Heart block, or atrioventricular (AV) block, means that the electrical signal in a person’s heart does not travel through it correctly. Second degree heart block type I means that the signal sometimes does not reach the ventricles, causing skipped beats.

There are three subtypes of second-degree heart block. Type I is often benign and may not require treatment, whereas the others are more serious.

Learn more about second degree heart block type I, including symptoms, causes, treatment, and how it differs from other types of second degree heart block.

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The heart has a complex electrical system. In heart block, the electrical signal does not travel from the top part of the heart — the atria — to the bottom part of the heart — the ventricles. This affects heart rhythm.

Doctors can determine the type of heart block by checking the rhythm and electrical activity of a person’s heart using an electrocardiogram (EKG). The “P wave” on the test shows when the atria contracts. The QRS complex — a combination of the Q, R, and S waves — shows when the ventricles contract.

Second degree heart block is a missed heartbeat with either a sudden or progressive delay in how long it takes for the signal to move across the atria.

Experts break this form of heart block into three subtypes: Second degree heart block Mobitz type I — also called Wenckebach heart block — second degree heart block Mobitz type II, and second degree high grade.

  • Second degree Mobitz type I: The heart’s electrical signal gets weaker, and eventually, it cannot send an electrical impulse between the atria and ventricles, causing the heart to skip a beat. This form usually does not require treatment.
  • Second degree Mobitz type II: Structural damage, such as that from a heart attack, causes heart cells to fail to conduct an electrical impulse. This can lead to unstable blood flow and worsening heart block.
  • Second degree high grade: This is two or more blocked P waves in a row. However, some electrical impulses travel between the ventricles and atria, differentiating it from third degree heart block.

Learn more about the types of heart block.

Second degree heart block type I is often harmless and does not require treatment. It sometimes occurs in athletes, individuals with very healthy hearts, and during sleep.

In second degree heart block type II, issues with the heart’s electrical signal come on suddenly rather than intermittently. Rather than slowing the heart rate or causing skipped beats, it causes sudden changes in the rhythm below the atria.

This increases the risk of dangerous heart rhythms, fainting, and sudden cardiac death. People with type II are also more likely to have symptoms such as fatigue, exercise intolerance, and dizziness or fainting.

Second degree heart block type II may also progress to third degree heart block, which is a complete heart block.

Second degree heart block type I may not cause any symptoms at all.

If it does, a person may experience:

Second degree heart block type I can occur in people with no heart disease, including trained athletes.

However, second degree heart block type II usually occurs in people with an underlying condition that damages the heart. Examples include:

Some medications used to treat heart disease may also increase the risk of heart block. These include:

A 2019 study identified several risk factors for heart block, including:

However, these risk factors may be absent in people with second degree heart block type I. This type can occur in athletes and people with otherwise healthy hearts.

Experts do not know how common second degree heart block is due to no large population studies measuring the prevalence.

No specific research has consistently found it to be more common in certain populations. Some smaller studies have found a higher risk of heart block in males, but this research is not definitive.

People with second degree heart block type I are less likely to progress to complete heart block than those with second degree heart block type II.

If they have no symptoms, they may not need treatment. However, if the condition causes a slow heartbeat or dangerously low blood pressure, a person may need medication, such as the drug atropine. In some cases, they may need a pacemaker.

Heart-healthy lifestyle changes designed to reduce blood pressure and cardiovascular risk factors can also be helpful. If a person develops a heart block due to another disease, a doctor may prescribe additional treatments.

The outlook for people with second degree heart block type I is good.

People often have no symptoms, and this type of heart block does not typically progress to more severe forms of heart block. They may need no treatment at all.

Second degree heart block type I is often a benign condition that does not usually progress to more serious types of heart block. However, in some people, it may cause symptoms or indicate that a person has an underlying medical condition.

People who have signs of heart disease or an irregular heart rhythm should seek medical care.