What is Second-Degree Atrioventricular Block ?

The heartbeat is governed by an electrical impulse that starts in the sinoatrial node, traveling through the atria to the atrioventricular node, then down along the His-Purkinje system to reach the ventricles. This causes them to contract, creating a heartbeat. This entire process is seen on an ECG (a test that measures the electrical activity of the heart) as the PR interval leading to a QRS complex.

If there is a delay in this conduction system, it results in a block which stops or slows down the electrical signals going through the heart. This is seen on the ECG as a prolonged PR interval. These blocks can be classified as first, second, or third-degree blocks. This document particularly focuses on the second-degree block.

Within the category of second-degree blocks, there are two types: Mobitz type I (also known as Wenckebach) and Mobitz type II. People with second-degree blocks may either experience no symptoms or they may feel faint or light-headed. Depending on the severity of the problem in the conduction system, the second-degree heart block could be temporary or permanent.

What’s especially noteworthy is that Mobitz type II block has the possibility of advancing to a complete heart block, a condition which prevents the heart from beating properly. If undetected and untreated, this could potentially be fatal.

What Causes Second-Degree Atrioventricular Block ?

Mobitz type I, also known as Wenckebach, is a type of heart rhythm that is usually normal and often seen in people with a high “vagal tone” (the activity level of the vagus nerve, which influences heart rate) without any existing heart disease. However, it can sometimes be caused by poor blood supply to the lower part of the heart, effects of certain medications, high levels of potassium in the blood, a condition known as cardiomyopathy caused by Lyme disease, or after heart surgery.

Certain medications, like beta-blockers, digoxin, calcium channel blockers, and many drugs that control irregular heart rhythms, can cause a condition known as AV block, which is a disturbance in the electrical signals of the heart. An immunosuppressant drug called fingolimod, used for treating relapsing multiple sclerosis, has also been found to cause AV block.

In contrast, Mobitz type II is rarely seen in people without heart disease. It is often related to poor blood supply to the heart and scarring or hardening of the heart muscle. This rhythm can often develop into third-degree atrioventricular block, which is a complete blockage of electrical signals from the upper chambers of the heart to the lower chambers.

There can be other causes of AV block as well, including:

* Diseases that cause buildup in the heart, like amyloidosis and hemochromatosis
* Acute rheumatic fever, which is a serious complication of strep throat
* Types of cancer, like lymphoma
* Thyroid conditions – both overactive and underactive thyroid
* Diseases related to the body’s connective tissues
* Heart tumors
* After having a special kind of heart valve placed using a catheter
* Ethanol injection for a heart condition called hypertrophic cardiomyopathy, where the heart muscle becomes abnormally thick

Some people may inherit a tendency to develop AV block due to mutations in a gene called SCN5A.

Risk Factors and Frequency for Second-Degree Atrioventricular Block

Mobitz type I or II atrioventricular blocks are heart conditions for which large-scale studies have not been conducted to determine how common they are in the general population. So far, no specific age group, race, or gender has been found to be more likely to have these conditions. These types of heart block may occur in athletes or in individuals who were born with heart disorders.

Signs and Symptoms of Second-Degree Atrioventricular Block

When examining someone who could have Mobitz type II – a type of heart block – doctors typically ask about any past or present heart conditions, whether the patient has had any recent heart procedures, and what medications they are currently taking. The common symptoms of Mobitz type II include feeling tired, shortness of breath, chest pain, feeling faint or actually fainting, and sometimes a sudden cardiac arrest.

During a physical examination, doctors often find that these patients have a slow heart rate. Those who have a particularly slow heart rate, leading to low blood pressure and reduced blood flow, may look pale and sweaty. If the heart block is due to heart muscle damage, these patients may present with chest pain.

Testing for Second-Degree Atrioventricular Block

If your doctor suspects you have a heart condition known as second-degree AV block, they might use a tool called an ECG (electrocardiogram) to confirm it. This tool records the electrical signals in your heart. If there are signs that your heart might not be getting enough blood (ischemia), based on your symptoms and the ECG, other tests might be needed.

