PR interval: what it measures and how to calculate it
The PR interval measures the time from the beginning of atrial depolarization (start of P wave) to the beginning of ventricular depolarization (start of QRS complex). It represents conduction time through the atria, AV node, Bundle of His, and proximal bundle branches — the entire supraventricular conduction pathway.
Normal PR interval: 120–200 ms (3–5 small boxes on standard ECG paper at 25 mm/s). Measure from the beginning of the P wave to the first deflection of the QRS complex (whether Q or R wave). Use the same lead consistently for serial comparisons — lead II is standard.
PR interval should be measured in the lead where P wave onset is clearest and QRS begins earliest. At faster heart rates, the PR interval normally shortens slightly; at slower rates, it may lengthen mildly. This rate-dependence is normal and distinct from pathologic PR prolongation.
Abnormal PR intervals: prolonged (AV block) and short (pre-excitation)
Prolonged PR (> 200 ms): first-degree AV block. All beats conduct but with delayed AV nodal conduction. Common causes: inferior MI, vagal tone, medications (digoxin, beta-blockers, CCBs), Lyme disease, aging. Usually benign but warrants monitoring for progression.
Progressively lengthening PR: Wenckebach (Mobitz I second-degree AV block). PR lengthens until a QRS is dropped, then resets. Occurs at the AV node level — usually benign, commonly seen with inferior MI, high vagal tone.
Constant PR before sudden dropped QRS: Mobitz II second-degree AV block. Infranodal block — dangerous, may progress to complete heart block without warning. Requires urgent pacemaker evaluation.
Short PR (< 120 ms): pre-excitation (WPW syndrome). The accessory pathway conducts before the AV node, producing early ventricular activation — short PR plus delta wave. Short PR without delta wave may represent accelerated AV node conduction (LGL syndrome).