Sinus Node Dysfunction (Sick Sinus Syndrome)

Definition

• A disease characterised by abnormal sinus node functioning with resultant bradycardia and cardiac insufficiency.

Causes

• May be multi-factorial in origin.
• Causes can be considered either intrinsic or extrinsic.

Intrinsic

• 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

Sinus arrest:

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

 

Example 2:  Bradycardia-tachycardia syndrome

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

Further Reading

• 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 Reference Sites on the WEB – the best of the rest

Author Credits

References

• Surawicz B, Knilans TK. Chou’s Electrocardiography in Clinical Practice. 6th Edition. Saunders Elsevier 2008.
• Chan TC, Brady WJ, Harrigan RA, Ornato JP, Rosen P. ECG in Emergency Medicine and Acute Care. Elsevier Mosby 2005.
• Brady WJ, Truwit JD. Critical Decisions in Emergency & Acute Care Electrocardiography. Wiley Blackwell 2009.