How do you measure the PR interval in second-degree atrioventricular (AV) block, including 2:1 AV block?

How to Measure the PR Interval in Second-Degree AV Block and 2:1 AV Block

In second-degree AV block, measure the PR interval from the conducted P waves only—specifically, measure the interval from the beginning of the P wave to the beginning of the QRS complex for beats that successfully conduct to the ventricles.

Key Measurement Principles by Block Type

Mobitz Type I (Wenckebach)

• Measure all conducted PR intervals before the blocked P wave
• The PR intervals progressively lengthen until a P wave fails to conduct 1
• Compare each PR interval to identify the pattern—the increase may be subtle in the last cycles before the block 1
• The shortest PR interval typically follows the blocked P wave 1
• Only measure from P waves that have an associated QRS complex

Mobitz Type II

• Measure the PR intervals of conducted beats only
• The PR intervals remain constant before and after the blocked P wave 1, 2
• This constant PR interval is the defining characteristic that distinguishes it from Type I 2
• An unchanged PR interval after the block is a sine qua non of Mobitz type II 3

2:1 AV Block - The Special Case

In 2:1 AV block, you can only measure one PR interval—the interval of the conducted beats 2, 4. This is the critical limitation that prevents classification as Type I or Type II.

• Measure from the P wave to the QRS complex on every other beat (the conducted beats)
• The blocked P waves have no QRS to measure to, so no PR interval exists for them
• 2:1 block cannot be classified as Type I or Type II because you need at least two consecutive conducted PR intervals to determine if they are changing or constant 2, 5, 3

Clinical Context for Interpretation

The QRS width provides crucial information when interpreting the measured PR interval in 2:1 block 5, 6, 7:

• Normal PR + wide QRS → suggests infranodal (His-Purkinje) disease 6, 7
• Prolonged PR + narrow QRS → suggests AV nodal disease 6, 7
• Normal PR + normal QRS → suspect intra-Hisian block 6, 7

Common Pitfalls to Avoid

1. Don't try to measure PR intervals from blocked P waves—by definition, they have no associated QRS complex
2. Don't classify 2:1 block as "Type I" or "Type II"—this violates accepted definitions and is diagnostically incorrect 5, 3
3. Don't assume a constant PR means Mobitz II without excluding other causes—ensure stable sinus rate, as vagal surges can mimic Type II block 3
4. Watch for concealed His bundle or ventricular extrasystoles that can create pseudo-AV block patterns 3

Determining the Pattern in Ambiguous Cases

When 2:1 block is present, look for transitions to other conduction patterns on the same or serial ECGs 5:

• If 2:1 block transitions to 3:2 or 4:3 with progressive PR lengthening → likely AV nodal (Type I pattern)
• If 2:1 block transitions to sudden block with constant PRs → likely infranodal (Type II pattern)
• "2:1 AV block can often be determined by seeking the company 2:1 AV block keeps" 5

The bottom line: You measure PR intervals only on conducted beats, and in 2:1 block, this means measuring the single repeating PR interval of every other P wave that successfully conducts.