November 8, 2013

REBEL ECG of the Week #1

68 year old hispanic female with a chief complaint of weakness/syncope.  PMH of type 2 diabetes mellitus and hypertension.  She was brought in via EMS due to the family calling 911 as she had an episode of syncope while at the dinner table.  At the time of arrival:

VS: 96/48   43     18     99% on RA  98.7

ECG obtained at arrival is shown…

Before reading on, try to come up with your own interpretation of this ECG before moving on to the final impression

Complete Heart Block

  • Rate: Ventricular rate ~ 48
  • Rhythm: 3rd degree block with AV nodal escape rhythm
  • Axis: Normal to slight RAD
  • QRS: Barely wide (>0.12 msec), RBBB morphology
  • ST/T Waves: ST elevation V1 – V2, hyperacute t waves in V4 – V6
  • Final ECG interpretation: Likely ACS/AMI based on V1-V2 and V4-6 findings with CHB

Third-degree AV block or complete heart block occurs when the electrical impulse generated in the SA node in the atrium is not conducted to the ventricles.  When the atrial impulse does not conduct to the ventricles, an accessory pacemaker (escape rhythm) in the ventricles will typically activate a ventricular contraction.  Since two independent electrical impulses occur (SA node impulse & accessory pacemaker impulse), there is no apparent relationship between the P waves and QRS complexes on an ECG.  This can be best characterized as:

  1. P waves with a regular P to P interval (Red Arrows)
  2. QRS complexes with a regular R to R interval (Blue Arrows)
  3. The PR interval will appear variable because there is no relationship between the P waves and the QRS Complexes

Complete Heart Block

Example of Complete Heart Block (CHB)

Key Point: Atrioventricular (AV) dissociation is not the same thing as 3rd degree heart block (complete heart block)

  • AV dissociation: Atria and ventricles beating regularly, but independent of each other
  • AV dissociation: Ventricular rate is the same or faster than the atrial rate
  • Complete heart block (CHB): No atrial beats are conducted to the ventricles
  • AV dissociation without CHB:  Some atrial beats conducted to ventricle (Look for changing QRS morphologies)
Cite this article as: Salim Rezaie, "REBEL ECG of the Week #1", REBEL EM blog, November 8, 2013. Available at:
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Salim Rezaie

Emergency Physician at Greater San Antonio Emergency Physicians (GSEP)
Creator & Founder of R.E.B.E.L. EM
  • Ken Grauer, MD
    Posted at 14:12h, 08 November Reply

    Interesting case and 12-lead ECG. Agree this is 3rd degree (complete) AV block – because there is a regular atrial rate; regular ventricular rate – and complete AV dissociation with none of the P waves conducting DESPITE having adequate opportunity to do so (which usually requires a ventricular rate of 50/min or less – which is the rate here). There is complete RBBB (QRS is actually a bit over 0.12 second). In addition – there is abnormal ST coving in V1,V2 – which shouldn’t be with simple RBBB. ST-T waves are abnormal in leads II and V3 – and the T waves in V4,V5,V6 are hyperacute (much taller and broader than they should be with simple RBBB in these leads) – so I strongly suspect recent acute MI in this patient. ST segment is also elevated in lead aVR.

    I find it best to think of AV Dissociation as the “generic” – and complete AV block as one of the 3 types of complete AV dissociation (the other 2 being AV dissociation by “default” or “usurpation”). You can have on a single rhythm strip absolutely complete AV dissociation in which the ventricular rate might be slightly slower than the atrial rate (with isorhythmic AV dissociation, there may be a “back-and-forth” between sinus rate slightly more or slightly less than AV junctional rate). The KEY determinant is whether or not P waves “have a chance” to conduct – and still fail to do so. Sometimes with AV dissociation – there may be no conduction, but because P waves are always occurring at a point when they have no chance to conduct (ie, too short of a PR; within the ST segment) – they may not have an opportunity to conduct – in which case no AV block can be diagnosed from that single rhythm strip. And with complete AV block at the AV nodal level – there may not necessarily be any change in QRS morphology between junctional escape beats vs sinus-conducted beats. Lots of potential caveats … That said – this tracing does appear to manifest complete AV block with escape morphology showing RBBB from recent infarction. VERY interesting case!

    • Salim Rezaie
      Posted at 14:32h, 08 November Reply

      Hello Ken,
      TY for coming on to the site. You were correct on the RBBB being complete and not incomplete. That was a typo on my end….fixed now.
      Unfortunately, I never did get follow up on this case, I agree the morphology appears to be acute myocardial infarction (STE V1 & V2), with hyper acute t waves in V4 – V6, but I have no lab work or cath report to follow up on. However this was a great opportunity to solidify the fact that AVD does not equal CHB. One of my residents kept interchanging AVD & CHB and I wanted to put this up as a reminder that they are not the same thing. Please feel free to always comment and correct on future posts. TY again, and great to see you on here.


      • Ken Grauer, MD
        Posted at 18:49h, 08 November Reply

        I agree 100% – AV Dissociation is NOT the same as Complete AV Block – : )

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