Case No. 4 D

A 59 yr old white male presented to his primary care physician with complaints of weakness and exertional dyspnea. The following ECG was recorded by EMS called to the scene.

Again we see the preference in RCA occlusion for a junctional rather than ventricular escape pacemaker. The low voltage baseline activity in leads I, II, and III appears artifactual, however a close examination of the precordial leads demonstrates a small blip consistantly 200ms after the beginning of the QRS, suggesting the possibility of retrograde atrial depolarization buried within the ST-segment. These findings have been marked with the red arrows below.

The disturbances marked with the blue arrows should also be noted, perhaps representing atrial depolarizations with “P-mitrale” morphology. Under this interpretation, the EKG in fact demonstrates a sinus rhythm with marked 1st degree block. Unfortunately, no additional tracings are available for study.

This pt survived to reach the cath lab, but no further is known.

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2 responses

  1. VinceD

    Wonderful blog you have here! Sorry for the late comment, I’m working my way through all your old cases.

    What do you think is causing the last QRS on the tracing to appear different in appearance from the preceding complex? Fusion beat?

    May 27, 2011 at 10:01 am

    • Thank you for the comments and kind words, Vincent! You have made a fascinating observation here and it is leading me to reevaluate my interpretation of this case.

      Here are my thoughts:

      Previously I had noted an apparent absence of P waves before the QRS complexes and it seemed possible that a retrograde atrial depolarization might be responsible for the small yet consistent irregularity ~200ms into the ST segment, now highlighted with the red arrows. In light of the inferior ST elevation and reciprocal depressions in I and avL, these observations suggested sinoatrial ischemia with a subsequent junctional escape rhythm.

      I understand that for a fusion complex to arise, two independent electrical wavefronts instigated by two geographically disparate foci must simultaneously activate the same region of myocardium. If the 6th complex in the 12-lead represents a fusion beat between two foci, the question is where are the foci originating. When the wavefronts of an atrial and a ventricular pacemaker fuse, the morphology is part supraventricular and part idioventricular. But if an atrial and a junctional wavefront fuse, only the atrial myocardium between them will reflect the fusion. The morphology of ventricular depolarization, the QRS, should remain the same, provided the same junctional focus is involved.

      I am thus lead to believe that the 6th complex is either a competing junctional focus with an alternative rout of decent down the bundle branches, or (more likely) the result of a transient bundle branch conduction delay. The initial precordial complexes have a rightward axis in the transverse plane, but the final complex has a leftward axis. A rightward axis is suggestive of a right conduction delay, while a leftward axis can either be normal or pathological depending on the morphology.

      As an additional thought, I have also noted the disturbances marked with the blue arrows. If these represent atrial depolarizations with “P-mitrale” morphology, then perhaps this EKG in fact represents a sinus rhythm with marked 1st degree block.

      Unfortunately I do not have any additional tracings from this patient. I am very interested in your thoughts here and I thank you again for your generous insight.

      May 28, 2011 at 8:41 pm

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