The problem with blocks

As noted in my post about the new AHA guidelines for STEMI, new left bundle-branch block in the absence of any Sgarbossa’s criteria is no longer to be considered indicative of STEMI.  This is potentially a major change for those of us who have laboured under the premise that new LBBB = STEMI. So what are Sgarbossa’s criteria and how do we use them?

Quite aside from the difficulty that comes from trying to determine whether a STEMI is present in patients with a normal QRS, thanks to the subtle and ever changing patterns of funny little bumps on the ECG, the FSM has seen fit to load us with further encumbrances, in the form of STEMI equivalents and STEMI mimics.  STEMI equivalents are those things that show occlusion but don’t conform to our existing idea of 1mm ST elevation in limb leads, 2mm in chest leads (not that these numbers are still used…) These are things such as Wellen’s syndrome and De Winter waves and may be the subject of a future post.

STEMI mimics on the other hand are things that look like STEMI but are not, such as early repolarization and left bundle-branch block.
The most common mimic we come across seems to be left bundle-branch block (LBBB). The teaching around how to diagnose STEMI in the presence of a wide complex was was non-existent for me. I presume the reason is that it is considered too difficult. But it really isn’t, as we shall see.

The trouble with LBBB is that it naturally has a degree of ST elevation in some leads and so can be mistaken for STEMI. LBBB can sometimes also be caused by a lesion that affects the left bundle. Therefore it was generally considered that A) one could not diagnose STEMI in the presence of a LBBB and B) the block could have been caused by an infarct, therefore we should consider that any new LBBB should be treated as a STEMI.

There are a number of problems with this. First of all, unless the patient has a recently obtained ECG to compare, we don’t know if the block is new or old. Second, there is no particular correlation between LBBB and STEMI as compared to the rest of the population. And thirdly, it is not true that STEMI cannot be diagnosed in the presence of LBBB.

Sgarbossa’s Criteria

Fortunately we have, and indeed have had for many years, an algorithm that allows us to pick when STEMI is present in the setting of someone with a LBBB or paced rhythm. This algorithm is not particularly difficult, it is reasonably sensitive and specific and can dramatically reduce the number of false positive cath lab activations from the field (assuming that anyone has really been sending through blocks as ‘STEMIs’ in the first place)

So, on with the criteria. There are three pretty simple points to remember and the original criteria had these points weighted with a final score being given to determine if STEMI is present.

  1. Concordant ST elevation of >1mm in at least one lead with a positive QRS.  A ‘normal’ LBBB pattern will always have the ST segments that are the opposite direction to the QRS, so to see concordant ST segments is concerning
  2. ST depression of >1mm in V1, V2 or V3.  Remember that ST depression in these chest leads is always abnormal.
  3. Greater than 5mm of discordant ST elevation in at least one lead.  Whilst LBBB will always have some discordant ST elevation, too much is a bad thing.  However this does not take into account the importance of proportionality in ECG waves, so Dr Smith has recently modified this criterion to ST elevation that is greater than 20% of the size of the QRS (Fulltext PDF of Dr Smith’s paper here)

So, in picture form (with thanks to ems12lead.com): “Normal” LBBB pattern with discordant ST segments.T-wave_discordance

Sgarbossa’s Criteria

Sgarbossa

The originally published paper gave these criteria different weighting, (Concordant elevation: 5 points, Concordant depression: 3 points, Excessive discordance: 2 points)  with a final score of ≥ 3 being considered positive for STEMI.  However, my personal opinion is that it is appropriate to be concerned about the presence of any of these criteria being present and consider whether the ECG needs to be sent to cardiology and the cath lab activated.  We may still have a number of false positives, but again, this is not necessarily a bad thing and is certainly better than false negatives.  It also saves us from having to remember lists of numbers and scores, which can be difficult, whilst remembering these three criteria should be a fairly simple exercise.

So, there we have a simple set of criteria that allows us to diagnose STEMI in the presence of LBBB or a paced rhythm.  You can go out there, secure in the knowledge that you can confidently diagnose STEMI in these patients without having to waste the time of cardiology with every presumed ‘new’ left bundle-brunch block you come across.

I apologise for the lack of references tonight; I am struggling with the steam powered work computers.  I shall rectify this tomorrow when I am home from work and using something slightly newer than Windows 95.

Let me know what you think, ask questions, send general abuse my way or share whatever other musings you would care to via the comments.

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One Response to The problem with blocks

  1. Pingback: ECG’s and STEMI equivalents | Clinical Focus

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