Can you measure PR interval in atrial fibrillation?

• A. Yes
• B. No
• C. Sometimes
• D. Not applicable

Answer: (B)

In atrial fibrillation, you don’t have a clear, individual P wave to mark the start of atrial depolarization. The atria are firing chaotically, producing irregular, chaotic waves, and the ventricular rhythm is driven by variable conduction through the AV node, giving an irregularly irregular QRS pattern. Because the PR interval is defined as the time from the start of atrial depolarization (the P wave) to the start of ventricular depolarization (the QRS complex), there is no consistent P wave to anchor this measurement in AF. So you cannot measure a PR interval in atrial fibrillation.

Sometimes you might see a PR interval in rhythms that include brief sinus beats or when AF isn’t the only rhythm present, but in pure AF it’s not measurable. Not applicable would be a similar idea, but the standard phrasing here is that it cannot be measured.