ECG Interpretation Resource Practice Test

A first-degree AV block is characterized by a prolonged PR interval that exceeds 0.20 seconds while remaining constant across all beats. This delay in conduction through the atrioventricular (AV) node results in an elongated time from atrial depolarization to ventricular depolarization. The key feature of a first-degree AV block is that although the PR interval is lengthened, every atrial impulse successfully conducts through to the ventricles.

In contrast, the other choices describe different electrocardiographic phenomena. A PR interval shorter than 0.12 seconds indicates normal conduction rather than a block, while a widened QRS complex with a normal PR interval may suggest a bundle branch block or other ventricular conduction disturbance, not specifically a first-degree AV block. Lastly, an irregularly irregular rhythm without a consistent PR interval typically signifies atrial fibrillation or other arrhythmias rather than any type of AV block. Thus, the defining feature of a first-degree AV block is the sustained lengthening of the PR interval beyond the normal limit, providing a clear indication of this condition.