AV Block: Degrees, Causes, and Therapy

An AV block can have different characteristics. In addition to the division into three grades, there are two different types of grade II: Wenckebach and Mobitz. Find out more here

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Location of the AV node in the heart

© W & B / Jörg Neisel

AV block - briefly explained

In the case of an AV block, the conduction of excitation from the atrium to the ventricles is impeded or interrupted. In the worst case, this can lead to cardiac arrest. The doctor uses the EKG to diagnose AV block. In the case of acute problems, medication may be considered for treatment, but ultimately only a pacemaker will help with a high-grade and permanent AV block.

What is an AV block?

The diagnosis AV block (atrioventricular block) means that the conduction of excitation between the atria (atrium = atrium) and the heart chambers (ventricle = heart chamber) is more or less disturbed. While the heart typically beats 60 to 80 times in a minute, the heart rate (pulse) can drop below 40 beats per minute with AV block. If the excitation line is severely blocked, cardiac arrest - at least temporarily - is possible.

A collection of specialized cells in the right atrium, the so-called sinus node, determines the rhythm with which the heart muscle contracts and relaxes in a coordinated manner. The electrical excitation emanating from the sinus node spreads through the muscles of the two atria. Eventually it meets the so-called AV node (atrioventricular node). It is located in the upper part of the heart septum. From there, the excitation reaches the muscles of the heart chambers via other special muscle fibers (His bundles and Purkinje fibers).

In the case of an AV block, the transmission of stimuli from the atria to the ventricle in the AV node is slowed down or interrupted.

AV block: degrees of severity

Atrioventricular block can occur in three degrees of severity:

• First degree AV block: The transmission of stimuli still works, but is delayed.

• AV block II. Degree: There are two types here.

In the Wenckebach type, the conduction delay in the AV node increases slowly from beat to beat until atrial excitation is no longer transmitted.

The AV block II type Mobitz is defined by the sudden failure of a conduction of excitation without being preceded by an increasing conduction delay.

• AV block III. Degree: There is no longer any transfer of stimuli from the atrium to the ventricle.

causes

AV blocks tend to affect older people. Because this leads to an increased storage of fiber tissue and at the same time to the loss of specific muscle fibers. The male sex is more often affected. However, an AV block can also be caused by a variety of other causes or diseases. Some examples:

  • Increased excitability of the vagus nerve
  • Medication
  • inflammatory heart disease
  • Infections
  • Heart attack
  • Damage to the conduction system after heart surgery
  • Disturbances in the salt-water balance of the body
  • Congenital AV block

Increased excitability of the vagus nerve (vagus tone)

The vagus nerve is part of the parasympathetic nervous system that acts as an antagonist to the sympathetic nervous system. Both form the vegetative nervous system, which controls numerous body functions. The vagus nerve slows the heartbeat. If the excitability of the vagus nerve is acutely or chronically increased, this can lead to a reduced heart rate and / or to a slowed conduction of excitation in the AV node. This form of AV conduction disorder (AV block I or II Wenckebach type) is not uncommon in athletes. It usually normalizes with physical exertion.

By the way: The heart of athletes often beats more slowly, sometimes with only about 40 beats per minute when resting. However, this is not due to an AV block, but to the influence of the vagus nerve on the sinus node. The heart therefore works at a lower frequency.

Medication

In particular, agents that slow the heart rhythm can cause an AV block under certain circumstances. These include beta blockers, verapamil-type calcium channel blockers, and digitalis supplements.

Inflammatory heart disease and infections

Inflammation of the heart muscle (myocarditis), infectious diseases such as borreliosis, tuberculosis, toxoplamosis, syphilis and diseases such as amyloidosis or sarcoid can disrupt or impede the conduction of excitation in the heart.

Heart attack

If a coronary artery blockage affects tissue that transmits electrical impulses from the atria to the ventricles, an AV block can occur.

Damage to the conduction system after heart surgery

Cardiac surgery or catheter-guided (TAVI) replacement of an aortic valve can damage the conduction system. This can lead to an AV block.

Disturbances in the salt-water balance of the body

Blood salts (electrolytes) such as sodium and potassium always have a certain ratio to one another. They play an important role in the transmission of stimuli. Certain diseases (for example kidney disease) can change the amount of blood salts. For example, if there is too much potassium in the blood (hyperkalemia), it can lead to AV block.

Can an AV block go away again?

It depends on the cause. If, for example, there is medication behind the conduction disorder, the heart rhythm normally returns to normal after stopping it. This can also be the case after a posterior infarction. However, if the conduction system is permanently damaged, for example by an infarction of the anterior heart wall, the AV block remains.

Symptoms

People usually do not notice a first-degree AV block.

However, with second or third degree AV block, the heart rate slows down. The heart is no longer pumping enough blood around the body. The organs are poorly supplied with oxygen, which is particularly noticeable in the brain.

Symptoms of an AV block can be:

  • Tiredness, exhaustion
  • low physical resilience (e.g. difficulty climbing stairs)
  • dizziness
  • "Blackening" in front of the eyes
  • unconsciousness
  • in extreme cases cardiovascular arrest (Adam Stokes attack)

However, the symptoms are not very specific. Symptoms such as dizziness or chronic fatigue can also be an expression of other illnesses. In any case, it is advisable to see a doctor and have the cause clarified.

diagnosis

The symptoms of patients with higher-grade AV conduction disorder are initially rather unspecific. Those affected complain of dizziness, lightheadedness, poor concentration, and possibly fainting spells. The doctor can easily identify the slowed pulse. Then it is important for him to know whether and in what dosage the patient is taking pulse-slowing medication. These include, for example, beta blockers or digitalis preparations.

The most important diagnostic methods are the EKG (electrocardiogram) and the long-term EKG. Because an AV block often only occurs temporarily. With a long-term ECG, an ECG is recorded for 24 hours or longer with a small portable recording device and then evaluated. The transmission of the electrical excitation from the atrium (small "P" wave) to the ventricle (large "R" wave) can be precisely tracked in the EKG. If the time interval between the start of the P wave and the R wave is more than 200 milliseconds, this already indicates a conduction disturbance between the atrium and ventricle.

An electrophysiological examination with a cardiac catheter is only very rarely necessary to reliably detect a suspected AV block.