What does an ECG show during first-degree AV block?

A first-degree AV block is characterized by a delay in the conduction through the atrioventricular (AV) node, which manifests as a prolonged PR interval on the electrocardiogram (ECG). The PR interval is the time from the beginning of atrial depolarization to the beginning of ventricular depolarization, and in first-degree AV block, this interval exceeds the normal range of 120-200 milliseconds.

The extension of the PR interval indicates that while the electrical signals are still being transmitted from the atria to the ventricles, they are doing so more slowly than usual. Importantly, this condition typically does not affect the relationship between the P waves (atrium contraction) and the QRS complexes (ventricle contraction), resulting in consistently spaced QRS complexes following each P wave.

Thus, identifying a prolonged PR interval on an ECG is key to diagnosing a first-degree AV block, making this the correct and most informative option in understanding the electrical conduction abnormalities associated with this condition.