

This ECG demonstrates some of the key features of propranolol overdose:

1st degree AV block.
Signs of sodium-channel blockade: QRS broadening (> 100 ms) and positive R’ wave in aVR (> 3mm).

NB. Normally the heart rate would be slower than this in a propranolol overdose (this is actually an ECG of flecainide poisoning — another sodium-channel blocking agent).