In determining the cardiac rhythm on an electrocardiogram, which step is used to assess atrioventricular conduction?

• A. Calculate the atrial (P wave) rate
• B. Calculate the ventricular (QRS complex) rate
• C. Determine the regularity and shape of the P waves
• D. Measure the PR interval

Answer: (D)

Measuring the PR interval is how you assess atrioventricular conduction. The PR interval spans from the start of the P wave to the start of the QRS complex and reflects the time it takes for an impulse to travel from the atria through the AV node and into the ventricles. A normal interval is about 0.12 to 0.20 seconds. If the PR interval is prolonged, AV nodal delay is indicated (first-degree block); if it gradually lengthens with dropped beats, that points to Wenckebach (Mobitz I); a consistently long PR with intermittent non-conducted beats suggests Mobitz II. Short PR intervals can hint at accessory pathways bypassing the AV node, though the key idea is the conduction time through the AV node.

Other steps don’t specifically measure AV conduction: calculating atrial (P wave) rate tells you how fast atrial depolarizations occur, not how quickly the impulse moves to the ventricles; calculating the ventricular (QRS) rate shows ventricular response but not the atrioventricular transmission timing; assessing the regularity and shape of P waves informs atrial rhythm and P-wave morphology rather than AV conduction timing.