Sinus Bradycardia

Sinus Bradycardia Overview

Sinus rhythm with a resting heart rate of < 60 bpm in adults, or below the normal range for age in children.

Normal heart rates in children

• Newborn: 110 – 150 bpm
• 2 years: 85 – 125 bpm
• 4 years: 75 – 115 bpm
• 6 years+: 60 – 100 bpm

Causes of bradycardia

Non-pharmacological

• Normal during sleep
• Increased vagal tone (e.g. athletes)
• Vagal stimulation (e.g. pain)
• Inferior myocardial infarction
• Sinus node disease
• Hypothyroidism
• Hypothermia
• Anorexia nervosa
• Electrolyte abnormalities – hyperkalaemia, hypermagnesaemia
• Brainstem herniation (the Cushing reflex)
• Myocarditis

Pharmacological

• Beta-blockers
• Calcium-channel blockers (verapamil & diltiazem)
• Digoxin
• Central alpha-2 agonists (clonidine & dexmedetomidine)
• Amiodarone
• Opiates
• GABA-ergic agents (barbiturates, benzodiazepines, baclofen, GHB)
• Organophosphate poisoning

Differential Diagnosis

• Sinus bradycardia may be indistinguishable from type II sino-atrial block.

ECG Example

Sinus bradycardia secondary to anorexia nervosa

• Sinus bradycardia (35 bpm) in a 15-year old girl with anorexia nervosa.
• Note the prominent U waves in the precordial leads, a common finding in sinus bradycardia.

Related Topics

• Sinus node dysfunction (Sick sinus syndrome)
• Sinus arrhythmia
• Sinus tachycardia

Advanced Reading

Online

• Wiesbauer F, Kühn P. ECG Mastery: Yellow Belt online course. Understand ECG basics. Medmastery
• Wiesbauer F, Kühn P. ECG Mastery: Blue Belt online course: Become an ECG expert. Medmastery
• Kühn P, Houghton A. ECG Mastery: Black Belt Workshop. Advanced ECG interpretation. Medmastery
• Rawshani A. Clinical ECG Interpretation ECG Waves
• Smith SW. Dr Smith’s ECG blog.

Textbooks

• Zimmerman FH. ECG Core Curriculum. 2023
• Mattu A, Berberian J, Brady WJ. Emergency ECGs: Case-Based Review and Interpretations, 2022
• Straus DG, Schocken DD. Marriott’s Practical Electrocardiography 13e, 2021
• Brady WJ, Lipinski MJ et al. Electrocardiogram in Clinical Medicine. 1e, 2020
• Mattu A, Tabas JA, Brady WJ. Electrocardiography in Emergency, Acute, and Critical Care. 2e, 2019
• Hampton J, Adlam D. The ECG Made Practical 7e, 2019
• Kühn P, Lang C, Wiesbauer F. ECG Mastery: The Simplest Way to Learn the ECG. 2015
• Grauer K. ECG Pocket Brain (Expanded) 6e, 2014
• Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric 6e, 2008
• 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