What is one of the ECG features that indicates a possible arrhythmic cause of syncope?

Persistent sinus bradycardia is associated with a higher risk of arrhythmias, particularly in situations where it is symptomatic, such as syncope. In the context of syncopal episodes, bradycardia can lead to inadequate cardiac output, which may result in reduced cerebral perfusion and, consequently, loss of consciousness.

In contrast, sinus tachycardia, while indicating increased heart activity, is often a physiological response to stress, fever, or other non-arrhythmic causes rather than a direct indicator of an arrhythmic event. A normal QT interval suggests a lower risk of arrhythmias since prolonged QT intervals are directly associated with increased risk for Torsades de Pointes and other serious arrhythmias. First-degree AV block typically does not present immediate hazards in clinical practice as it often is an incidental finding that does not significantly affect heart function in the absence of other conditions.

Thus, persistent sinus bradycardia stands out as a feature that may signal serious underlying arrhythmic complications capable of leading to syncope.