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

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

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).

Related Topics

• Tricyclic antidepressant overdose (sodium-channel blocker toxicity)
• Digoxin toxicity
• Carbamazepine toxicity
• Quetiapine toxicity

References

• Dr Smith’s ECG Blog – Differential diagnosis of severe bradycardia (case discussions)

Advanced Reading

Online

• Wiesbauer F, Kühn P. ECG Yellow Belt online course: Become an ECG expert. Medmastery
• Wiesbauer F, Kühn P. ECG Blue Belt online course: Learn to diagnose any rhythm problem. Medmastery
• Rawshani A. Clinical ECG Interpretation ECG Waves
• Smith SW. Dr Smith’s ECG blog.

Textbooks

• Mattu A, Tabas JA, Brady WJ. Electrocardiography in Emergency, Acute, and Critical Care. 2e, 2019
• Brady WJ, Lipinski MJ et al. Electrocardiogram in Clinical Medicine. 1e, 2020
• Straus DG, Schocken DD. Marriott’s Practical Electrocardiography 13e, 2021
• Hampton J. The ECG Made Practical 7e, 2019
• Grauer K. ECG Pocket Brain (Expanded) 6e, 2014
• Brady WJ, Truwit JD. Critical Decisions in Emergency and Acute Care Electrocardiography 1e, 2009
• Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric 6e, 2008
• Mattu A, Brady W. ECG’s for the Emergency Physician Part I 1e, 2003 and Part II
• Chan TC. ECG in Emergency Medicine and Acute Care 1e, 2004

LITFL Further Reading

• ECG Library Basics – Waves, Intervals, Segments and Clinical Interpretation
• ECG A to Z by diagnosis – ECG interpretation in clinical context
• ECG Exigency and Cardiovascular Curveball – ECG Clinical Cases
• 100 ECG Quiz – Self-assessment tool for examination practice
• ECG Reference SITES and BOOKS – the best of the rest