Third-degree AV block vs Sinus bradycardia
How to tell Third-degree AV block and Sinus bradycardia apart when a board question makes them look alike.
Step 2 CK pairs are in open beta while clinical review finishes. Spot an issue? Tell us.
P waves marching through QRS complexes without a consistent PR interval identifies complete heart block.
Side by side
Complete atrioventricular dissociation with independent P waves and escape QRS rhythm indicates third-degree AV block.
Slow but regular sinus rhythm with one P wave preceding each QRS indicates sinus bradycardia.
No atrial impulses conduct to ventricles, so a junctional or ventricular escape rhythm maintains perfusion.
Reduced sinus node automaticity slows rate while preserving AV conduction.
ECG shows fixed P-P intervals and fixed R-R intervals with no PR relationship; ventricular escape rate is often below 40/min.
ECG shows sinus rhythm below 60/min with constant PR interval and intact one-to-one AV conduction.
The trap
Symptomatic complete heart block is mistaken for benign sinus bradycardia and treated with observation alone. AV dissociation with hypotension or syncope requires immediate pacing pathway activation.
The repair move
Next best step: initiate transcutaneous pacing immediately for unstable complete heart block, give atropine only as a temporary bridge if responsive, and arrange urgent transvenous pacing followed by permanent pacemaker placement. Stable sinus bradycardia is managed by treating reversible causes first.
Try a free question
CoreA 79-year-old patient with syncope has heart rate 30/min and ECG showing complete AV dissociation. What is the next best step in management?
Show answer and explanation
Answer: A. Immediate transcutaneous pacing. Unstable third-degree AV block requires immediate pacing support.
Why the other choices are tempting
- B. Wrong. Observation alone is unsafe in unstable complete heart block.
- C. Wrong. Beta blockers can worsen bradycardia and conduction failure.
- D. Wrong. Outpatient monitoring is inappropriate in acute instability.
- E. Wrong. Adenosine can worsen AV block and is not indicated.
Flashcards
Tap a card to flip it.
What ECG feature confirms third-degree AV block in seconds?
Atrioventricular dissociation with independent atrial and ventricular rhythms.
What ECG pattern distinguishes sinus bradycardia from complete block?
Sinus bradycardia has a P wave before every QRS with constant PR interval.
Quick answers
What is the fastest way to tell Third-degree AV block from Sinus bradycardia?
P waves marching through QRS complexes without a consistent PR interval identifies complete heart block.
What is the key clue for Third-degree AV block?
Complete atrioventricular dissociation with independent P waves and escape QRS rhythm indicates third-degree AV block.
What is the key clue for Sinus bradycardia?
Slow but regular sinus rhythm with one P wave preceding each QRS indicates sinus bradycardia.
What is the common trap with Third-degree AV block vs Sinus bradycardia?
Symptomatic complete heart block is mistaken for benign sinus bradycardia and treated with observation alone. AV dissociation with hypotension or syncope requires immediate pacing pathway activation.