6th March 2024

Atrioventricular block

This occurs due to impaired conduction between the atria and ventricles. This may be at the level of the AV node (intranodal) or within His-Purkinje system (infranodal). Causes are predominantly due to idiopathic fibrosis with increasing frequency in old age. Other causes include ischaemia, infiltration (e.g., amyloidosis, haemochromatosis), infections such as endocarditis and diphtheria, negative chronotropic and antiarrhythmic drugs. There are three types of intranodal AV blocks:

• 1st degree: prolongation of the PR interval (>200ms or 5 small squares on a standard ECG at 25mm/s)
• 2nd degree: dropped QRS complexes following non-conducted P waves. This is divided into:
  • Mobitz 1 (Wenckebach): PR interval gradually prolongs until a QRS complex is dropped.
  • Mobitz 2: characterised by a fixed block without PR prolongation; often two or three P waves precede a successfully conducted QRS complex (termed 2:1 or 3:1 block respectively).
• 3rd degree or complete block is where there is complete dissociation between the P waves and QRS complexes. Here, the P-P and R-R intervals are preserved, but the two are not associated with each other. The resulting heartbeat originates from an ectopic focus and is known as a junctional escape beat. 

In addition to the ECG changes in complete heart block, clinical examination may reveal a variable S1 due to the uncoordinated nature of atrial and ventricular contraction, resulting in changes in preload and blood flow from atria to ventricles in each cardiac cycle. Assessment of the JVP will reveal cannon a-waves caused by the atria contracting against a closed tricuspid valve.