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Sinus Node Dysfunction

(Sick Sinus Syndrome)

By

L. Brent Mitchell

, MD, Libin Cardiovascular Institute of Alberta, University of Calgary

Last full review/revision Jan 2021
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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 electrocardiography. 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 pause is temporary cessation of sinus node activity, seen on electrocardiography (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.

Etiology of Sinus Node Dysfunction

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 of Sinus Node Dysfunction

Diagnosis of Sinus Node Dysfunction

Prognosis for Sinus Node Dysfunction

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

Treatment of Sinus Node Dysfunction

  • Pacemaker

Treatment of sinus node dysfunction is pacemaker implantation Cardiac Pacemakers The need for treatment of arrhythmias depends on the symptoms and the seriousness of the arrhythmia. Treatment is directed at causes. If necessary, direct antiarrhythmic therapy, including antiarrhythmic... read more Cardiac Pacemakers . 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 Drugs for Arrhythmias The need for treatment of arrhythmias depends on the symptoms and the seriousness of the arrhythmia. Treatment is directed at causes. If necessary, direct antiarrhythmic therapy, including antiarrhythmic... read more 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.

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