First-degree AV block is diagnosed by which ECG finding?

• A. Prolonged QRS Duration
• B. Absence of P Waves
• C. PR Interval > 0.2 Seconds
• D. Prolonged QT Interval

Answer: (C)

The key idea is a slowed conduction through the AV node that lengthens the time from the atrial to the ventricular activation, while keeping every atrial impulse conducted to the ventricles. On the ECG, this shows as a PR interval longer than 0.20 seconds, with each P wave followed by a QRS complex in a consistent 1:1 relationship. Normal QRS width is preserved because the problem is at the AV node, not in the ventricles.

So, first-degree AV block is diagnosed by a prolonged PR interval (>0.20 seconds) with all P waves still getting through to produce QRS complexes. This contrasts with other patterns: if there are no P waves, that suggests atrial inactivity or a junctional/ventricular rhythm; a markedly prolonged QT interval points to a repolarization issue; and a widened QRS with normal PR can indicate intraventricular conduction delay rather than AV nodal delay.

Other practical notes: it’s often benign and can be caused by increased vagal tone or AV-nodal blocking drugs (like certain beta-blockers, non-dihydropyridine calcium channel blockers, or digoxin).