Wednesday, January 4, 2012

ECG Interpretation Review #34 (Acute MI - vs Nonspecific Changes - ST Elevation/Depression)

The 12-lead ECG shown in Figure 1 was obtained from a 72-year-old woman seen in the ED (emergency department) with new-onset chest discomfort.  No prior ECGs were available for comparison.  The tracing was interpreted as “showing no acute changes”.  Do you agree?  What else do you see?
Figure 1 – 12-lead ECG from a 72-year-old woman with new-onset chest discomfort.  
Do you agree that there are “no acute changes”?  
NOTE Enlarge by clicking on FiguresRight-Click to open in a separate window.
INTERPRETATION:  The rhythm is sinus at a rate of ~75/minute.  All intervals and the axis are normal.  There is no chamber enlargement.  With regard to Q-R-S-T Changes – there are several findings of note:
  • There are Q waves (QS complexes) in anteroseptal leads V1-thru-V3.  A small positive deflection (r wave) finally develops by lead V4.  Thus, transition is delayed (only occurring between leads V4-to-V5).
  • There is subtle (but real) ST segment elevation in lead aVL.
  • Support that the subtle ST elevation in aVL is a real finding is forthcoming from the ST segment flattening and subtle (but real) ST depression seen in each of the inferior leads (II,III,aVF).
CLINICAL IMPRESSION:  Given the clinical history of a 72-year-old woman presenting to the ED with new-onset chest discomfort and no prior tracing available for comparison – the findings in Figure 1 are clearly of concern.  Lack of any r wave at all until lead V4 suggests prior anteroseptal infarction.  The subtle (but real) ST elevation in lead aVL with equally subtle (but real) “reciprocal changes” in leads II, III and aVF suggest that the patient may be in process of evolving an acute high lateral MI (myocardial infarction).
  • IF this patient is truly in process of evolving an acute STEMI (ST-Elevation MI) – this should be known shortly.  ECG changes of acute MI may evolve quickly – sometimes in less than an hour.  We would repeat the 12-lead ECG in short order.
  • Of the 5 lateral leads (I,aVL; V4,V5,V6) – lead aVL views the heart from the highest and most peripheral perspective (looking down at the heart from the left shoulder).  As a result – lead aVL may sometimes be the only lateral lead to show acute changes (as may be the case in Figure 1 … ).
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ADDITIONAL POINTS ABOUT ASSESSING SUBTLE ST-T WAVE CHANGES:
Determining ST-T wave changes is essential to ECG interpretation.  It is with respect to the PR segment that ST segment deviations (elevation or depression) are judged (Figure 2).  The PR segment is the connecting line that extends from the end of the P wave – until the beginning of the QRS complex.  We define:
  • ST ElevationIF the ST is above the PR baseline.
  • ST DepressionIF the ST is below the PR baseline.
NOTE:  The PR baseline is more difficult to see when the heart rate is faster (the PR shortens with tachycardia).  It may be especially challenging to identify the PR baseline when there is baseline wander …
Figure 2 – Use of the PR segment baseline to assess for ST segment deviations. 
(Figure reproduced from ECG-2011 Pocket Brain - pg 46).
As noted – the changes in the 12-lead ECG shown in Figure 1 are subtle.
  • The shape of ST elevation is more important than the amount of elevation.  Acute MI may occur with only minimal ST elevation.
  • History is ever important.  The fact that the patient in this case is of a certain age (72 years old) and that she presents to the ED with new-onset chest discomfort places the onus on us to prove that she is not presenting with ACS (acute coronary syndrome) rather than the other way around.
In Figure 3 – we blow-up the key leads of concern.  Although the amount of ST segment deviation (above and below the baseline) is not a lot – careful attention to the PR segment baseline (horizontal red lines in Figure 3) reveals that there is ST elevation in lead aVL and reciprocal ST depression in II, III and aVF.
Figure 3 – Blow-up of leads II,III,aVL,aVF from Figure 1.  
NOTE Enlarge by clicking on FiguresRight-Click to open in a separate window.
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BOTTOM LINE:  We admittedly are in no way certain that the patient in this case is in process of evolving an acute high-lateral MI.  However, in view of the history and the ECG changes described above – we have to assume this is occurring until proven otherwise.  Careful observation – presumptive initial treatment measures – serum troponins – repeating the ECG in short order – and a little bit of time should suffice to clarify the true picture.
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- See ECG Blogs #6 and #7 – and our Acute MI Review PDF.
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