- Beta-blockers: Atenolol, metoprolol, propranolol, sotalol.
- Cardioselective calcium-channel blockers: Verapamil and diltiazem.
Effects on the ECG
- Sinus bradycardia.
- 1st, 2nd 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.
Two of the beta-blockers have additional important toxic effects:
- 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 blocks myocardial potassium channels, causing QT prolongation and Torsades de Pointes in overdose.
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.
Slow junctional rhythm:
- Regular rhythm at 30 bpm.
- Narrow QRS complexes
- No visible P waves
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 – Sotalol poisoning
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 – Propranolol poisoning
This ECG demonstrates some of the key features of propranolol overdose:
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).
Learn From The Experts!
Consolidate your learning with lessons from the masters of ECG interpretation. Follow the links below for expert commentary, video lessons, case-based discussion and detailed explanations to take your learning to the next level.
- Amal Mattu’s ECG Video of the week — A case of unstable bradycardia (video lesson)
- Dr Smith’s ECG Blog — Differential diagnosis of severe bradycardia (case discussions)
- ECG BASICS — Waves, Intervals, Segments and Clinical Interpretation
- ECG CLINICAL CASES — Your favourite ECG’s placed in clinical context with a challenging Q&A approach
- ECG and Cardiology Eponymous Syndromes — Cheats guide to eponymous emancipation
- ECG Exam Template — a framework for the FACEM part 2 exam.
- ECG Reference Sites on the WEB — the best of the rest
- Murray L, Daly F, Little M, Cadogan M. Toxicology Handbook (second edition). Elsevier, 2011.