AV Block ECG Patterns

The term “AV block” describes conduction abnormalities between the atria and ventricles. These can range from a simple delay (first-degree block) to intermittent failure of conduction (second-degree block) to complete dissociation (third-degree block). ECG interpretation focuses on the relationship between P waves and QRS complexes, and on the PR interval.

First-degree AV block is a conduction delay (not a “dropped beat”). Every atrial impulse conducts to the ventricles, but more slowly than normal.

ECG features: PR interval is > 200 ms and constant across beats. P waves are followed by QRS complexes in a 1:1 pattern. QRS complexes are usually narrow unless there is additional conduction disease.

Type I second-degree AV block is characterised by progressively increasing conduction delay until an impulse fails to conduct and a ventricular beat is dropped.

ECG features: PR interval progressively lengthens beat-to-beat until a non-conducted P wave occurs (a “dropped” QRS). After the dropped beat, the cycle repeats. The conduction abnormality is often within the AV node.

Type II second-degree AV block occurs when conduction fails intermittently without prior progressive PR prolongation. Some atrial impulses conduct normally while others do not conduct at all.

ECG features: PR interval is typically constant for conducted beats, but intermittent non-conducted P waves occur (dropped QRS complexes). This often appears in a pattern (e.g., 2:1, 3:1). The block is more likely within the His-Purkinje system and may be associated with a wider QRS if bundle branch disease is present.

Third-degree AV block is complete failure of conduction from atria to ventricles. The atria and ventricles beat independently: atrial activity is driven by the SA node, while the ventricles are driven by an escape rhythm.

ECG features: AV dissociation with P waves and QRS complexes occurring regularly but with no consistent relationship between them. Ventricular rate depends on the escape focus: junctional escape rhythms are often faster than ventricular escape rhythms, and QRS width may vary accordingly.