Bradycardia

How the heart's electrical system works

Each heartbeat begins in the sinoatrial (SA) node, a cluster of cells in the right upper chamber that acts as the natural pacemaker. The signal spreads across the atria (upper chambers), then passes through the atrioventricular (AV) node and down the His-Purkinje fibers to the ventricles (lower chambers), which contract and pump blood.

Bradycardia develops when:

• the SA node fires too slowly or pauses (sinus node dysfunction), or
• the signal is delayed or blocked on its way to the ventricles (AV block or heart block)

When the ventricles are not activated, a backup ("escape") pacemaker lower in the heart may take over, but at a slow rate of 20 to 40 bpm.

Types of bradycardia

See heart block for more on conduction block.

Causes

Intrinsic (problems in the heart itself)

• Age-related fibrosis of the conduction system, the most common cause in older adults
• Coronary artery disease and heart attack, especially inferior heart attacks
• Cardiac surgery, valve replacement (including TAVR) or catheter ablation
• Infiltrative diseases: sarcoidosis and amyloidosis
• Infections: Lyme carditis, myocarditis, endocarditis with abscess
• Congenital heart block, sometimes linked to maternal lupus antibodies
• Inherited conduction diseases

Extrinsic (outside influences, often reversible)

• Medicines: beta-blockers, non-dihydropyridine calcium channel blockers, digoxin, amiodarone and other antiarrhythmics, ivabradine, clonidine, lithium, donepezil
• Electrolytes: high potassium (hyperkalemia)
• Hypothyroidism
• Obstructive sleep apnea
• High vagal tone: athletes, vomiting, pain
• Hypothermia
• Raised intracranial pressure

Symptoms

Bradycardia only causes symptoms when the slow rate reduces blood flow:

• dizziness, lightheadedness and fainting
• fatigue and reduced exercise capacity
• breathlessness
• chest pain
• confusion or poor concentration
• worsening heart failure

Symptoms may be intermittent, especially with sick sinus syndrome or intermittent block, which makes monitoring important.

Diagnosis

1. 12-lead ECG: confirms the rate and shows the type of block.
2. Ambulatory monitoring: a 24 to 48 hour Holter monitor, a patch monitor for up to 2 weeks, an event monitor for a month, or an implantable loop recorder for up to 3 years when episodes are rare.
3. Exercise testing: checks whether the heart rate rises with exertion (chronotropic incompetence).
4. Blood tests: potassium, thyroid function, drug levels and troponin if a heart attack is possible.
5. Echocardiogram: looks for structural heart disease.
6. Sleep study: if sleep apnea is suspected.
7. Electrophysiology study: occasionally used to test the conduction system directly.

The goal is to link symptoms to the slow rhythm. A slow rate without symptoms often needs no treatment, apart from some types of advanced heart block.

Treatment

Treat reversible causes first

• Reduce or stop the responsible medicine with your doctor's guidance
• Correct potassium and thyroid levels
• Treat sleep apnea with CPAP
• Antibiotics for Lyme carditis, which often recovers fully

Emergency treatment

For dangerous bradycardia with low blood pressure, doctors use atropine, medicines such as dopamine or epinephrine infusions, and temporary pacing through skin patches or a wire into the heart, until the cause is fixed or a permanent pacemaker is placed.

Permanent pacemaker

A pacemaker is the main long-term treatment for symptomatic sinus node disease and for Mobitz II or complete heart block. It is a small device, usually placed under the skin below the collarbone under local anesthetic, with one or two wires (leads) into the heart. Leadless pacemakers, about the size of a large vitamin capsule, sit inside the heart without wires. The device senses the heart's own beats and only paces when the rate drops too low.

Most people go home the same or next day. You avoid raising the arm on that side above shoulder height for a few weeks, and the device is checked every 6 to 12 months, often remotely. Batteries last around 7 to 15 years. Modern pacemakers are compatible with most household electronics, and many are MRI-conditional.

Outlook

Bradycardia caused by fitness or sleep needs no treatment. Reversible causes often resolve completely. People with a pacemaker for sinus node disease or heart block usually have a normal or near-normal life expectancy and quality of life, though outcomes also depend on any underlying heart disease.

Frequently asked questions

What causes bradycardia?

The most common causes are age-related wear of the heart's electrical system, heart medicines such as beta-blockers, heart attacks, an underactive thyroid, high potassium and sleep apnea. In athletes and during sleep, bradycardia is a normal sign of a strong heart and high vagal tone.

How is bradycardia treated?

Harmless bradycardia needs no treatment. When symptoms occur, doctors first look for reversible causes such as medicines, thyroid disease or potassium problems. If the conduction system itself is failing, a pacemaker is fitted to keep the heart rate from falling too low.

When does bradycardia need a pacemaker?

A pacemaker is recommended when a slow rhythm from sinus node disease causes symptoms such as fainting or breathlessness, and for Mobitz II second-degree or complete heart block, even sometimes without symptoms. It is not needed for harmless sinus bradycardia or first-degree block.

Can bradycardia be cured?

Bradycardia from medicines, an underactive thyroid, high potassium, Lyme disease or sleep apnea often resolves when the cause is treated. Age-related conduction disease does not reverse, but a pacemaker controls it effectively.

Is bradycardia the same as heart block?

No. Bradycardia means a slow heart rate from any cause. Heart block is one cause, in which electrical signals are delayed or blocked between the upper and lower chambers. Other causes include a slow sinus node and medicines.