What does a progressively increasing PR (P-R) interval followed by an eventual skipped QRS complex indicate?

Mobitz Type I (Wenckebach) Second-Degree Atrioventricular Block

A progressively increasing PR interval followed by an eventual skipped QRS complex is diagnostic of Mobitz Type I (Wenckebach) second-degree atrioventricular block, which typically occurs at the level of the AV node and generally has a favorable prognosis compared to other forms of heart block.

Definition and Characteristics

Mobitz Type I second-degree AV block is characterized by:

• Progressive prolongation of the PR interval before a blocked P wave
• The largest increment in PR prolongation typically occurs between the first and second conducted beats
• The pause containing the blocked P wave is less than twice the normal P-P interval
• The PR interval shortens after the blocked beat
• Usually associated with narrow QRS complexes (when the block is at the AV node)

Anatomical Location and Pathophysiology

The block typically occurs at the level of the AV node in most cases, especially when associated with narrow QRS complexes 1. This is important because:

• AV nodal blocks generally have better prognosis than infranodal blocks
• The block represents decremental conduction through the AV node, where each successive impulse encounters increasingly refractory tissue
• When Wenckebach occurs with wide QRS complexes, the block may be either in the AV node or within/below the His bundle, requiring further evaluation 1

Clinical Significance and Prognosis

The clinical significance of Mobitz Type I block varies:

• Generally considered benign when occurring at the AV node level 1
• Common and benign in athletes and during sleep 1
• Rarely progresses to complete heart block when isolated to the AV node 1
• When associated with wide QRS complexes, further evaluation may be needed to determine the exact location of the block 1

Diagnostic Considerations

When evaluating a patient with suspected Mobitz Type I block:

• Confirm the progressive PR prolongation pattern
• Differentiate from Mobitz Type II block, which has constant PR intervals before the blocked P wave 1
• Note that 2:1 AV block cannot be classified as Type I or II based on surface ECG alone 1
• Be aware that atypical Wenckebach patterns are common (up to 86% of spontaneous Wenckebach periods may be atypical) 2

Management Recommendations

Management depends on symptoms and associated conditions:

• Asymptomatic patients with isolated Mobitz Type I block generally do not require permanent pacing 1
• Symptomatic patients (syncope, presyncope) may require pacemaker implantation 1
• Monitoring may be considered on an individual basis but is generally not required for typical Wenckebach block 1
• Type I second-degree AV block with wide QRS complexes warrants further evaluation to determine if the block is infranodal, which may require pacing 1

Important Clinical Pearls

• The presence of Mobitz Type I block with narrow QRS almost always indicates AV nodal block 3
• Coexistence of both Type I and Type II narrow QRS block in the same recording is exceedingly rare 3
• Wenckebach patterns with higher conduction ratios (>6:5) are almost always atypical 2
• Vagal surges can cause simultaneous sinus slowing and AV nodal block that may superficially resemble Type II block 3

Potential Pitfalls

• Mistaking pseudo-Mobitz II for true Mobitz II block (pseudo-Mobitz II shows ≥20 ms PR shortening after blocked complexes) 4
• Failing to recognize that a 2:1 AV block cannot be classified as Type I or II without additional information 3
• Not considering reversible causes such as medication effects, increased vagal tone, or electrolyte abnormalities 1
• Overlooking the possibility of infranodal disease in patients with bundle branch block and Wenckebach pattern 1

Understanding the characteristics and implications of Mobitz Type I AV block is essential for appropriate clinical decision-making and avoiding unnecessary interventions in patients with this generally benign conduction abnormality.