ECG Interpretation Review #33 (Bundle Branch Block - PACs - Blocked - Aberrant Conduction - RBBB)

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

• Can you explain the irregularity?

Figure-1: Right-sided MCL-1 monitoring lead rhythm strip. 

Can you explain the irregularity? 

INTERPRETATION: The easiest way to approach interpretation of challenging arrhythmias is to start with what is known.  Save more difficult parts of the tracing until later.  We always look first to see if there is an underlying rhythm.  In Figure-1 — the underlying rhythm is sinus, as determined by beats #1, 3, 5, 7, 9, and 11 which are all preceded by a similar-morphology biphasic P wave with fixed PR interval.  All QRS complexes except for beat #4 are narrow.  

• Note the interesting bigeminal periodicity of the rhythm — with alternating short-long cycles. Thus, every-other-QRS complex occurs early. Every other QRS complex is a PAC (Premature Atrial Contraction = Figure-2).

Figure-2: – Adding arrows facilitates recognition of multiple PACs (See text). 

Additional Points about this Rhythm:

The rhythm in Figure-1 is sinus with multiple PACs. We highlight a number of additional interesting points about this rhythm:

• P wave morphology of each PAC in Figure-2 (RED arrows) is slightly different in being primarily positive compared to the biphasic P waves of each sinus beat.  This is as it should be since PACs by definition originate from a different site in the atria than the sinus node.
• Not all PACs in Figure-2 are conducted. Note that no QRS complex follows the PACs that occur just after beats #2, 6, and 10. These PACs are “blocked” (nonconducted) – because they arise so early in the cycle as to occur during the ARP (Absolute Refractory Period) when conduction to the ventricles from an impulse arriving early at the AV node is not possible (See ECG Blogs #14 and #15 for Review of this concept).
• Whether blocked PACs also occur and are hidden within the ST-T wave of beats #4 and 8 can not be determined with certainty from Figure-2 — but the slight ‘blip’ near the beginning of the ST segment of beat #4 suggests that this may be the case.
• The widened QRS complex in Figure-2 (beat #4) is not a PVC (Premature Ventricular Contraction). Instead — it too is a PAC conducted with aberration.  Note that beat #4 is also preceded by a premature P wave (RED arrow before beat #4) — which confirms that this beat is an aberrantly conducted PAC.
• In fact — beats #2, 4, 8 and 10 are all aberrantly conducted PACs!  Normally — QRS morphology of PACs will be virtually identical to QRS morphology of sinus beats. PACs merely arrive earlier than anticipated at the AV node — but once there, they typically conduct to the ventricles in normal fashion. It is only when PACs arrive especially early (or when the relative refractory period is for some reason prolonged) that PACs may manifest aberrant conduction.
• Beats #2, 4, 8 and 10 are all PACs that manifest different degrees of aberrancy.  Of these beats — it is beat #4 that manifests the greatest degree of aberrant conduction (in the form of a complete right bundle branch block pattern). This makes sense because the coupling interval of the PAC preceding beat #4 (distance between this P wave and beat #3) is shorter than the coupling interval for the other PACs. The P wave preceding beat #4 therefore arrives earlier at the AV node at a time when it is more likely to encounter the conduction system in a relatively refractory state.  In contrast — the coupling intervals of the P waves following beats #2, 6 and 10 are even shorter. Physiologically, the P waves following beats #2, 6 and 10 are presumably occurring during the absolute refractory period, which is why these PACs are “blocked”.
• Beat #6 does not conduct with aberration.  Measurement with calipers shows its coupling interval (from the P preceding beat #6 until beat #5) is slightly longer than the coupling interval preceding premature beats #8 and 10 (ergo slightly more time for recovery and therefore more normal conduction of beat #6).

FINAL THOUGHTS: The rhythm in Figure-1 is sinus with multiple PACs.  Some of these PACs are blocked, while others are manifest varying degrees of aberrant conduction. The importance of being comfortable with recognizing blocked PACs and aberrant conduction was highlighted in our Blogs #14 and #15 which illustrate how such PACs may simulate AV block and ventricular tachycardia …

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- See ECG Blogs #14 and #15 for Review on Aberrant Conduction.

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