Welcome to the Apocalypse

Well, 21/12/2012 is here, and what an eventful day it is. Notwithstanding the whole end of the world thing happening today, it is also my birthday (I’m pretty sure I didn’t cause the apocalypse, but no guarantees I’m afraid) And, perhaps of greater note, the AHA has released it’s 2013 guidelines for the management of STEMI (1)
These are obviously of note for all paramedics, as they essentially set the scene for the ERC, ILCOR and ARC, which will then have a flow on effect to us in the field.

So, what is new? Well, I’m sorry to say, that as it is my birthday I have only had the briefest of reads through the paper, so I will note a couple of points I have seen so far, and come back to the rest of it when I have had time to sink my teeth in.

Of note for paramedics, concerning the diagnosis of STEMI:

  • “Diagnostic ST elevation… is defined…  as new ST elevation at the J point in at least 2
    contiguous leads of 2 mm (0.2 mV) in men or 1.5 mm (0.15 mV) in women in leads V2–V3 and/or of 1 mm (0.1 mV) in other contiguous chest leads or the limb leads”
  • “New or presumably new LBBB at presentation occurs infrequently, may interfere with ST-elevation analysis, and should not be considered diagnostic of acute myocardial infarction (MI) in isolation”
  • “ST depression in 2 precordial leads (V1–V4) may indicate transmural posterior injury.”
  • “Multilead ST depression with coexistent ST elevation in lead aVR has been described in patients with left main or proximal left anterior descending artery occlusion.”
  • “Rarely, hyperacute T-wave changes may be observed in the very early phase of STEMI, before the development of ST elevation.”

These are all on page 6 of the document.  I hope that most of these do not come as a surprise to you.

There are, however some subtle, but important differences to the criteria for STEMI that we are taught.  Most obviously is the difference in the amount of elevation required between males and females.  This is actually a simplification of a reasonably wide range of figures, but it is worth noting the lowered level of ST elevation required to diagnose STEMI in females.  It is also worth noting that they are talking about J-point elevation.  I have heard it put around by some paramedics that elevation is measured 2mm after the J-Point.  This is not true and would result in a massive over-diagnosis.  Some false positives are acceptable, and there is a reasonable amount of data that this happens frequently in patients coming from the ED (2), so we shouldn’t be too worried about the odd one ourselves.  However messing around with where we read elevation to be would result (in my opinion) in unacceptably high levels of inappropriate activation.

It is still enshrined in our protocol that “new” left bundle branch block (LBBB) is diagnostic of STEMI and we are expected to activate the cath lab when we find one.   This is generally not true and not appropriate (3), so with luck we will lose this edict in the near future.   LBBB certainly can make diagnosis of STEMI difficult, and it may be of concern either way as it appears to be a predictor of mortality (3) but LBBB ≠ STEMI.  I will talk about distinguishing between LBBB with STEMI and LBBB without in another post.

I would hope that MICA paramedics at least would be well aware of the signs of posterior infarction.  Isolated posterior infarct is not necessarily that rare, but may be frequently missed.   Remember that ST depression in V1-V3 is never normal.  If you see it, grab some posterior leads, and keep in mind that voltages around there are very low, so we aren’t looking for 2mm of elevation: 0.5mm is probably diagnostic of STEMI. (4)

ST elevation in aVR with widespread ST depression throughout the other leads should also be something that makes paramedics very nervous.  If not, the AHA provide a clear answer as to why you probably should have a high pucker factor when you see it.  An ostial occlusion, left main occlusion, proximal LAD occlusion or triple vessel disease can all cause an elevated aVR (as it is a sign of widespread sub-endocardial ischaemia).  None of these are good things and most of them quickly lead to death.

Finally, I am sure that paramedics are well aware of the significance of hyperacute T-waves in the setting of someone we suspect of having an ACS.  It is important to remember that the size of the T-wave is relative to the size of the QRS (which also goes back to Sgarbossa’s criteria which we will talk about at a later date) and it pays to keep in mind other conditions like hyperkalemia, which can also cause acute T wave changes.

So, there are a couple of interesting points to have a think about.  There is a lot of other stuff to go through so I will try to dissect the rest of the paper and see what there is of interest to ambulance drivers down-under.

Robbie

 

  1. O’Gara, P. T., Kushner, F. G., Ascheim, D. D., Casey, D. E., Jr., Chung, M. K., de Lemos, J. A., et al. 2013 ACCF/AHA Guideline for the Management of ST-Elevation Myocardial Infarction: A Report of the American College of Cardiology Foundation/American Heart Association Task Force on Practice Guidelines. Circulation.
  2. McCabe, J. M., Armstrong, E. J., Kulkarni, A., Hoffmayer, K. S., Bhave, P. D., Garg, S., et al. Prevalence and factors associated with false-positive ST-segment elevation myocardial infarction diagnoses at primary percutaneous coronary intervention-capable centers: a report from the Activate-SF registry. Arch Intern Med, 172(11), 864-871.
  3. Mehta, N., Huang, H. D., Bandeali, S., Wilson, J. M., & Birnbaum, Y. Prevalence of acute myocardial infarction in patients with presumably new left bundle-branch block. J Electrocardiol, 45(4), 361-367
  4. Wung, S. F., & Drew, B. J. (2001). New electrocardiographic criteria for posterior wall acute myocardial ischemia validated by a percutaneous transluminal coronary angioplasty model of acute myocardial infarction. Am J Cardiol, 87(8), 970-974; A974.
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