ECG Interpretation Review #57 (Mobitz I - Mobitz II - Wenckebach - Blocked PAC - Pause - 2nd Degree AV Block)

Interpret the lead MCL-1 rhythm strip shown in Figure 1.

• Does this rhythm represent Mobitz I (Wenckebach) or Mobitz II AV block?
• Is a pacemaker likely to be needed?

Figure 1: Lead MCL-1 rhythm strip. Is this Mobitz I or Mobitz II?

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INTERPRETATION:  Neither Mobitz I nor Mobitz II is present. Rather than AV block – the rhythm in Figure 1 is an insightful example of the “mischief” that blocked PACs (Premature Atrial Contractions) can cause, especially when PACs are frequent.

     We have previously reviewed the basics of the 2nd degree AV blocks (See ECG Blog #19 – Blog #20 – Blog #21 – and Blog #22). Essential to the diagnosis that some type of AV block is present are 2 ECG findings:

• Finding #1: A consistent underlying atrial rhythm (usually sinus) – which is established by similar morphology of P waves on the tracing. Occasional PACs or junctional beats may be seen – but constantly changing P wave morphology is much more suggestive of other phenomena (wandering pacemaker; sinus pauses or arrest; multifocal atrial tachycardia) than of “AV block”.
• Finding #2: A regular (or at least fairly regular) atrial rhythm should be seen when some form of AV block is present. Clearly – there may be underlying sinus arrhythmia. In addition – slight variation in regularity of the underlying sinus rhythm may be the result of the AV block itself (known as “ventriculophasic sinus arrhythmia”) – in which the P‑P interval tends to shorten for P waves that sandwich a QRS complex (thought to be due to transiently increased perfusion immediately following ventricular contraction). However, gross variation in the P‑P interval is usually not seen when the primary problem is AV block.

WHY FIGURE 1 is Not AV BLOCK: AV block is not present in Figure 1 – because the above 2 ECG findings are absent. This becomes obvious in Figure 2, in which red arrows highlight each P wave:

Figure 2: Red arrows highlight each P wave in Figure 1.

• P wave morphology changes in Figure 1 (and in Figure 2). Sinus P waves are seen as a biphasic (small pointed positive followed by rounded negative) deflection preceding beats #1, 2, 3, 4, 5 and 6. In contrast – P waves buried within the ST-T wave of beats #1-thru-6 are triphasic (small negative-then positive-then narrow negative) deflections that clearly look different in morphology than the sinus P waves. These triphasic-deflection P waves arise from an atrial site other than the sinus node.
• Red arrows in Figure 2 make it obvious that the P-P interval varies. In fact there is a pattern to this P‑P variation (alternating short-long cycles) produced by the fact that every-other-P wave is early (premature). The underlying rhythm is atrial bigeminy (every other beat is a PAC).

PEARL: The Commonest Cause of a Pause is a Blocked PAC

     We introduced the concept of “blocked” PACs in ECG Blog #14. Depending at what point within the refractory period a premature beat occurs – a PAC may conduct: i) Normally; ii) With aberrant conduction (if part of the ventricular conduction system is still refractory); or iii) The PAC may occur so early as to fall within the absolute refractory period when no conduction is possible. This is what is occurring in Figure 2 – in which every-other-P-wave is blocked (non-conducted)!

• The commonest cause of a pause is a blocked PAC!  Blocked PACs are far more common than any form of heart block. Although sometimes subtle – blocked PACs can be identified if looked for. Close inspection of T waves at the beginning of a relative pause will usually reveal a notch or other small deformity not evident in the T waves of normally conducted sinus beats.

BEYOND-the-CORE: What is Happening with Beat #7?

     Unlike the PACs occurring within the T waves of beats #1-thru-5 (which are non-conducted) – the PAC that notches the T wave of beat #6 is conducted (ie, beat #7)!

• The reason for QRS widening and the different QRS morphology of beat #7 – is that this PAC conducts with LBBB (Left Bundle Branch Block) aberration. It presumably occurs during the RRP (Relative Refractory Period) – as illustrated in Figure 2 of ECG Blog #14.
• The reason for the longer-than-anticipated PR interval preceding beat #7 – is that the PAC that occurs within the T wave of beat #6 encounters a still partially refractory AV node, resulting in delay of conduction to the ventricles.

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BOTTOM LINE: The commonest cause of a pause is a blocked PAC. Remembering to think of this truism whenever you assess a tracing for possible AV block will prove invaluable in uncovering the real reason for the rhythm disturbance in a surprising number of cases. Blocked PACs occur far more often than any form of AV block.

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- For more information – GO TO:

1) ECG Blogs on AV Block: Blog #19 – Blog #20 – Blog #21 – Blog #22.

2) ECG Blogs on Aberrant Conduction: Blog #14 – Blog #15 –

3) ECG Basics of AV Block (from ECG-2014-ePub) -

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