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Normal Sinus Rhythm With Aberrantly-Conducted PACs

Dawn's picture
Tue, 04/02/2013 - 22:00 -- Dawn

This is a normal 12-Lead ECG with two PACs that are aberrantly conducted in a right bundle branch block pattern. (Sixth and ninth beats). In the PACs, the QRS is slightly wider than the normal beats.  The aberrantly-conducted beats have an rsR' pattern in V1, and a wide little S wave in aVL. No PACs are seen in Lead I to demonstrate the wide S wave.  This represents a right bundle branch block pattern, which is a common form of aberrancy, and is rate-related.  That is, the PAC occurs early in the cycle, catching the right bundle branch is a refractory state and unable to depolarize.  Slower beats are easily acommodated by the right bundle branch.

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Comments's picture

Perhaps the most helpful lesson learned from review of this tracing is the manner in which use of all 12-leads on an ECG may prove invaluable to arrhythmia diagnosis.

As per Dawn - there are 2 premature beats (PACs) seen in the long lead II rhythm strip at the bottom of the tracing (beats #6 and #9). That said - I would not have been certain these are PACs if the only information I had was this lead II rhythm strip. Baseline ST-T wave morphology varies somewhat among the normal sinus beats - and in lead II there is NOT a convincing premature P wave preceding beats #6 and 9. Although there is a resemblance of QRS morphology for beats #6 and 9 compared to the other beats on this tracing - a beat should always be assumed "guilty" (ie, a PVC) until proven otherwise - and there is no convincing evidence in favor of aberrant conduction in this lead II ....

On the other hand - Using the 12-lead ECG:

  • Simultaneously occurring leads aVR, aVL and aVF all show pronouncement in the T wave preceding the early beat compared to the T wave of the sinus beat in these leads.
  • We clearly see a notch in the T wave preceding beat #9 in lead V2 - which unmistakenly is a PAC.
  • QRS morphology in lead V1 is "classic" for RBBB aberration (the 9th beat manifests an rSR' with similar initial deflection, S wave descending below the baseline, and taller right rabbit ear R' ).
  • Consistent with RBBB aberration - is the wide terminal S wave in lead aVL for the 6th beat.

The above evidence from the simultaneously recorded 12-lead overwhelmingly supports a diagnosis of PACs with RBBB-aberration for early beats #6 and #9.

Ken Grauer, MD   [email protected] 

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