What Do You Think 23

In my last “What do you think?”, I showed this ECG of junctional rhythm.

With “tongue in cheek”, I referred to the premature beat as a sinus node “ectopic” (red highlight).

I felt it prudent to investigate this “ectopic” further. 

Here is another example (red highlight).

Is this really an ectopic?

What do you think?

Because the latter is a Holter monitor recording, we can investigate the rhythms further.

The rhythm is sinus. There isa pause due to sino-atrial block (ghosted red arrow) and an escape junctional beat (yellow highlight).

If the sino-atrial block (red ghosted arrows) is long enough, then an escape junctional rhythm dominates.

When we discuss sino-atrialblock, we usually recognise pauses in “groups” and forget about an escape rhythm.

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Let us revisit sino-atrialblock!

• The normal sinus impulse leaves the node and depolarizes the atrium.
• It is seen on the ECG as a P wave.
• With a delay or block at the sinus node, we do not see a P wave on the ECG.
• As with AV block, there are three types of sino-atrial block:‍
  • 1st Degree: Delay at the sino-atrial junction. Not seen on the ECG.‍
  • 2^nd Degree:
    • Type I - sino-atrial Wenckebach
    • TypeII – delay of one, two or three P waves‍
  • 3^rd Degree:  Sinus pauses or arrest.      

How do we differentiate second and third degree sino-atrial block?

Type I, sino-atrial Wenckebach.

• Fulfils the criteria for Wenckebach block, but is within the sinus node and therefore “pre-P wave”
• There is a progressive fatigue of sino-atrial conduction, culminating in sinus block to the atrium    
• Sinus node rests and conduction returns‍
• The greatest increase in sino-atrial conduction is between the first and second beat with the “increment increase” of each conducted beat becoming shorter‍
• The P-P interval progressively shortens with each beat of the cycle

The P-P intervals shorten with the greatest increment between the first and second P waves (40 ms). The P-P interval before the dropped beat is the shortest (690 ms) and the one after the dropped beat is the longest (840 ms).

Type II sino-atrial block.

The pauses are multiples of the P-P intervals.

Sinus rhythm (red arrow) with pauses, which are twice the P-P intervals of the regular sinus beats. The dropped P waves are shown as ghosted red arrows.

Sinus pauses or arrest.

The pauses are not necessarily multiples of the P-P intervals.

Unlike our case (and those inthe literature, the above examples have no junctional escape beats or rhythm to“muddy the waters”.

Because of these escape rhythms, it is near impossible to determine the type of sino-atrial block.