AV NODE DISORDERS Flashcards
Which feature on an ECG suggests Mobitz I (Wenckebach) second-degree AV block?
A. Fixed PR interval before dropped QRS
B. Progressive PR interval prolongation before dropped QRS
C. Complete AV dissociation
D. Narrow QRS escape rhythm without dropped beats
Answer: B. Progressive PR interval prolongation before dropped QRS
Rationale: Mobitz I (Wenckebach) block is characterized by progressive lengthening of the PR interval before a dropped QRS complex. This block typically occurs at the AV node and is often benign.
Which type of AV block is most likely to progress to complete heart block and requires permanent pacing?
A. Mobitz I second-degree AV block
B. First-degree AV block
C. Mobitz II second-degree AV block
D. Wenckebach block
Answer: C. Mobitz II second-degree AV block
Rationale: Mobitz II block involves a sudden failure of conduction without PR interval prolongation before the dropped QRS. It is usually infranodal (below the AV node) and is associated with an unreliable escape rhythm. It has a high risk of progression to third-degree (complete) heart block, making permanent pacing necessary.
Which ECG finding is most suggestive of complete (third-degree) AV block?
A. Progressive PR interval prolongation before a dropped QRS
B. PR interval remains constant but with intermittent non-conducted P waves
C. Complete dissociation between atrial and ventricular activity
D. Shortened PR intervals followed by grouped QRS complexes
Answer: C. Complete dissociation between atrial and ventricular activity
Rationale: In third-degree AV block, no atrial impulses reach the ventricles, leading to AV dissociation. The atria and ventricles beat independently at different rates. The ventricular rate is slower than the atrial rate, and the escape rhythm’s width and stability depend on the block’s location.
AV block is more frequent in which type of acute myocardial infarction (MI)?
A. Anterior MI
B. Inferior MI
C. Lateral MI
D. Posterior MI
Answer: B. Inferior MI
Rationale: AV block occurs more commonly in inferior MI because the RCA supplies the AV node. First-degree and second-degree AV block (Mobitz I) are the most common types. In contrast, anterior MI is associated with infranodal blocks (His-Purkinje level) and carries a worse prognosis.
What is the typical level of AV block in inferior MI?
A. Distal to the AV node (His-Purkinje system)
B. In the AV node
C. In the left bundle branch
D. In the right bundle branch
Answer: B. In the AV node
Rationale: In inferior MI, AV block typically occurs within the AV node. These blocks tend to be more stable and often present with narrow QRS escape rhythms that do not require temporary pacing.
Which of the following statements is true regarding AV block in anterior MI?
A. It usually occurs in the AV node and is reversible
B. It is associated with high mortality and unstable escape rhythms
C. It presents with a narrow complex junctional escape rhythm
D. It does not usually require temporary pacing
Answer: B. It is associated with high mortality and unstable escape rhythms
Rationale: In anterior MI, AV block is typically distal to the AV node (in the His bundle or bundle branches) and results in wide complex, unstable escape rhythms. This is associated with extensive infarction, high mortality, and often requires temporary pacing.
Why does high-grade AV block in anterior MI indicate a poor prognosis?
A. It is usually transient and resolves quickly
B. It indicates ischemia of the AV node, which has good collateral circulation
C. It suggests extensive infarction and failure of the His-Purkinje system
D. It responds well to pharmacologic therapy alone
Answer: C. It suggests extensive infarction and failure of the His-Purkinje system
Rationale: In anterior MI, AV block is usually due to extensive myocardial necrosis affecting the His-Purkinje system, which results in wide complex, slow, and unreliable escape rhythms. This is associated with poor prognosis and a high risk of progression to complete heart block.
What is the most appropriate management for a high-grade AV block in the setting of anterior MI?
A. Observation and pharmacologic therapy
B. Immediate percutaneous coronary intervention (PCI) only
C. Temporary pacing
D. Permanent pacemaker implantation
Answer: C. Temporary pacing
Rationale: In anterior MI, high-grade AV block often occurs below the AV node and results in unstable escape rhythms. Temporary pacing is typically required due to the high risk of hemodynamic instability and sudden cardiac arrest.
Which infectious disease is the most common cause of AV block?
A. Lyme disease
B. Chagas’ disease
C. Viral myocarditis
D. Toxoplasmosis
Answer: A. Lyme disease
Rationale: Lyme disease, caused by Borrelia burgdorferi, is the most common infectious cause of AV block, typically at the AV node level with a narrow escape rhythm (>40 bpm). AV block in Lyme disease is often reversible with antibiotic therapy.
Which type of myocardial infarction (MI) is most commonly associated with transient AV block?
A. Anterior MI
B. Inferior MI
C. Lateral MI
D. Posterior MI
Answer: B. Inferior MI
Rationale: Inferior MI is more likely to cause transient AV block, typically at the AV node level, leading to narrow escape rhythms that are usually stable and reversible without pacing.
Which of the following indicates the need for pacing in the setting of acute MI?
A. First-degree AV block
B. Transient second-degree AV block without bundle branch block
C. Persistent symptomatic second- or third-degree AV block
D. Isolated fascicular block
Answer: C. Persistent symptomatic second- or third-degree AV block
Rationale: Pacing is indicated in acute MI if there is:
Persistent symptomatic second- or third-degree AV block
Transient second- or third-degree AV block with bundle branch block
In contrast, isolated first-degree AV block or fascicular blocks do not require pacing.
