Which ECG abnormality is associated with increased risk of arrhythmic causes of syncope?

Mobitz type II second-degree AV block is characterized by intermittent non-conducted P waves that occur in a regular pattern, leading to dropped QRS complexes without a preceding progressive lengthening of the PR interval. This type of block is particularly concerning because it can progress to complete heart block, which significantly increases the risk of severe bradycardia and subsequently arrhythmias, which can lead to syncope.

Patients with Mobitz type II are more likely to experience syncope due to these arrhythmic events, which are often unpredictable and may occur suddenly. This contrasts with other types of blocks, such as Mobitz type I, where the risk of sudden progression to dangerous arrhythmias is lower, and the patient often exhibits warning signs due to the progressive nature of the block.

In summary, the association of Mobitz type II AV block with arrhythmic causes of syncope stems from its potential for rapid progression to complete block and bradyarrhythmias, making it a critical abnormality to recognize in clinical practice.