Second degree AV Block Type I is also known as AV Wenckebach or Mobitz I. Did you know that there are two (2) types of AV Wenckebach? It could either be Typical or Atypical AV Wenckebach. This is how to recognize these ECG Patterns:
Second Degree AV Block Type I (Typical)
- · The P wave is normal.
- · The PR interval progressively lengthens until a P wave is not followed by a QRS.
- · As the PRI lengthens, there is shortening of the RR interval.
- · The RR interval containing the dropped P wave is less than 2x of the shortest RR interval.
- · The PRI of the first conducted P wave (may be normal or prolonged) is shorter than the PRI of the last conducted P wave.
- · The largest increment in the PRI is usually on the second conducted P wave.
- · There is "group-beating" on the ECG.
Figure 1 – Sinus rhythm, second degree AV block type I (Mobitz I/Wenckebach) with 3:2 AV conduction. There are regular sinus P waves at a rate of about 100 bpm. There is group-beating of the QRS with a P to QRS ratio of 3:2. The first conducted P waves are marked with green arrows with a PR interval (PRI) of 0.20 seconds. The next conducted P waves are marked with a blue arrow with a PRI of 0.28 seconds. The non-conducted P waves are marked with red arrows. As can be seen there is prolongation of the PRI interval until a non-conducted P wave. This is typical AV Wenckebach.
Second Degree AV Block Type I (Atypical)
- · The P wave is normal.
- · When the conduction ratio exceeds 6:5 (6P and 5 QRS) or 7:6, the PR interval increment becomes unpredictable.
- · The PRI may remain the same (prolonged), then increase, and then the dropped beat.
- · As with second degree AV block typical type I, the PRI of the first conducted P wave is shorter compared to the PRI of last conducted P wave.
- · A long strip is needed for the correct interpretation.
- · It may look like a first degree AV block if examined using a short strip.
Figure 2 – Sinus tachycardia (~107 bpm), Second degree AV block type I (Mobitz I/Wenckebach). P waves are upright in II, III, aVF and inverted in aVR (sinus in origin) at a rate of about 107 bpm/min. There is group-beating of the QRS. In this strip, there is a P to QRS ratio of 5:4, 6:5 and 5:4. In the highlighted portion, the first conducted P wave is marked with green arrow/s. The PR interval (PRI) is 0.20 seconds. The next conducted P waves are marked with blue arrows with a PRI of about 0.28 seconds. The non-conducted P wave is marked with red arrow. As can be seen, the PRI after the first conducted remained the same (but prolonged). This is atypical AV Wenckebach.
Figure 3 – Sinus rhythm, first degree AV block with atypical AV Wenckebach. For long-cycle AV Wenckebach, often you only see the non-conducted P wave in a 10 second strip. As can be seen, the clue that this is AV Wenckebach is the noted shorter PRI of the first conducted P vs. the last conducted P wave (0.24 seconds vs. 0.28 seconds).
Figure 4A to 4C – Sinus rhythm, first degree AV block, atypical AV Wenckebach. As seen on timestamp, the full cycle is about 14 minutes (2258 -2312). This means that it took this time before you can see the non-conducted P wave. Strip like this will look like first degree AV block as you can see in Figure 4B.