Which AV block is defined by a prolonged PR interval?

Which AV block is defined by a prolonged PR interval?

• A. First degree AV block
• B. Wenckebach (Mobitz I)
• C. Mobitz II (Mobitz II)
• D. Third-degree AV block

Answer: (A)

Long conduction delay at the AV node that affects every atrial impulse is the hallmark. In first-degree AV block, every P wave is followed by a QRS, but the interval from the start of the P wave to the start of the QRS (the PR interval) is longer than normal—typically over 0.20 seconds. The rhythm stays regular and QRS width is usually normal, reflecting that conduction to the ventricle is preserved, just slowed. This condition is often benign and can result from increased vagal tone, certain medications, or mild conduction system disease. The other patterns involve actual drops or dissociation: a gradually lengthening PR with eventual missing QRS points to a Wenckebach pattern; a constant PR with sudden non-conducted P waves indicates Mobitz II; and complete AV dissociation with independent atrial and ventricular rates defines third-degree block.

Long conduction delay at the AV node that affects every atrial impulse is the hallmark. In first-degree AV block, every P wave is followed by a QRS, but the interval from the start of the P wave to the start of the QRS (the PR interval) is longer than normal—typically over 0.20 seconds. The rhythm stays regular and QRS width is usually normal, reflecting that conduction to the ventricle is preserved, just slowed. This condition is often benign and can result from increased vagal tone, certain medications, or mild conduction system disease.

The other patterns involve actual drops or dissociation: a gradually lengthening PR with eventual missing QRS points to a Wenckebach pattern; a constant PR with sudden non-conducted P waves indicates Mobitz II; and complete AV dissociation with independent atrial and ventricular rates defines third-degree block.