Atrioventricular (Heart) Block

The PR interval represents the time taken for depolarisation to spread from the sinoatrial node to the ventricles i.e. it measures the conduction of the atrioventricular node. If there is a problem here, it results in a heart block.

The Conduction System of the Heart

Just as a reminder, the conduction system is as follows:

1. Sinoatrial node (SAN)
2. Atrial depolarisation
3. Atrioventricular node (AVN)
4. Bundle of His
5. Left and right bundles (LBB/RBB)
6. Purkinje Fibres
7. Ventricular depolarisation

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Conduction System of the Heart

Atrioventricular block, also called heart block, occurs when there is a disease in the AVN which results in a prolonged delay in the conduction of depolarisations of the SAN towards the ventricles.

First Degree Heart Block

• Depolarisations of the SAN are conducted to the ventricles, but this occurs slowly.
   
• Results in a prolonged PR interval – remember, the PR interval represents time taken for depolarisation to spread from the sinoatrial node to the ventricles.
   
• This may be a normal variant, but it can also occur in individuals with increased vagal tone such as athletes.

Second Degree Heart Block

In second degree heart block, there are some depolarisations of the SAN that fail to be conducted through to the ventricles. There are two types of second-degree heart block.

Mobitz Type I (Wenckebach)

• The PR interval gets progressively longer, and then a ventricular beat will drop i.e. you’ll get a P wave followed by an absent QRS complex.
   

Mobitz Type II

• The PR interval is constant, and every so often there will be a dropped beat i.e. P wave followed by absent QRS complex.
   
• Mobitz type II occurs due to a defect below the level of the AVN e.g. at the bundle of His or in the bundle branches. This can result in wide QRS complexes (QRS complex is demonstrating the depolarisation of the ventricle). There is a greater risk of Mobitz II progressing to third degree heart block compared to Mobitz I.
   

Advanced Heart Block

• This is when a set number of P waves are conducted to the ventricles e.g. every second P wave is conducted. That means for every 2 P waves, there is 1 QRS complex (2:1 heart block). This too, carries a risk of progression into third-degree heart block.
   

Third Degree Heart Block

In third degree heart block, no SAN depolarisations conduct through to the ventricles, and the ventricles are relying on an ‘escape rhythm’ i.e. rhythm which is not arising from the SAN. Usually, this rhythm is generated by the AVN, bundle of His or Purkinje fibres and beats slower than the atria.

In this case, there is no pattern with the PR interval and there is no relationship between the P waves and QRS complexes because the atria and ventricles are beating independently.
 

Causes

First-degree and Mobitz I may be simple benign variants, whilst Mobitz II and third-degree heart block usually indicate a more serious underlying cause. This can be:

• Following a myocardial infarction e.g. the right coronary artery supplies the AVN, so an inferior MI can result in an AVN block.
• Idiopathic fibrosis
• Myocarditis
• Drugs which slow conduction through the AVN
  • Non-dihydropyridine calcium channel blockers
  • Beta-blockers
  • Digoxin
  • Amiodarone
• Lyme’s disease
• Hyperkalaemia

Clinical Features

The symptoms of heart block can vary depending on the degree of heart block. For example, first degree heart block may cause no symptoms at all. Some patients with higher degrees of heart block may complain of lightheadedness, chest pain, dyspnoea, fatigue, and confusion.

Sometimes, patients can present with syncope which occurs suddenly without warning. These are sometimes known as Stokes Adams attacks, which are characterised by sudden collapse followed by a short period of unconsciousness and a rapid recovery.  They are usually caused by a third-degree heart block but other causes do exist.

Investigations

12-lead ECG is the main investigation of choice for diagnosis.

Management

Some types of AVN heart block i.e. first-degree and Mobitz I usually do not require treatment, especially if patients are asymptomatic. If patients do develop symptoms, drugs which block the AVN should be stopped e.g. beta-blockers. For especially severe symptoms however, treatment may be necessary.

Higher grade heart blocks such as Mobitz type II or third-degree heart block usually warrant management with cardiac pacing. NICE recommends that patients with symptomatic bradycardia as a result of atrioventricular block receive dual-chamber cardiac pacing. If the patient is particularly frail, has specific co-morbidities, or has continuous atrial fibrillation in addition to atrioventricular block, single chamber pacing of the ventricle may be more appropriate.

References

https://www.nice.org.uk/guidance/ta88/chapter/1-guidance

Davidson's Principles of Medicine 23rd Edition