3. Blocks and conduction disorders, diagnosis and drug treatment

Atrioventricular blocks

Definition

Atrioventricular (AV) block is characterised by a partial or complete block of conduction from the atria to the ventricles, usually at the level of the AV node. There are multiple different types, but the major causes of all are similar:

  • Idiopathic
  • Myocarditis
  • Hyperkalaemia
  • Drugs slowing AV node conduction (beta blockers, digoxin, CCBs)
  • Ischaemic heart disease
  • Hypothyroidism
  • Infiltrative heart diseases (amyloidosis, sarcoidosis)

We distinguish multiple different types:

  • 1st degree AV block – the conduction of atrial impulses to the ventricles is delayed
  • 2nd degree AV block – some atrial impulses fail to reach the ventricles
    • Mobitz type I (Wenkebach)
    • Mobitz type II
  • 3rd degree AV block – all atrial impulses fail to reach the ventricles

1st degree AV block

1st degree AV block is characterised by a PQ/PR interval which is longer than normal (> 200 ms), but all atrial impulses reach the ventricles. It can occur in normal, healthy people.

1st degree AV block is almost always asymptomatic and harmless, not requiring treatment.

2nd degree AV block Mobitz type I

In Mobitz type I 2nd degree AV block the PQ interval is progressively longer with each beat, until a beat is dropped (a P wave is not followed by a QRS complex). It is also called Wenckebach block or Wenckebach pattern.

Like 1st degree AV block, this can occur in healthy people and is often asymptomatic.

2nd degree AV block Mobitz type II

In Mobitz type II 2nd degree AV block some atrial impulses (P waves) are suddenly not conducted to the ventricles (not followed by a QRS), often at a regular interval, like 3:2, 2:1, 4:3, etc. If more than one atrial impulse in a row is not conducted, it’s called “high grade AV block” and is more severe.

Most people with Mobitz type II are symptomatic. They experience typical symptoms of AV block, like fatigue, dyspnoea, presyncope, syncope, or even sudden cardiac arrest.

Mobitz type II can quickly progress to 3rd degree AV block, and so all patients must be continuously monitored. If the patient is haemodynamically unstable, they should be treated with 0,5 mg IV atropine and temporary cardiac pacing. Atropine increases AV conduction and may partially reverse the block.

While being monitored and once stabilised, reversible causes of AV block should be excluded and treated if present. If no reversible cause can be found and treated, the patient requires a permanent pacemaker as the risk of sudden cardiac death is high.

3rd degree AV block

In 3rd degree AV block (complete heart block), no atrial impulses are conducted to the ventricles. This causes an escape rhythm to kick in, originating somewhere more downstream from the site of the block. With the atria being paced by the SA node and the ventricles being paced by some other focus, the P waves and QRS waves will be independent of each other on the ECG.

The rate and morphology of the escape rhythm depends on where it originates from. Junctional or AV nodal escape rhythms have narrow QRS and have a rate of about 40 to 60 bpm. Ventricular escape rhythms have wide QRS and a rate of less than 40 bpm.

3rd degree AV block is almost always symptomatic. The symptoms depend on the rate of the escape rhythm, and include fatigue, dyspnoea, presyncope, syncope, or even sudden cardiac arrest.

If the patient is haemodynamically unstable, they should be treated with atropine and temporary cardiac pacing. Once stabilised, reversible causes of AV block should be excluded and treated if present. If no reversible cause can be found and treated, the patient requires a permanent pacemaker.

Bundle branch blocks

Definition

Bundle branch blocks are conduction blocks in the left or right bundle branches. The Bundle of His divides into the left bundle branch and right bundle branch. Blocks in these conducting pathways cause the depolarisation of part of the ventricle to occur later than the rest, resulting in a wide QRS complex.

Bundle branch blocks rarely cause symptoms or need treatment in and of themselves, but they’re clinically significant in that they are often signs of heart disease. As such, patients with LBBB or RBBB must be evaluated for heart disease.

Etiology

  • Idiopathic slowly progressive degeneration of the conduction system
  • Hypertensive cardiomyopathy
  • Coronary artery disease (especially acute myocardial infarction)
  • Cardiomyopathy
  • Myocarditis

Left bundle branch block

In left bundle branch block (LBBB) a block in the left bundle branch causes the left ventricle to depolarise later than the right. This causes certain characteristic ECG features.

The characteristic ECG features include:

  • Wide QRS (> 120 ms)
  • Wide and notched R wave in leads I, aVL, V5, V6 (forming an “M” shape)
  • Absent Q waves in leads I, aVL, V5, V6
  • Deep (sometimes notched) S wave in leads V1, V2, V3 (sometimes forming a “W” shape)

The ECG findings can be explained. The delay in depolarisation of the left ventricle causes the Q wave to be absent in leads corresponding to the left ventricle (I, aVL, V5, V6). The delayed activation of the left ventricle prolongs the leftward progression of the depolarisation, leading to a positive and widened R wave in the same leads. The deep S wave in the leads corresponding to the right ventricle may perhaps be explained by the lack of a counterpart depolarisation in the left ventricle.

LBBB is a sort of “ST-elevation-equivalent” in the diagnosis of acute myocardial infarction. The definition of a STEMI includes ECG evidence of either ST elevation or a new onset LBBB. As such, a person with a new onset LBBB and positive troponin dynamics is considered to have STEMI, even in the absence of ST elevation.

Right bundle branch block

In rights bundle branch block (RBBB) a block in the right bundle branch causes the right ventricle to depolarise later than the left. This causes certain characteristic ECG features.

In addition to the above mentioned etiologies, RBBB is associated with lung pathologies like pulmonary hypertension, embolism, or COPD. This is because these pathologies often put strain on the right ventricle and the right bundle branch.

The characteristic ECG features include:

  • Wide QRS (> 120 ms)
  • RSR’ pattern in leads V1, V2 (forming an “M” or “rabbit ears” shape)
  • Wider S wave than R wave in leads I, V5, V6

The first R in leads V1 and V2 correspond to the depolarisation of the left ventricle, while the second R (the R’) in the “rabbit ears” is delayed and corresponds to the depolarisation of the right ventricle, which occurs after the depolarisation wave has spread from the left ventricle to the myocardium to the right ventricle.


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