Atrioventricular (AV) Blocks Flashcards

On the ECG, which measurement defines a normal PR interval?

Answer: 120-200 ms

The normal PR interval ranges from 120 to 200 ms (3 to 5 small boxes on standard ECG paper), representing atrial depolarization and AV nodal conduction time.

In high-degree AV block with a 4:1 conduction ratio, what is the ventricular rate if the atrial rate is 80 bpm?

Answer: 20 bpm

With a 4:1 AV conduction ratio and an atrial rate of 80 bpm, only 1 in 4 P waves conducts, giving a ventricular rate of 80 divided by 4 = 20 bpm.

Which characteristic of the QRS complex in complete heart block provides a clue about the location of the escape pacemaker?

Answer: Narrow QRS suggests a junctional escape; wide QRS suggests a ventricular escape

Junctional escape pacemakers activate the ventricles via the normal His-Purkinje system, producing narrow QRS complexes, while ventricular escape foci cause wide, aberrant QRS complexes.

Which medication is a common pharmacological cause of AV block?

Answer: Digoxin

Digoxin enhances vagal tone and slows AV nodal conduction, and toxicity can cause all degrees of AV block including complete heart block.

AV dissociation that is NOT due to complete heart block can occur in which situation?

Answer: Accelerated junctional rhythm that exceeds the sinus rate

AV dissociation without complete block occurs when a lower pacemaker (junctional or ventricular) accelerates to exceed the sinus rate, causing the two chambers to beat independently without true block.

In a rhythm strip, you observe regular P waves at 75 bpm and regular wide QRS complexes at 35 bpm with no consistent PR relationship. Which rhythm is present?

Answer: Third-degree (complete) AV block with ventricular escape rhythm

Regular P waves and regular wide QRS at a slow independent rate with no consistent PR interval is the hallmark of complete heart block with a ventricular escape rhythm.