Which describes the key features of first-degree AV block on an ECG?

First-degree AV block is characterized by a prolonged PR interval on the ECG, which indicates a delay in the conduction through the atrioventricular (AV) node. In this condition, despite the delay, every atrial impulse is still conducted to the ventricles, resulting in the consistent relationship between the P waves and QRS complexes. This means that while the PR interval is longer than the normal range (greater than 200 milliseconds), each P wave is followed by a QRS complex, demonstrating that conduction is still intact. Therefore, the key feature highlighting first-degree AV block is a prolonged PR interval accompanied by the presence of conducted QRS complexes.

This distinguishes it from other arrhythmias or conduction abnormalities, where the lack of P waves or varying conduction patterns might be observed. Regularly spaced QRS complexes can occur in various rhythms, making it too vague to describe first-degree AV block specifically. Irregular rhythms signify different types of block or arrhythmias, and wide QRS complexes typically suggest a different conduction issue unrelated to first-degree AV block.