Case No. 4 A: Atypical Symptoms with a Critical Lesion

A 64 yr old white female presented to EMS with n/v/d times three days and a recent episode of orthostatic syncope. She had no complaints of chest discomfort or shortness of breath. She was pale in apearance and found to be hypotensive on exam.

Although AV dissociation can be appreciated in the first three tracings, the fourth appears consistent with undifferentiated 2nd degree conduction block and prolonged (>200 ms) PR interval with bigeminal junctional escape. Close examination of II and V4-6 reveal a subtle morphological variation in the complexes, supporting the argument for multiple depolarization foci. A narrow-complex junctional escape rhythm is typical of a culprit RCA lesion resulting in often transient ischemia to the superior portion of the nodal tissue. Wide-complex 3rd degree block more frequently reflects a significant LCA infarction which has resulted in distal, more inferior conduction system damage that is less likely to recover. Note that the ST elevation in III is greater than that in II— a finding that has been correlated with RV involvement, although I know of no EBM trials to support this. It is regrettable that right-sided and posterior leads were not recorded.

AV conduction abnormalities can be appreciated in as much as 30% of inferior wall MIs owing to the ~70% prevalence of the RCA as the dominant vessel supplying the AV nodal branch. Left dominant coronary systems present a variation to the predictability of progressive conduction pathway ischemia but constitute only 10% of the populace. This leaves 20% with co-dominant systems. In the absence of confounding factors, it would be interesting if there has been a study demonstrating a decreased incidence of AV conduction blocks in pts with redundant AV nodal circulation who present with acute coronary syndrome.

Despite what appear to be convalescent ECG changes over the course of these three 12-leads, the pt. deteriorated rapidly in the ED and required pressor support and intubation before she could be transported to the cath lab. The outcome is unknown.

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2 responses

  1. Michele

    The determination of the rhythm in the last ECG is very interesting, and makes me wish there were a much longer rhythm strip available to appreciate the type of block reflected here. Since this patient seems to be alternating between this and a third degree block, I am tempted to believe that the AV pathology is type two, since this would be more likely to progress to third degree. Semantically, I wonder if we can call the junctional escape bigeminal, since technically there is a dropped QRS — the beat comes after every one ventricular complex, but after every two atrial contractions. Great commentary on the RCA lesion.

    October 19, 2010 at 2:03 pm

  2. Your argument concerning the differentiation of the second degree block here is exceptionally discerning and I am grateful for your commentary. Given that a 2:1 AV ratio is at play, we cannot specify a Type I vs. Type II Mobitz block. Your insight that in light of the pt’s advanced electrophysiologic pathology and poor clinical presentation a Type II represents the greater probability demonstrates, I believe, sound reasoning and good medical judgment. I am always admiring of practitioners who can apprehend a pt’s future and past simply by looking closely at where the pt is at present.

    I also want to take this opportunity to discuss the possibility of a coincidentally synchronized or “isorhythmic” AV dissociation situation. Theoretically, if the atrial and ventricular rates are sufficiently close, for example with a 2:1 ratio as seen above, it might be difficult to distinguish a 2nd degree block from pure dissociation. Were this the case here, however, we would expect to see a regular atrial rate coincidentally superimposed on a regular ventricular rate of half the speed. Yet in the rhythm above, the ventricular rate is regularly irregular, again supporting a Mobitz type interpretation.

    October 24, 2010 at 1:45 am

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