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Sinus Node Dysfunction

(Sick Sinus Syndrome)

By L. Brent Mitchell, MD, Professor of Medicine, Department of Cardiac Services, Libin Cardiovascular Institute of Alberta, University of Calgary

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Sinus node dysfunction refers to a number of conditions causing physiologically inappropriate atrial rates. Symptoms may be minimal or include weakness, effort intolerance, palpitations, and syncope. Diagnosis is by ECG. Symptomatic patients require a pacemaker.

Sinus node dysfunction includes

  • Inappropriate sinus bradycardia

  • Alternating bradycardia and atrial tachyarrhythmias (bradycardia-tachycardia syndrome)

  • Sinus pause or arrest

  • Sinoatrial (SA) exit block

Sinus node dysfunction affects mainly the elderly, especially those with another cardiac disorder or diabetes.

Sinus pause is temporary cessation of sinus node activity, seen on ECG as disappearance of P waves for seconds to minutes. The pause usually triggers escape activity in lower pacemakers (eg, atrial or junctional), preserving heart rate and function, but long pauses cause dizziness and syncope.

In SA exit block, the SA node depolarizes, but conduction of impulses to atrial tissue is impaired.

  • In 1st-degree SA block, the SA node impulse is merely slowed, and ECG is normal.

  • In type I 2nd-degree SA (SA Wenckebach) block, impulse conduction slows before blocking, seen on the ECG as a P-P interval that decreases progressively until the P wave drops altogether, creating a pause and the appearance of grouped beats; the duration of the pause is less than 2 P-P cycles.

  • In type II 2nd-degree SA block, conduction of impulses is blocked without slowing beforehand, producing a pause that is a multiple (usually twice) of the P-P interval and the appearance of grouped beats.

  • In 3rd-degree SA block, conduction is blocked; P waves are absent, giving the appearance of sinus arrest.


The most common cause of sinus node dysfunction is

  • Idiopathic SA node fibrosis, which may be accompanied by degeneration of lower elements of the conducting system.

Other causes include drugs, excessive vagal tone, and many ischemic, inflammatory, and infiltrative disorders.

Symptoms and Signs

Many patients with sinus node dysfunction are asymptomatic, but depending on the heart rate, all the symptoms of bradycardias and tachycardias can occur.


  • ECG

A slow, irregular pulse suggests the diagnosis of sinus node dysfunction, which is confirmed by ECG, rhythm strip, or continuous 24-h ECG recording. Some patients present with atrial fibrillation (AF), and the underlying sinus node dysfunction manifests only after conversion to sinus rhythm.


Prognosis of sinus node dysfunction is mixed; without treatment, mortality is about 2%/yr, primarily resulting from an underlying structural heart disorder. Each year, about 5% of patients develop AF with its risks of heart failure and stroke.


  • Pacemaker

Treatment of sinus node dysfunction is pacemaker implantation. Risk of atrial fibrillation is greatly reduced when a physiologic (atrial or atrial and ventricular) pacemaker rather than a ventricular pacemaker is used. Newer dual chamber pacemakers that minimize ventricular pacing may further reduce risk of AF. Antiarrhythmic drugs may prevent paroxysmal tachyarrhythmias after pacemaker insertion.

Theophylline and hydralazine are options to increase heart rate in healthy, younger patients who have bradycardia without syncope.