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:

• The sequential lengthening of the PR interval
• The dropped beat
• first PR interval after the pause being the shortest.

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.

• A sinus impulse is assumed once it leaves the node and depolarizes the atrium resulting in a P wave with a normal frontal plane axis (all P waves upright).
• A delay or block at the sinus node is not seen on the ECG

Types of sino-atrial blocks:

• 1^st Degree: Delay at sino-atrial junction. No ECG characteristics
• 2^nd Degree: Type I - sino-atrial Wenckebach
• Type II – delay of one, two or three P waves
• 3^rd degree: Sinus arrest

For sino-atrial Wenckebach block, the sequence is hidden in the sino-atrial node or “pre-P wave”.What are the footprints for recognition?

• Progressive fatigue of sino-atrial conduction culminating in sinus block to the atrium
• Sinus node rests and conduction returns
• Greatest increase in sino-atrial conduction is typically between first and second beat with “increment increase” of each conducted beat becoming shorter
• The P-P interval progressively shortens with each beat of the cycle

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?

Differential diagnosis?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.Harry Mond