Bradycardia Part 2

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Teaching Points:

  • Basics on the AV node
  • Causes of AV blocks
  • Different types of AV blocks and how they can be differentiated by the ECG.

AV conduction disorders

In the healthy heart, the AV node is the only structure involved in the conduction of electric impulses from the atria to the ventricles. It is directly influenced by the vegetative nervous system, that is the sympathetic and the parasympathetic nervous systems. Like all cells in the cardiac excitation system, its activity also depends on the surrounding electrolyte concentrations and on the levels of catecholamines circulating in the blood.

The AV node is the heart’s natural filter for all preliminary excitations. If it didn’t exist, then an episode of atrial fibrillation, the single most frequent heart rhythm disturbance, would lead directly to ventricular fibrillation. Problems really only arise if the AV node does not perform its job properly, meaning that the spread of excitation from the atria is delayed or blocked, or that rapid atrial excitations pass into the ventricles unfiltered. In the first two situations, we speak of an AV block. Like the SA block, this can be intermittent or permanent.

The major cause of AV block is structural heart disease, in particular ischemic cardiomyopathy and hypertensive heart disease.

Because of the anatomical proximity of the AV node to the left ventricular outflow tract, AV block often occurs after aortic valve repair or replacement.

As with sinuatrial block, AV block is divided up into three degrees. The second degree is split into two further, more differentiated types. Considering the different prognoses and therapeutic consequences of these two types, it is surprising why we don’t simply refer to four degrees instead.

First-degree AV block

First degree AV block is defined by a PR interval of more than 200 milliseconds. This delay is due to a slowing of the spread of excitation from the atria to the ventricles. It is important to remember that in first and second-degree AV block, the QRS duration remains unchanged. If a widening of the QRS complex is seen as well, this points to an additional bundle branch block or other intraventricular conduction disorder.

The prognosis of first-degree AV block is excellent! It is often found in healthy people, in particular athletes with high vagotonia.

Even a PR interval of 300 milliseconds or more rarely is an indication for a pacemaker, since no progression or acute transformation into symptomatic AV blocks is to be expected.

However, PR intervals from around 250 milliseconds may be hemodynamically relevant. As always, if there are symptoms of bradycardia, such as dizziness or syncopy, further diagnostic tests must be carried out, in particular if further conduction disturbances, such as a bifascicular block, are present. But in all cases remember: first-degree AV block is not the sole reason for clinical symptoms.

Second degree AV block, type 1

As explained earlier, second degree AV Block is further divided into two sub-types:

In second degree AV block type 1 – also referred to as Mobitz type 1 or Wenckebach – the PR interval extends with each atrial excitation until one cyle of conduction is completely blocked.

This is shown on the ECG by the presence of a P-wave that is not followed by a QRS complex. The pattern then starts again from the beginning. In some cases PR intervals can’t be measured reliably. Therefore, we must consider another ECG criterion.

In second degree AV block type 1, the PR interval AFTER the blocked P-wave must be SHORTER than the PR interval BEFORE the blocked P-wave.

In this scenario, even if the length of the PQ intervals before the block are similar, we still consider the pattern to be that of type 1.

Second degree AV block, type 2

Second degree AV block is also known as Mobitz type 2. Here, every 2nd, 3rd, or n-th atrial excitation is conducted to the ventricles. In the ventricles, we talk about a 2:1, 3:1, or n:1 conduction. Since the AV node is influenced by the vegetative nervous system, and the sympathetic nerve is positively dromotropic, only a small degree of physical activation is necessary to increase the rate of conduction. So an excitation ratio of 3:1 can suddenly change to a ratio of 2:1. On occasion, the number of transmitted excitations changes several times within the space of a few heart beats.

Remember: 2:1 conduction from the atria to the ventricles can occur in both second degree AV block, Mobitz type 1 and in second degree AV Block, Mobitz type 2! However, differentiation between the two is impossible in the surface ECG. In general, though, most AV blocks with a 2:1 conduction ratio are Mobitz type 2.

Second degree AV block, type 1 is considered benign, since it originates in the AV node itself, meaning that the origin is above the bundle of His. Progression to third degree AV block is rare.

By contrast, the origin of type 2 is usually more distal to the bundle of His. For this reason, type 2 has a worse prognosis. Even without symptoms, the presence of type 2 can be an indication for an artificial pacemaker.

Third degree AV block = complete heart block

If the AV node is completely blocked, no signals can be passed from the atria into the ventricles. This is referred to as third degree AV block or total AV block. In this condition, the atria and the ventricles are depolarized independently. If the sinus node is functioning normally, it generates a continuous sinus rhythm, which does not correlate with the observed waves of ventricular depolarization. This results in a so-called atrioventricular dissociation, or AV dissociation for short.

When we look at the surface ECG, we see that the PR intervals are irregular. The P waves may be hidden by the replacement rhythm, so that some P waves seem to be missing.

All the aforementioned criteria for the AV block can only be used with an existing replacement rhythm in the ventricles. Without a replacement rhythm, we still have AV block, but must consider this asystole, even if the atrium produces P waves.

The rate and origin of the replacement rhythm depend directly on the location of the block. To be of any functional use, the origin of the replacement rhythm must be distal to the block. The most common cause of total AV block is infrahisian or trifascicular block. In this case, the ventricular replacement rhythm generates a very slow heart rate, and is associated with a broad QRS complex, which is often longer than 120 milliseconds.

Congenital AV-blocks are usually found in the AV-node itself. The replacement rhythm is often found in the bundle of His, and has a more narrow QRS-complex.

Clinically, third degree AV block can cause a variety of typical bradycardiac symptoms, including weakness, dizziness, and syncope.

Third degree AV block is a medical emergency.

Regardless of the symptoms, the implantation of a cardiac pacemaker is indicated, and patients should be transferred to an intensive care unit, as there is a risk of hemodynamically relevant bradycardia or even asystole.

This is especially true for patients whose heart show structural disease, such as ischemic cardiomyopathy or severe aortic stenosis.