Sinus Node Dysfunction

How to Cite This Chapter: Acosta Velez JG, Amit G, Hernández Ruiz EA, Trusz-Gluza M, Leśniak W. Sinus Node Dysfunction. McMaster Textbook of Internal Medicine. Kraków: Medycyna Praktyczna. Accessed April 02, 2020.
Last Updated: April 23, 2019
Last Reviewed: August 5, 2019
Chapter Information

Definition, Etiology, PathogenesisTop

Sinus node dysfunction refers to pathologies resulting in inappropriately low heart rates that are insufficient for the current physiologic needs and thus cause clinical symptoms, arrhythmias, or both.

Disorders of sinus node automaticity and conduction may be intermittent or persistent; persistent disorders are referred to as sick sinus syndrome (SSS). In patients with bradycardia following episodes of supraventricular tachycardia (most commonly atrial fibrillation [AF]), tachycardia-bradycardia syndrome is diagnosed.

Causes: Ischemic heart disease, systemic connective tissue disease, postoperative complications, idiopathic degeneration related to aging, functional sinus node dysfunction (in athletes or due to vagal reflexes, disturbances in serum electrolyte levels [hypokalemia or hyperkalemia], metabolic abnormalities [hypothyroidism, hypothermia, anorexia nervosa], neurologic conditions [elevated intracranial pressure, central nervous system tumors], obstructive sleep apnea), drugs (beta-blockers, diltiazem or verapamil, digitalis, class I antiarrhythmic drugs [see Table 3.4-1], amiodarone, lithium).

Sinus node dysfunction is frequently accompanied by loss of a normal chronotropic response to exercise, that is, inability to achieve 85% of the maximum heart rate predicted for age, and in 20% to 30% of patients, by atrioventricular or intraventricular conduction disturbances.

Clinical Features and Natural HistoryTop

Signs and symptoms: see Disorders of Automaticity and Conduction.

Sinus node dysfunction may be transient/intermittent (eg, following myocardial infarction, drug induced) or persistent. Prognosis primarily depends on the underlying condition, concomitant tachyarrhythmias, and risk of thromboembolic complications (stroke or peripheral embolism) if the patient has concomitant AF (see Atrial Fibrillation).


Diagnostic Criteria

Electrocardiography (ECG):

1) Sinus bradycardia: Sinus rhythm with heart rates <50 beats/min when awake.

2) Sinus pause: No sinus P wave in a period longer than 2 PP intervals of baseline rhythm. The pause is not a multiplication of the normal PP intervals.

3) Sinoatrial (SA) block:

a) Mobitz type I (Wenckebach): A progressive increase in the SA conduction time until one beat is blocked, which is reflected in a progressive reduction of PP intervals (and prolongation of PR intervals) with a P wave eventually nonconducting to the ventricle.

b) Mobitz type II: Periodic loss of AV conduction at a 2:1 or 3:1 ratio. The resulting pause is a multiplication of baseline sinus rhythm and may result in an escape atrial, nodal, or (less commonly) ventricular contraction.

4) Tachycardia-bradycardia syndrome: Commonly seen as a prolonged postconversion pause after an episode of AF transitioning to sinus rhythm.

The diagnosis of SSS requires documented symptoms during an episode of bradycardia <40 beats/min or pauses >3 seconds; however, documentation is often difficult. Prolonged Holter monitoring is the best diagnostic tool. Atrial flutter and AF may mask sinus node dysfunction, which is revealed only after cardioversion.

Differential Diagnosis

Other causes of syncope (see Syncope).


1. Management of symptomatic bradycardia: see Figure 3.2-4.

2. Long-term management:

1) In the case of patients involved in sports, stop training, perform a follow-up assessment, and decide about the possible continuation of training based on the results.

2) Optimize treatment of the underlying condition and discontinue drugs causing bradycardia.

3) Theophylline may be useful in some patients but it is rarely suitable for long-term therapy and not used in Canada.

4) Implantation of a cardiac pacemaker (see Disorders of Automaticity and Conduction) in patients with persistent bradycardia causing unequivocally documented symptoms or with intermittent bradycardia caused by sinus pauses or SA block. The recommended first-line mode of stimulation is DDDR (or DDD in patients with persistent bradycardia and no chronotropic incompetence) with delayed ventricular stimulation.

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