Have a look at this Holter monitor strip.
There is a pause about twice the cycle length of the R to R intervals and no P, QRS or T waves.
This is a type 1 sino-atrial Wenckebach sequence.
How do I deduce that? This is one of the most difficult explanations in ECG interpretation and you have to revisit the fundamentals or “footprints” of Wenckebach AV conduction.
We all know about the:
However, there is another footprint which is not well known and essential to understanding Wenckebach block outside AV conduction. It only occurs with “typical” sequences and these represent about 15% of Wenckebach AV block.
This poorly understood footprint relates to the shortening of the R-R intervals during a typical Wenckebach sequence.
Let us review:
The greatest increase in the PR interval is typically between the first and second beat with the “increment increase” of each conducted beat becoming shorter.
In this 3:2 Wenckebach AV sequence, the PR interval increases from 210 ms to 380 ms or by an increment of 170 ms (red highlight). The next increment is 160 ms. The R-R interval thus shortens from 1010 to 1000 ms, whereas the P-P interval remains the same (limited by sinus arrhythmia). Let us review it on a ladder diagram:
R1 to R2 is composed of S1 to S2 plus 170 ms which is 1010 ms. It shortens to 1000 ms for R2 to R3.
To summarise: In all typical Wenckebach sequences, the cycle length before the dropped beat is shorter than the one preceding it.
Now let us return to sino-atrial block.
Types of sino-atrial blocks:
For sino-atrial Wenckebach block, the sequence is hidden in the sino-atrial node or “pre-P wave”.
What are the footprints for recognition?
Let us make it easy: Just remember P-P interval before the pause is shorter than the one before it (yellow highlight).
How do I differentiate the other levels of sinus pauses?
Sinus arrhythmia: All the R-R intervals are different.
Vaso-vagal episodes: Profound bradycardia with no multiples of block.
Non-conducted atrial ectopics:
The commonest cause of a pause (Henry Marriott)
The pause is a compensatory pause.
Look for the embedded P wave (yellow highlight).
It’s all in the timing.
In 49+ years as a practicing cardiologist, Dr Harry Mond has published 260+ published manuscripts & books. A co-founder of CardioScan, he remains Medical Director and oversees 500K+ heart studies each year.
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