CRAT Atrioventricular (AV) Blocks 2 — Questions and Answers
Question 1: In first-degree AV block, which ECG finding is diagnostic?
- PR interval greater than 200 ms with every P wave conducted (Correct answer)
- Dropped QRS complexes with progressive PR lengthening
- Complete dissociation between P waves and QRS complexes
- PR interval less than 120 ms
Correct answer: PR interval greater than 200 ms with every P wave conducted
First-degree AV block is defined by a PR interval exceeding 200 ms (one large box) with every P wave successfully conducting to the ventricles.
In first-degree AV block, conduction through the AV node is delayed but not blocked. Every P wave is followed by a QRS complex. The PR interval is consistently prolonged beyond 200 ms. It is often benign but may indicate underlying conduction system disease, medication effect, or increased vagal tone.
Question 2: Mobitz type I (Wenckebach) second-degree AV block is best described by which pattern?
- Progressive PR lengthening until a QRS is dropped, then the cycle resets (Correct answer)
- Fixed PR interval with intermittently dropped QRS complexes
- Complete AV dissociation
- Constant PR interval with every other P wave blocked
Correct answer: Progressive PR lengthening until a QRS is dropped, then the cycle resets
Wenckebach is characterized by progressive PR interval lengthening with each beat until one P wave is not conducted (dropped QRS), after which the cycle resets.
In Mobitz type I block, each successive impulse through the AV node finds it less recovered, progressively delaying conduction. Eventually the AV node is completely refractory and one P wave is blocked. The cycle then resets. The PR increment decreases with each beat, so the R-R intervals actually shorten before the pause.
Question 3: Mobitz type II second-degree AV block differs from Mobitz type I primarily because it has which characteristic?
- Constant PR interval with sudden unexpected dropped beats (Correct answer)
- Progressively lengthening PR intervals
- Variable PR intervals
- No visible P waves
Correct answer: Constant PR interval with sudden unexpected dropped beats
In Mobitz type II, the PR interval remains constant and QRS complexes are dropped suddenly without warning, indicating infranodal disease and higher risk of complete heart block.
Mobitz type II block occurs below the AV node, usually in the bundle of His or bundle branches. Because the AV node itself is functioning normally, PR intervals are constant. The sudden failure of conduction without progressive PR lengthening reflects structural damage and carries a significant risk of progressing to complete (third-degree) heart block.
Question 4: In third-degree (complete) heart block, how do atria and ventricles relate to each other?
- They beat completely independently at their own intrinsic rates (Correct answer)
- P waves conduct to ventricles after a very long PR interval
- P waves and QRS complexes maintain a fixed but abnormal ratio
- Ventricular rate equals atrial rate
Correct answer: They beat completely independently at their own intrinsic rates
In complete heart block, no atrial impulses are conducted to the ventricles, so each chamber maintains its own independent pacemaker and rate.
Third-degree AV block results in complete AV dissociation. The SA node fires at its own rate for the atria, while an escape pacemaker (junctional at 40-60 bpm or ventricular at 20-40 bpm) drives the ventricles independently. P waves and QRS complexes bear no consistent relationship to each other.
Question 5: A patient has second-degree AV block with a 3:2 conduction ratio in Wenckebach pattern. How many P waves appear for every 2 QRS complexes?
- 3 P waves (Correct answer)
- 2 P waves
- 4 P waves
- 1 P wave
Correct answer: 3 P waves
A 3:2 conduction ratio means 3 P waves for every 2 conducted QRS complexes — one P wave in each cycle is blocked.
In AV block notation, the ratio refers to atrial to ventricular events. A 3:2 ratio means 3 P waves occur for every 2 QRS complexes. In Wenckebach, you would see two PR intervals progressively lengthening followed by one blocked P wave, then the cycle repeats.
Question 6: Which of the following AV block types is most likely to require immediate pacemaker implantation?
- Mobitz type II second-degree AV block (Correct answer)
- First-degree AV block
- Mobitz type I second-degree AV block
- Fascicular block without AV block
Correct answer: Mobitz type II second-degree AV block
Mobitz type II carries high risk of sudden progression to complete heart block and usually requires permanent pacemaker implantation.
Mobitz type II block occurs at or below the bundle of His and can abruptly progress to complete heart block without warning. Because the escape rhythm in complete infranodal block is often an unreliable ventricular escape at 20-40 bpm, patients are at high risk for syncope or cardiac arrest. Permanent pacemaker implantation is standard management.
CRAT Atrioventricular (AV) Blocks 2
In first-degree AV block, which ECG finding is diagnostic?