Second-degree heart block is a potentially serious but treatable condition affecting the heart’s electrical system. Treatment may require a pacemaker to stabilize the heart’s rhythm.

Electrical impulses that travel from the heart’s upper chambers (atria) down to the lower chambers (ventricles) keep your heart beating in a steady, efficient manner. Second-degree heart block is a condition in which the impulses from the atria occasionally fail to reach the ventricles.

Heart blocks may also be called atrioventricular (AV) blocks.

There are two different kinds of second-degree heart block: Mobitz type 1 and Mobitz type II. Mobitz type II may require a pacemaker or other treatments to maintain a healthy heart rhythm.

If not treated effectively, second-degree heart block can lead to ongoing symptoms, such as fainting and low blood pressure, or to more serious complications if the condition worsens into third-degree heart block.

This article takes a closer look at the two types of second-degree heart block, along with symptoms, causes, and treatment.

The two types of second-degree heart block, Mobitz type I and Mobitz type II, are both related to intermittent signals traveling from the sinus node in the atria down to the ventricles.

The sinus node is the portion of the heart where the electrical activity responsible for your heartbeat originates. When an electrical impulse reaches the ventricles, it causes them to contract, pumping blood out to the lungs and the rest of the body.

Mobitz type I

A diagnosis of Mobitz type I — also called “Wenckebach heart block” — means the impulses from the sinus node get slower and slower. Occasionally, the heart skips a beat entirely. Then the impulses start up again and they become increasingly slow until the heart skips another beat.

Mobitz type II

With Mobitz type II, the heart randomly drops a beat. This differs from Mobitz I, in which there’s a predictable lengthening of a portion of the cardiac cycle before a beat is dropped.

Heart blocks are categorized by severity

First-degree heart block is the mildest type. It means the impulses from the atria are slowing down significantly, but are still reaching the ventricles. First-degree heart block often requires no treatment.

Second-degree heart block is a condition in which the impulses from the atria occasionally fail to reach the ventricles. A pacemaker and other treatments may be needed to maintain a healthy heart rhythm.

Third-degree heart block is the most serious and is often considered a medical emergency. Electrical impulses no longer travel from the atria to the ventricles. It’s sometimes referred to as “complete heart block.”

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Second-degree heart block rates vs other typesShare on Pinterest
Normal heart rate vs first-degree, second-degree, and third-degree heart block (AV) rates. Illustration by Paul Lawrence.

Some people with second-degree heart block have no noticeable symptoms. But when symptoms are present, they can include:

  • chest pain
  • fatigue
  • lightheadedness
  • shortness of breath
  • nausea

Shortness of breath unrelated to physical exertion and unexplained chest pain should be treated as medical emergencies, as they can both be signs of a heart attack or other serious health problems.

If you experience episodes of lightheadedness or are feeling more tired than usual, talk with a doctor right away.

The primary cause of second-degree heart block is underlying heart disease or damage to the heart, such as heart attack, congenital heart disease, or heart surgery. Other possible second-degree heart block causes include:

Some babies are born with congenital heart block. This is usually a complication associated with an autoimmune disease of the mother.

People with heart disease are at high risk for any type of heart block. A 2019 study also suggests that people with high blood pressure and high blood glucose levels may also have an elevated risk of developing heart block.

Advancing age is another risk factor, even for people who haven’t yet received a diagnosis of any heart problems.

Mobitz type II is usually found in people with structural heart problems, often the result of a heart attack or an autoimmune disease that caused sclerosis (scarring) of the myocardium, the thick tissue of the heart muscle.

Doctors typically diagnose second-degree heart block with an electrocardiogram (ECG). An ECG is a noninvasive, painless test that uses soft electrodes placed on the skin of the chest to assess the heart’s electrical activity. ECG readings can reveal whether a heart is experiencing Mobitz type I or Mobitz type II.

If a standard ECG can’t detect second-degree heart block — sometimes due to episodes not occurring when an individual is undergoing an ECG — doctors may recommend an implantable loop monitor or a wearable Holter monitor. These devices can pick up abnormal electrical activity at any time.

A diagnosis also includes a review of your medical history and current symptoms, as well as a physical examination.

If Mobitz type I is asymptomatic, you may not need any treatment.

If you have symptomatic Mobitz I or II and the cause is reversible, such as an electrolyte abnormality or a medication side effect, correcting the underlying cause is the treatment. If the heart block goes away, a pacemaker isn’t necessary.

However, for irreversible causes, your doctor will likely recommend you have a pacemaker implanted.

A pacemaker is a small, battery-powered device placed just under the skin near the collarbone. It sends electrical signals though very thin wires or leads to the heart to keep it beating in a steady rhythm.

You may also need to take medications to get your blood pressure and blood glucose levels into healthy ranges.

Because second-degree heart block can reduce blood flow throughout the body, especially to the brain, you may experience lightheadedness and fainting. Fainting is associated with falls and fall-related injuries.

Reduced blood flow may also damage certain organs if the drop in circulation is substantial or if reduced blood flow is allowed to continue too long.

One of the main complications of Mobitz type II is that it has a greater risk of developing into third-degree heart block.

A 2022 case report suggests that in rare cases, the reduction in robust blood flow to the brain caused by second-degree heart block may cause some cognitive impairment.

With treatment, the outlook for second-degree heart block is generally quite good. However, the type of second-degree heart block you have can affect your outlook.

Mobitz type I is often asymptomatic, and in those cases, little or no interventions are necessary to maintain heart health or quality of life.

The outlook for Mobitz type II depends largely on the cause of the problem. Having a pacemaker implanted may help reduce the risk of complications and alleviate symptoms.

Second-degree heart block is a potentially serious, but treatable condition affecting the heart’s electrical system. It means the impulses necessary to keep the heart beating in a steady and robust manner don’t always move properly through the heart’s electrical pathways.

Following a prescribed medication regimen and having regular pacemaker checks with your doctor, if needed, will give you the best chances of maintaining healthy heart function for a long time.