Heart Block (Atrioventricular Block)

How the heart's electrical system works

Each heartbeat starts in the sinus node in the right atrium. The signal spreads across the atria, then reaches the AV node, which briefly delays it so the ventricles can fill. It then passes through the bundle of His and the left and right bundle branches to the ventricles, which contract.

Heart block happens when this signal is slowed or stopped at the AV node or in the conducting pathway below it. Blocks lower down (in the His bundle or both bundle branches) are more dangerous, because the backup rhythm there is slow and unreliable. A block in only one bundle branch is a separate condition, bundle branch block.

Types of heart block

"2:1 block", where every second beat is dropped, and "high-grade block", where two or more beats in a row are dropped, are forms of second-degree block that doctors treat cautiously.

Symptoms

Mild heart block usually produces no symptoms and is found on a routine ECG. When the heart rate drops enough to reduce blood flow, symptoms include:

• dizziness or lightheadedness
• fainting (syncope), sometimes without warning ("Stokes-Adams attacks")
• tiredness and reduced exercise tolerance
• shortness of breath and ankle swelling from heart failure
• chest pain
• confusion, especially in older adults
• a sense of skipped beats or a pounding in the neck

The pulse in complete heart block is typically 30 to 45 beats a minute when the backup rhythm comes from below the AV node.

Causes and risk factors

Acquired causes (most common)

• Age-related fibrosis: gradual scarring of the conduction system (Lenègre and Lev disease) is the commonest cause in older adults.
• Heart attack: especially an inferior heart attack, which often causes temporary AV block, or an anterior one, which can cause permanent block.
• Medicines: beta blockers, non-dihydropyridine calcium channel blockers (verapamil, diltiazem), digoxin, amiodarone and some other heart rhythm drugs. Combinations are a common cause in older adults.
• Heart surgery and procedures: valve replacement, including TAVR, septal ablation and congenital heart repairs.
• Infections: Lyme disease can cause heart block that is usually reversible with antibiotics. Also endocarditis, myocarditis and rheumatic fever.
• Infiltrative and inflammatory disease: cardiac sarcoidosis (suspect it in anyone under 60 with unexplained heart block), amyloidosis, lupus.
• Electrolyte and hormone problems: high potassium (hyperkalemia) and severe hypothyroidism.
• High vagal tone: athletes and people during sleep can show first-degree or Mobitz I block, which is usually normal.

Congenital heart block

Some babies are born with complete heart block, most often because the mother has anti-Ro (SSA) antibodies linked to lupus or Sjögren syndrome. Pregnant women with these antibodies may have fetal heart monitoring.

How heart block is diagnosed

• 12-lead ECG: identifies the type of block.
• Holter monitor or event recorder: records your rhythm over 24 hours to several weeks to catch intermittent block. An implantable loop recorder can monitor for up to 3 years when fainting is unexplained.
• Exercise test: block that worsens with exercise suggests disease below the AV node.
• Blood tests: potassium, thyroid function, troponin, and Lyme serology when relevant.
• Echocardiogram: checks heart muscle and valves.
• Cardiac MRI or PET: for suspected sarcoidosis or other infiltrative disease.
• Electrophysiology study: occasionally used to locate the level of block.

Heart block treatment

No treatment needed

First-degree block and asymptomatic Mobitz I usually need only monitoring and a review of medicines.

Treat the reversible cause

Stopping or reducing heart-slowing medicines, correcting potassium, treating Lyme disease with antibiotics, or treating a heart attack can resolve the block. A temporary pacemaker wire may support the heart in the meantime.

Emergency treatment

For a dangerously slow heart, doctors give atropine, adrenaline-like drug infusions or external pacing through pads on the chest, then a temporary pacing wire if needed.

Permanent pacemaker

A pacemaker is recommended for Mobitz II, high-grade and third-degree block, and for any type that causes symptoms without a reversible cause. A small generator is placed under the skin below the collarbone, with leads into the heart (or a leadless capsule inside it). It is usually done under local anesthetic, and most people go home the same day or the next morning. You avoid heavy lifting with the arm on that side for about 4 to 6 weeks, and the device is checked regularly, often remotely.

Living with heart block

• Tell every doctor and dentist you have heart block or a pacemaker, and carry your device card.
• Check new prescriptions with your doctor or pharmacist for heart-slowing effects.
• Most people with pacemakers drive (after a short break set by local rules), exercise and live normally.
• Keep phones and headphones at least 15 cm (6 inches) from the pacemaker; household appliances are safe. Tell security staff before walking through scanners.

Frequently asked questions

Is first-degree heart block serious?

Usually not. First-degree heart block means the signal is delayed but every beat gets through. It causes no symptoms, is common in older adults and athletes, and rarely needs treatment. Your doctor may review medicines that slow the heart and repeat an ECG over time.

What is the most dangerous type of heart block?

Third-degree (complete) heart block and second-degree Mobitz type II are the most serious. In complete block no signals reach the ventricles, so the heart relies on a slow, unreliable backup rhythm, causing fainting and risk of sudden cardiac arrest. Both usually need a pacemaker.

Can heart block go away?

Yes, when the cause is reversible. Block caused by medicines, high potassium, Lyme disease or a recent inferior heart attack often resolves once the cause is treated. Block caused by age-related scarring of the conduction system is usually permanent and tends to progress.

What is the life expectancy with heart block?

Untreated complete heart block carries a high risk of fainting, heart failure and sudden death. With a pacemaker, life expectancy is generally close to that of people of the same age without heart block, depending on any underlying heart disease. First-degree block alone does not shorten life in most people.

What causes heart block in young people?

In young people, common causes are high vagal tone from athletic training, Lyme disease, myocarditis, cardiac sarcoidosis, congenital heart block from maternal antibodies, and heart surgery for congenital defects. Unexplained heart block under age 60 is usually investigated for sarcoidosis.

Can you exercise with heart block?

Most people with first-degree or Mobitz I block can exercise normally. People with higher-grade block should avoid strenuous exercise until assessed. After a pacemaker, most activities are allowed once the leads settle, usually after about 6 weeks, apart from heavy contact sports that could damage the device.