Monday, March 3, 2014

Pseudo High Lateral STEMI -- How not to be deceived by ST elevation in aVL

This ECG was shown to me with no other information:

My response was: this is normal variant ST elevation in I and aVL. 


"Why?" I was asked.  First, I will say that many of my interpretations are subjective, based on pattern recognition that is not always translatable into a rule.

But I believe I can at least partly transform my interpretation into a rule here:   Even though there is some minimal reciprocal ST depression in lead III, the ST elevation is in the setting of a well-formed R-wave in aVL.  Moreover, there is a proportionally (proportional to the R-wave) small T-wave.  T-wave size and morphology is at least as important as ST elevation in diagnosing MI.  Also, there are well-formed J-waves in I and aVL.

High lateral MI is the MI location that is most difficult to diagnose largely because the R-wave voltage in Lead aVL is often very small, and thus any ST elevation or T-wave amplitude may also be small, though when scrutinized it is often proportionally excessive.  This does not apply here.  There is plenty of R-wave amplitude with which to judge the ST segment and T-wave.

There are no other findings on the ECG to support MI:

1. Lateral MI is frequently associated with posterior MI, but here there is no precordial ST depression

2. High lateral MI from first diagonal occlusion (D1) is frequently associated with ST elevation in V2.  Well there is indeed ST elevation in V2, but this is normal variant ST elevation:  if you use the Smith formula, STE60 V3 = 1.5, computerized QTc = 392, and R-wave amplitude is 10.5 mm; formula = 21.50 (significantly less than 23.4, so very unlikely to be MI).

3.  There is no ST elevation in V5 and V6.

Pretest Probability

If this were a person at risk for STEMI: risk factors, older age, crushing chest pain, I would highly recommend serial ECGs, immediate formal echo, etc.  But not cath lab activation.

In this case, as it turned out, the patient was less than 40, had no risk factors, and had primarily abdominal pain and vomiting. 

Certainly I would not obect to serial ECGs, but no more aggressive investigation is warranted.

Also, do not hesitate to consult someone with more expertise.  This is usually, but not always, a cardiologist

Outcome:

It turns out that the cath lab was activated, coronaries were normal, and the patient ruled out.

Unfortunately, this happened on a late in the evening, so that the cath team had to be called into the hospital and it was an unfortunate use of resources.

All serial ECGs were identical.  The patient ruled out by serial troponins.

Lessons

1. Use cath lab resources wisely, especially depending on your institution's own resources
2. Remember the importance of proportionality
3. Remember the importance of the T-wave in STEMI
4. Soft ECG findings should be more deeply scrutinized when the pretest probability is low
5. f you are worried, use other resources, especially immediate high quality echo, to look for wall motion abnormality.








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