Why does the PR interval remain constant in Mobitz type II second-degree atrioventricular block?

Why the PR Interval Remains Constant in Mobitz Type II AV Block

The PR interval remains constant in Mobitz Type II AV block because the conduction defect occurs in the His-Purkinje system (infranodal location), where tissue exhibits "all-or-nothing" conduction properties rather than the decremental conduction characteristic of the AV node. 1, 2

Electrophysiologic Basis

The fundamental reason lies in the distinct conduction properties of different cardiac tissues:

• The His-Purkinje system demonstrates all-or-nothing conduction, meaning impulses either conduct completely or fail completely without gradual deterioration 3, 4
• The AV node exhibits decremental conduction, where impulses progressively slow before blocking, which explains the incrementally prolonging PR intervals seen in Mobitz Type I 4
• Mobitz Type II block occurs within or below the His bundle (infranodal), not in the AV node, which is why conducted beats maintain constant PR intervals 2, 5

Anatomic Location Determines Pattern

The location of the block dictates the electrocardiographic pattern:

• When impulses successfully traverse the AV node and reach the His-Purkinje system, they produce a constant PR interval because the AV nodal conduction time remains stable 1
• The block occurs suddenly and unpredictably in the diseased His-Purkinje tissue without warning signs of progressive conduction delay 3
• This is typically associated with wide QRS complexes due to the infranodal location of the pathology 1, 2

Cellular Mechanism

Research has elucidated the microscopic basis:

• Ischemic damage to the His-Purkinje system causes functional dissociation of cellular activation, where some cells fail to activate while others conduct normally 6
• At faster rates, action potential amplitude decreases and cellular dissociation increases until complete block occurs, but the conducted beats maintain consistent timing through the AV node 6
• The block site shows dynamic spatial shifting but the supraventricular conduction time (PR interval) remains constant because the AV node itself is functioning normally 6

Critical Diagnostic Point

An unchanged PR interval after the blocked beat is sine qua non (essential requirement) for diagnosing Mobitz Type II block 3. This distinguishes it from:

• Atypical Wenckebach patterns where PR intervals may appear constant but show subtle increments 3
• Vagally-mediated blocks that can mimic Type II but involve the AV node 3
• 2:1 AV block cannot be classified as Type I or Type II based on ECG alone since you need to observe PR interval behavior over multiple conducted beats 2

Clinical Significance

The constant PR interval has important prognostic implications:

• Type II block indicates infranodal disease with high risk of progression to complete heart block, warranting permanent pacemaker even in asymptomatic patients 2, 3
• The all-or-nothing nature means sudden progression to complete block without warning, unlike the more benign Type I block 5