What is a 2nd degree Mobitz type 1 (Wenckebach) heart block?

Second Degree Mobitz Type I (Wenckebach) Heart Block

Second degree Mobitz type I (Wenckebach) heart block is characterized by progressive lengthening of the PR interval before a blocked P wave and generally reflects disease within the atrioventricular (AV) node, with a typically benign prognosis that rarely requires intervention in asymptomatic individuals. 1

Characteristics and Diagnosis

• Mobitz type I (Wenckebach) is a form of second-degree AV block where some, but not all, atrial impulses are conducted to the ventricles 1
• The classic pattern shows progressive prolongation of the PR interval on consecutive beats followed by a blocked P wave (non-conducted atrial impulse) 2
• The block typically occurs within the AV node (proximal conduction system) rather than below it 1
• It can be distinguished from Mobitz type II block, which shows constant PR intervals before blocked beats and typically occurs in the infranodal (distal) conduction system 3

Clinical Significance and Prognosis

• Wenckebach block generally has a benign prognosis compared to Mobitz type II block 1
• It is commonly observed in well-trained endurance athletes, especially during sleep 1, 4
• The condition is often vagally mediated and may be transient 4, 5
• Unlike Mobitz type II, Wenckebach rarely progresses to complete heart block or sudden death 1

Evaluation

For patients with Wenckebach block, evaluation should include:

• Assessment for symptoms attributable to the block (syncope, presyncope, dizziness) 1
• Echocardiogram to exclude underlying structural heart disease 1
• Exercise stress test to observe the response of the conduction system to increased sympathetic tone 1, 4
• 24-hour ECG monitoring if the QRS complex is abnormal or the shortest PR interval is excessively prolonged (≥0.3 second) 1

Special Considerations

• Important caveat: In rare cases, Wenckebach block can occur in the infranodal conduction system, which carries a worse prognosis and may predict progression to complete heart block 6
• Electrophysiological studies (EPS) are rarely necessary but may be considered in selected cases to determine the site of block when there are concerning features 1
• Athletes with Wenckebach block and coexisting bundle-branch block should undergo EPS to identify potential intra-His-Purkinje or infra-His-Purkinje block that may require pacemaker therapy 1
• The block may worsen with increased vagal tone or medications that slow AV conduction 5

Management

• Asymptomatic patients with isolated Wenckebach block generally require no specific treatment 1
• Monitoring may be considered in patients with Wenckebach block but is generally not required 1
• For symptomatic patients with marked first-degree or second-degree Mobitz type I AV block with symptoms clearly attributable to the block, permanent pacing may be reasonable 1
• In athletes with Wenckebach block who have no symptoms or structural heart disease, participation in all competitive athletics is generally permitted 1

Differentiation from Mobitz Type II

• Mobitz type II shows constant PR intervals before blocked beats (no progressive PR prolongation) 3
• Mobitz type II typically occurs below the AV node and is considered abnormal 1
• Mobitz type II has a worse prognosis and is generally considered an indication for permanent pacemaker implantation 1
• It's important to distinguish 2:1 Wenckebach physiology from true Mobitz type II block, which can usually be achieved with a stress test 1