Sinus Node Dysfunction
(Sick Sinus Syndrome)
(See also Overview of Arrhythmias.)
Sinus node dysfunction includes
Sinus node dysfunction affects mainly older patients, especially those with another cardiac disorder or diabetes.
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.
Many patients with sinus node dysfunction are asymptomatic, but depending on the heart rate, all the symptoms of bradycardias and tachycardias can occur.
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.
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