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Atrial Pacing With Right Bundle Branch Block

Mon, 05/19/2014 - 12:32 -- Dawn

No instructor's collection should be without an atrial paced rhythm OR a right bundle branch block.  Here, you get both.  First, the atrial pacing.  This patient had a sinus node problem, but his AV conduction system was functional (if not perfect).  At this time, he is able to conduct impulses from the atria to the ventricles.  What he cannot do is reliably produce the impulse in his atria.   So, this pacemaker is currently pacing the right atrium, producing a paced "P" wave, which is then conducted to the ventricles.  The fifth beat on the strip shows a "native" beat - one produced by the patient.  No P wave is seen, so it is presumed to be a junctional beat.

As for conduction through the ventricles, there is a right bundle branch block.  The left bundle branch is ensuring that the ventricles receive the depolarization "message", and the ventricles are depolarizing and contracting.  However, the right ventricle gets the message a little late, since is arrives from the left ventricle, and not through a functioning right bundle branch.  This produces a terminal wave on each QRS that represents this delayed depolarization of the right ventricle.  In leads oriented to the left side of the heart, like I and V6, it is seen as a wide little S wave.  In V1, which is oriented to the patient's right, we see an R prime (R'), producing the easily-recognizable rSR' pattern of RBBB.

For your more advanced students, this patient has atypical T waves for RBBB.  Normally, the T waves axes should be OPPOSITE that of the terminal portion of the QRS.  So, Lead V1 correctly shows an inverted T wave, since the R' is a positive deflection.  There are inverted T waves in Leads III, aVF (II is biphasic), as well as in V4, V5, and V6.  We expected upright T waves here. Because we do not have clinical information for this patient, we will call them "non-specific" T wave changes, remembering that inverted T waves can be a sign of ischemia.

ALSO:  As noted in Dave Richley's comment below, there is a left axis deviation, with a negative Leads II, aVF and III, and a positive I and aVL.  This  indicates left anterior fascicular block, which is rather common with RBBB, since the right bundle branch and the left anterior fascicle share a blood supply. So, this person as a "bi-fascicular block". 


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Dave Richley's picture

It's probably worth pointing out that in addition to what Dawn describes there is also a big left axis deviation typical of left anterior fascicular block.

Dave R

Submitted by Dawn on

Yes, I omitted that accidentally, as it is rather glaring. I will add an addendum. Thanks so much.'s picture

     As described - this tracing shows atrial pacing. But ventricular complexes are not paced - so we can fully analyze them for QRS and ST-T wave morphology.

  • As stated - there is complete RBBB (Right Bundle Branch Block) and LAHB (Left Anterior HemiBlock).
  • Criteria for typical RBBB are clearly met - rSR' with taller right "rabbit ear" - and wide terminal S waves in leads I,V6.
  • Criteria for LAHB are clearly met - with marked LAD (Left Axis Deviation) evidenced by the predominantly negative QRS in the inferior leads.
  • However - ST-T wave changes are definitely not as they should be. The ST depression in V1,V2 are as expected for RBBB - but note how the T inversion then gets DEEPER by lead V3. The ST segment in V4,V5 is coved - and deep T inversion persists through to V5. This is MUCH MORE than expected for RBBB. These are ischemic T waves.
  • Similarly - the ST coving and fairly deep, symmetric T inversion in the inferior leads is also abnormal and suggests ischemia.
  • There is some semantics in our terminology. That said - rather than "nonspecific" - the changes we see here are more than that. They do not indicate an acute event (they don't give us any idea of how long they have been present for) - but these changes could be new, and they could reflect acute ischemia. If given this ECG to interpret - my response would be, "How is this patient doing right now at this time?"
OTHER FINDINGS on this tracing:
  • Voltage for LVH (tall R >12 in aVL - which loses some specificity in the setting of LABH, but still is enough to call).
  • Beat #4 is spontaneous (NOT preceded by a paced P wave). This actually IS important - because it confirms that we CAN state all of the above by confirming that QRS morphology is the same on this tracing regardless of whether or not the atria are paced.

Ken Grauer, MD   [email protected] 

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