Sinus Node Dysfunction (Sick Sinus Syndrome)

Sinus Node Dysfunction Overview

Intrinsic Causes

• Idiopathic Degenerative Fibrosis (commonest).
• Ischaemia.
• Cardiomyopathies.
• Infiltrative Diseases e.g. sarcoidosis, haemochromatosis.
• Congenital abnormalities.

Extrinsic Causes

• Drugs e.g. digoxin, beta-blockers, calcium channel blockers.
• Autonomic dysfunction.
• Hypothyroidism.
• Electrolyte abnormalitites — e.g. hyperkalaemia.

ECG in Sinus Node Dysfunction

ECG abnormalities can be variable and intermittent. Multiple ECG abnormalities can be seen in sinus node dysfunction including:

• Sinus Bradycardia.
• Sinus Arrhythmia — associated with sinus node dysfunction in the elderly in the absence of respiratory pattern association.
• Sinoatrial Exit Block.
• Sinus Arrest — pause > 3 seconds.
• Atrial fibrillation with slow ventricular response.
• Bradycardia – tachycardia syndrome.

Bradycardia – tachycardia syndrome

• Alternating bradycardia with paroxysmal tachycardia, often supraventricular in origin.
• On cessation of tachyarrhythmia may be a period of delayed sinus recovery e.g. sinus pause or exit block.
• If significant this period of delayed recovery may result in syncope.

Clinical Manifestations

• Commonly seen in the elderly but sinus node dysfunction can affect all age groups.
• Symptoms are due to decreased cardiac output and end-organ hypoperfusion associated with cardiac rhythm abnormality.
• Wide range of clinical symptoms including syncope, near-syncope, dizziness, fatigue and palpitations.

Treatment

• Correction / removal of extrinsic causes e.g. non-essential drugs.
• Pacemaker insertion – requires correlation of both ECG abnormalities and clinical symptoms.

Recommendation for Pacing in Sinus Node Dysfunction

Class I – Evidence and/or agreement that permanent pacing is useful and effective.

• Sinus node dysfunction with documented symptomatic bradycardia, including frequent sinus pauses that produce symptoms. In some patients, bradycardia is iatrogenic and will occur as a consequence of essential long-term drug therapy of a type and dose for which there are no acceptable alternatives.
• Symptomatic chronotropic incompetence.

Class IIa – Conflicting evidence/ divergence of opinion but weight of evidence / opinion in favour

• Sinus node dysfunction occurring spontaneously or as a result of necessary drug therapy, with heart rate less than 40 bpm when a clear association between significant symptoms consistent with bradycardia and the actual presence of bradycardia has not been documented.
• Syncope of unexplained origin when major abnormalities of sinus node function are discovered or provoked in electrophysiological studies.

Class IIb – Conflicting evidence/divergence of opinion where usefulness / efficacy is less well established

• In minimally symptomatic patients, chronic heart rate less than 40 bpm while awake.

Class III –  Permanent pacing  is not useful/effective and in some cases may be harmful.

• Sinus node dysfunction in asymptomatic patients, including those in whom substantial sinus bradycardia (heart rate less than 40 bpm) is a consequence of long-term drug treatment.
• Sinus node dysfunction in patients with symptoms suggestive of bradycardia that are clearly documented as not associated with a slow heart rate.
• Sinus node dysfunction with symptomatic bradycardia due to nonessential drug therapy.

ECG Examples

Example 1

Sinus arrest:

• Prolonged absence of sinus node activity (absent P waves) > 3 seconds.

Example 2

Bradycardia-tachycardia syndrome:

• Runs of tachycardia interspersed with long sinus pauses (up to 6 seconds).
• The sinus rate is extremely slow, varying from 40 bpm down to around 10 bpm in places.
• Sinus beats are followed by paroxysms of junctional tachycardia at around 140 bpm.

Related Topics

• Sinus Bradycardia
• Sinus Arrhythmia
• Sinoatrial Exit Block

References

• Da Costa D, Brady WJ, Edhouse J. Bradycardias and atrioventricular conduction block. BMJ. 2002 Mar 2;324(7336):535-8

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