Your doctor could order a cardiac biomarker test and a chest X-ray. Cardiac biomarkers are substances that are released into the bloodstream during a heart injury like a heart attack, making them useful in identifying heart problems. The chest X-ray can show the size and shape of your heart and if there’s any damage or blockages in the blood vessels.

They may also check levels of electrolytes (minerals that help regulate heart function), calcium, and magnesium in your blood. If you’ve been taking a medication called digoxin for your heart condition, your doctor might want to check the level of this drug in your blood too because if there’s too much, it can be harmful (toxic).

Usually, you won’t need any imaging (like an X-ray or MRI) for the initial checks for second-degree atrioventricular block.

Sometimes, your doctor might recommend a procedure called electrophysiologic testing. This test lets your doctor locate where the blockage in your heart’s electrical system is and decide if you need a pacemaker, a device that helps your heart keep a normal rhythm.

Treatment Options for Second-Degree Atrioventricular Block

Mobitz Type I and Type II are conditions that affects the electrical conduction system of your heart. If you have these conditions, your heart may beat irregularly or slowly, sometimes causing symptoms such as low blood pressure.

When you have Mobitz Type I, which is also known as Wenckebach phenomenon, treatment is often not needed unless it causes a slow heart rate (bradycardia) and low blood pressure (hypotension). If this happens, one of the possible medications that can be used is atropine, which helps to regulate your heart rhythm. If atropine doesn’t work, then your doctor might consider a procedure called pacing. This involves sending small electrical impulses to your heart to regulate its rhythm. Pacing methodology can be applied through your skin (transcutaneous) or a vein (transvenous).

If you’re taking any heart medications like beta blockers, calcium channel blockers, or digoxin, your doctor might adjust the dose or ask you to stop taking them, as they can also affect your heart rhythm. It’s important if you have this condition that you’re monitored at the hospital so doctors can ensure your heart rate is becoming more regular.

Mobitz Type II is another kind of irregular heartbeat but requires a more immediate intervention. Once identified, doctors will typically start pacing right away. The condition often implies some structure damage to the system in your heart that regulates rhythm. It may worsen into a complete heart block, which would disrupt the electrical signal from the top chambers of your heart to the bottom chambers. Similar to Mobitz Type I, patients with this condition will need pacing. However, they usually need a permanent pacemaker implanted, which is a small device placed in the chest or abdomen to control abnormal heart rhythms.

Unlike those with Mobitz Type I, patients with Mobitz Type II who experience bradycardia and hypotension often don’t respond to atropine. That means their doctors will have to seek out other treatment options.

When evaluating medical conditions with similar symptoms to one another, doctors may consider the following possibilities:

Congenital heart block (a heart condition present at birth)
Sinoatrial exit block (a block in the path of electrical impulses in the heart)
Non-conducted premature atrial complexes (irregular heartbeats)
First-degree atrioventricular block (a heart rhythm problem)
Myocardial infarction (heart attack)
Medication toxicity (harmful effects from medication)
Third-degree atrioventricular block (a serious heart rhythm problem)

What to expect with Second-Degree Atrioventricular Block

The chances of recovering from Mobitz type I (also known as Wenckebach) are excellent. This condition often shows no symptoms and no treatment is needed if the patient isn’t experiencing any discomfort.

On the other hand, the recovery chances from Mobitz type II can differ depending on what’s causing the irregular heart rhythm. You might need a device called a pacemaker implanted. This helps to prevent complications from Mobitz type II such as a decrease in the amount of blood your heart is able to pump, a slower heart rate that causes symptoms like fatigue or dizziness, or a sudden stop in the heart’s function.

Possible Complications When Diagnosed with Second-Degree Atrioventricular Block

Possible Symptoms:

Total loss of heart function
Fainting
Feeling lightheaded
Pain in the chest
Death

Recovery from Second-Degree Atrioventricular Block

Everyone who has AV block (a condition where the electrical signaling in your heart is slowed or blocked) will need ongoing check-ups for the rest of their lives.