Is the P (P wave) to P interval constant in Mobitz type 1 second-degree atrioventricular (AV) block?

P-to-P Interval in Mobitz Type 1 Second-Degree AV Block

Yes, the P-to-P interval is constant in Mobitz type 1 second-degree AV block, as defined by the American College of Cardiology/American Heart Association guidelines. 1

Understanding Mobitz Type 1 (Wenckebach) Block

Mobitz type 1 second-degree AV block has several key characteristics:

• Constant P-to-P interval: P waves occur at a constant rate (typically <100 bpm) 1, 2
• Progressive PR prolongation: The PR interval progressively lengthens with each beat until a P wave is not conducted 1, 2
• Dropped beat pattern: After the non-conducted P wave, the cycle repeats with a shorter PR interval 2
• Anatomical location: Usually occurs at the level of the AV node 1, 2

Differentiating Features from Mobitz Type 2

The constant P-to-P interval is a feature of both Mobitz type 1 and Mobitz type 2 blocks, but they differ in other important ways:

• Mobitz type 1:

  • Progressive PR prolongation before the blocked beat
  • PR interval shortens after the blocked beat
  • Usually involves the AV node (supra-Hisian)
  • Generally has better prognosis 1, 2

• Mobitz type 2:

  • Constant PR intervals before and after the blocked beat
  • Usually involves the His-Purkinje system (infra-nodal)
  • Higher risk of progression to complete heart block
  • Generally requires pacing 1, 2

Clinical Implications

The constant P-to-P interval in Mobitz type 1 block reflects normal sinus node function with the pathology occurring at the level of AV conduction. This distinguishes it from sinus arrhythmias or sinoatrial blocks where the P-to-P interval would vary.

While Mobitz type 1 is traditionally considered more benign than Mobitz type 2, research suggests that in adults over 45 years, it may not be as benign as previously thought 3. In rare cases, Mobitz type 1 can occur in the infranodal conduction system, which carries a higher risk of progression to complete heart block 4.

Important Caveats

• The presence of a wide QRS complex in a patient with Mobitz type 1 block should raise suspicion for infranodal disease, which carries a worse prognosis 1, 5
• In young athletes, Mobitz type 1 block may be a normal variant related to high vagal tone 6
• The distinction between Mobitz types is primarily descriptive; the anatomical location of the block is more important for prognosis and management decisions 5

Remember that while the P-to-P interval is constant in Mobitz type 1 block, the PR interval is not - it progressively increases until a P wave is not conducted, which is the hallmark feature that distinguishes it from Mobitz type 2 block.