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Heart block (slow or blocked heart signals)

Also called: Heart block, AV block, first degree heart block, complete heart block

The heart's electrical signals are slowed or blocked between the upper and lower chambers. Mild forms cause no symptoms; severe forms cause a very slow pulse, fainting, and need a pacemaker.

1st degreeUsually harmless
2nd degreeSome beats dropped
3rd degree (complete)Needs a pacemaker

Overview

Some medicines, Lyme disease, and heart attacks can cause it.

Athletes may have mild block that's normal.

Symptoms

Common signs:

  • Slow pulse
  • Slow heart rate

Less common

  • Dizziness
  • Fainting
  • Tiredness
  • Shortness of breath

Look up a symptom: Dizziness

When to get medical care

Call 911Fainting, chest pain, or shortness of breath.
Get care todayDizziness with a slow pulse.
See a doctor soonHeart block on an ECG.

Causes

Aging, medicines, heart disease.

Does it spread or run in families?

  • It isn't contagious. You can't catch it from someone else.

Who is more likely to get it

  • Older age
  • Heart disease

Diagnosis

An ECG shows the type.

Questions you may be asked

  • Medicines, fainting

Tests

Tests, if needed:

  • ECG:
  • Heart monitor:
  • Lyme test: Sometimes.

Treatment

Treatment depends on type.

Self-care

  • Review heart-slowing medicines
  • Report fainting
  • Check your pulse
  • Tell doctors about any fainting

A pacemaker for severe block.

Prevention

To lower your risk or prevent problems:

  • Treat heart disease

Which doctor treats this

  • Start with your primary care provider (family doctor or internist).
  • They may send you to a cardiologist (heart doctor).

Living with it

Support groups and helplines

Children, pregnancy, and older adults

  • Athletes: Mild block can be normal.

Pregnancy and breastfeeding

  • If you're pregnant or planning to be, tell your prenatal provider about this condition.
  • This condition needs closer checkups during pregnancy. Plan ahead with your doctor.
  • Don't start or stop any medicine on your own. Some are safe in pregnancy and some need to be changed.
  • Ask before taking any pain reliever. The FDA advises avoiding ibuprofen and naproxen after 20 weeks unless your doctor says to.
  • Breastfeeding: Most medicines are fine, but ask. The NIH LactMed database lists what's known about each one.

Outlook

Pacemakers work very well.

Getting ready for your visit

Bring a list of your symptoms and medicines.

Questions to ask

  • Do I need a pacemaker?
  • Is my medicine slowing my heart?

Research studies

Research studies test new ways to find and treat health problems. Some are looking for volunteers. Talk with your doctor before joining one.

Search ClinicalTrials.gov for studies on Heart block (slow or blocked heart signals)

Sources

Last updated 2026-11-12.