(See also Overview of Arrhythmias Overview of Arrhythmias The normal heart beats in a regular, coordinated way because electrical impulses generated and spread by myocytes with unique electrical properties trigger a sequence of organized myocardial... read more 
.)
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 older patients, especially those with another cardiac disorder or diabetes Diabetes Mellitus (DM) Diabetes mellitus is impaired insulin secretion and variable degrees of peripheral insulin resistance leading to hyperglycemia. Early symptoms are related to hyperglycemia and include polydipsia... read more .
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
Many patients with sinus node dysfunction are asymptomatic, but depending on the heart rate, all the symptoms of bradycardias and tachycardias Symptoms and Signs The normal heart beats in a regular, coordinated way because electrical impulses generated and spread by myocytes with unique electrical properties trigger a sequence of organized myocardial... read more can occur.
Diagnosis of Sinus Node Dysfunction
ECG
A slow, irregular pulse suggests the diagnosis of sinus node dysfunction, which is confirmed by ECG Electrocardiography The standard electrocardiogram (ECG) provides 12 different vector views of the heart’s electrical activity as reflected by electrical potential differences between positive and negative electrodes... read more , rhythm strip, or continuous 24-hour ECG recording. Some patients present with atrial fibrillation Atrial Fibrillation Atrial fibrillation is a rapid, irregularly irregular atrial rhythm. Symptoms include palpitations and sometimes weakness, effort intolerance, dyspnea, and presyncope. Atrial thrombi may form... read more (AF), and the underlying sinus node dysfunction manifests only after conversion to sinus rhythm.
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 
. 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.
