Sinus node dysfunction (sick sinus syndrome) and symptomatic bradycardia !

The heart's natural pacemaker fails and the pulse becomes too slow, pauses or alternates fast and slow. It can cause dizziness and fainting.

Updated 2026-11-01

Structures involved

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Why it happens

What it is

A disorder of the sinoatrial node, the heart's natural pacemaker, that produces inappropriate sinus bradycardia, sinus pauses or arrest, sinoatrial exit block, chronotropic incompetence or the tachycardia-bradycardia syndrome. Symptomatic bradycardia is a slow heart rate (classically under 60, and usually under 50 per minute when it causes problems) that causes symptoms or signs of low cardiac output, whatever its cause.

Who gets it

Mainly a disease of older adults, and one of the leading reasons for permanent pacemaker implantation. In Colombia and Latin America, Chagas cardiomyopathy is an important additional cause of sinus node and conduction disease in younger adults.

Causes

Intrinsic: age-related degenerative fibrosis of the sinus node and atrium (most common)
Ischaemia of the sinus node (sinus node artery, usually from the right coronary artery)
Infiltrative and inflammatory disease: Chagas disease, amyloidosis, sarcoidosis, myocarditis
Surgical injury after atrial surgery or repair of congenital heart disease
Extrinsic (reversible): beta-blockers, non-dihydropyridine calcium channel blockers, digoxin, antiarrhythmics; hypothyroidism, hyperkalaemia, hypothermia, raised intracranial pressure, sleep apnoea, increased vagal tone

Risk factors

Older age
Coronary disease and hypertension
Chagas disease
Rate-slowing medications
Atrial fibrillation (tachycardia-bradycardia syndrome)

Pathophysiology

Fibrosis or disease of the sinus node reduces automaticity and impairs conduction to the atrium. The heart rate falls, pauses appear and the rate does not rise with exercise, lowering cardiac output and brain perfusion. Diseased atrial tissue also favours atrial fibrillation or flutter; when these stop, the weak sinus node may take several seconds to recover, producing long pauses and syncope (tachy-brady syndrome).

Classification

Persistent sinus bradycardia not explained by drugs or physiology
Sinus pause or sinus arrest
Sinoatrial exit block
Chronotropic incompetence (inadequate rate rise with exercise)
Tachycardia-bradycardia syndrome

Symptoms and signs

Dizziness, presyncope or syncope
Fatigue and exercise intolerance
Palpitations during the fast phases
Confusion or memory problems in older people
Dyspnoea or worsening heart failure

Diagnosis

Clinical: The key step is correlating symptoms with the rhythm. Review all medications and look for reversible causes before labelling the disease as intrinsic.
Tests: 12-lead ECG; Holter or longer ambulatory and implantable loop recorders to capture pauses during symptoms; exercise testing for chronotropic incompetence; potassium, thyroid function and digoxin level when relevant; Chagas serology in people from endemic areas. An electrophysiological study is rarely needed.
Imaging: Echocardiography to assess structural heart disease and ventricular function.

Differential diagnosis

Physiological bradycardia in athletes or during sleep
Second- or third-degree atrioventricular block
Drug-induced bradycardia (beta-blockers, calcium channel blockers, digoxin)
Vasovagal (neurally mediated) syncope
Hypothyroidism

Goal of treatment

Treat unstable bradycardia promptly, remove reversible causes, and relieve symptoms with a permanent pacemaker when the dysfunction is intrinsic and symptomatic.

Treatment

Acute: Unstable bradycardia: atropine first, then transcutaneous pacing or an infusion of dopamine or epinephrine while preparing transvenous pacing.
Medical: Stop or reduce rate-slowing drugs when possible and correct hypothyroidism, hyperkalaemia, hypothermia or other reversible causes.
Medical: Tachycardia-bradycardia syndrome: rate-control drugs for the fast phases once a pacemaker protects against pauses, and anticoagulation according to stroke risk.
Procedure: Permanent pacemaker (usually dual-chamber or atrial-based) for symptomatic sinus node dysfunction; it is the definitive treatment.
Follow-up: Nursing priorities: continuous ECG monitoring, pulse and blood pressure before giving rate-slowing drugs, fall prevention, and after implantation, wound care, arm-movement restrictions and pacemaker education.

Complications

Syncope with falls and fractures
Atrial fibrillation and embolic stroke
Heart failure
Pacemaker complications (pocket infection, lead displacement, pneumothorax)

Prognosis

Sinus node dysfunction itself rarely causes sudden death; survival is mainly determined by the underlying heart disease. A pacemaker relieves symptoms well, but atrial fibrillation and stroke risk persist in many patients.

Prevention

Careful prescribing and review of rate-slowing drugs in older adults
Chagas disease control and screening in endemic areas
Regular follow-up of patients with known conduction disease

Test yourself

Think of the answer, then tap the question to see it.

What is the first drug for unstable symptomatic bradycardia?

Atropine, followed by transcutaneous pacing or a dopamine or epinephrine infusion if it fails.

What is the tachycardia-bradycardia syndrome?

Sinus node dysfunction in which episodes of atrial tachyarrhythmia, usually atrial fibrillation, alternate with bradycardia and long pauses when they stop.

What is the definitive treatment for symptomatic intrinsic sinus node dysfunction?

A permanent pacemaker.

Which two reversible causes should be excluded before blaming the sinus node?

Rate-slowing drugs (beta-blockers, calcium channel blockers, digoxin) and hypothyroidism; also hyperkalaemia and hypothermia.

Which coronary artery most often supplies the sinus node?

The right coronary artery, through the sinus node artery.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: Sick sinus syndrome: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026).

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Representative model, for educational purposes only.

Sinoatrial (SA) node highlighted on the Durgical 3D model