ECG Basics · Lesson 10 · Leads, axis and conduction
PR interval and AV block
AV block is about the relationship between P waves and QRS complexes. March out the P waves first, then ask which ones conduct.
In the strips below, a small triangle marks each P wave the engine generated, conducted or not.
First-degree: every P conducts, slowly
PR is longer than 200 ms and constant. Every P is followed by a QRS. Usually a delay within the AV node.
Second-degree Mobitz I (Wenckebach)
PR lengthens beat by beat until one P fails to conduct. The PR after the pause is the shortest. The result is grouped beating. The block is usually in the AV node and is often benign, for example with high vagal tone.
Second-degree Mobitz II
PR stays constant, then a P suddenly fails to conduct. The block is usually below the AV node (His–Purkinje), the QRS is often wide, and it can progress to complete block. It is generally an indication for pacing evaluation. With 2:1 conduction you cannot tell Mobitz I from II on that strip alone.
Third-degree (complete) block
No P wave conducts. P waves and QRS complexes run independently (AV dissociation), with the atrial rate faster than the ventricular rate. An escape rhythm keeps the ventricles going: junctional (narrow, 40–60 /min) or ventricular (wide, 20–40 /min).
Telling them apart
- All P conduct, PR > 200 ms → first-degree.
- PR grows until a dropped beat → Mobitz I.
- Fixed PR, sudden drop → Mobitz II.
- No relationship between P and QRS → complete block.
References
- Kusumoto FM, et al. 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay. Circulation 2019;140:e382–e482.
- Mason JW, Hancock EW, Gettes LS, et al. Recommendations for the standardization and interpretation of the electrocardiogram, Part II: electrocardiography diagnostic statement list. Circulation 2007;115:1325–1332.