Wednesday, December 31, 2008

Acute MI from LAD occlusion, or early repolarization?

3 hours of chest pain. Is it STEMI or is it normal (or early repolarization)?

Read Answer Below

This ECG looks quite normal, with only moderate ST elevation, upward concavity, good R-wave amplitude.  There is no ST depression or T-wave inversion.

But one easily identified characteristic makes the ST elevation unlikely to be from early repolarization: the computerized QTc is 455 milliseconds. In my study of early repolarization (ER) (n=167), only 2 of 167 (2%) cases of ER had a QTc greater than 455ms. The mean QTc was 394ms, compared to 420ms for MI (n=125) from LAD occlusion. Conversely, only 4% of LAD occlusion, vs. 40% of ER, had a QTc less than 380 ms. These clues were not appreciated by the clinicians. A very astute interpreter would not that the T-waves are also too symmetric to be early repol, which should have a steeper downslope than upslope, as demonstrated here:

The clinicians did not appreciate these subtle differences.

The patient continued to have chest pain.  70 minutes later, they repeated the ECG, which is shown here:
Note that now there are tiny Q-waves in V2-V4, making this unequivocally diagnostic of acute STEMI. 
These subtle Q-waves were not appreciated, but the clinicians were astute and ordered a stat echocardiogram, which confirmed anterior wall motion abnormality. The patient was taken to the cath lab and had a 100% LAD occlusion.

After reperfusion, the patients baseline ST-T complex was revealed. This is probably what they would have looked like prior to the LAD occlusion:

For those who want some more detail on differentiating ER from MI, see below:

Also useful was the mean (from V2-V4) R-wave amplitude, at a cutoff of 5 mm, with values less than 5 mm likely to represent MI. Interestingly, mean ST elevation (no matter how it was measured) was not as good a differentiator as the mean R-wave amplitude, but if mean R-wave was less than 5 mm OR the mean ST elevation (V2-V4, as measured at the J-point, STEJ) was greater than or equal to 2 mm, then it was very likely to be MI and very unlikely to be ER.

Even better was a formula derived with logistic regression, which also included the QTc :
(1.196 x STE60 in V3 in mm) + (0.059 x computerized QTc in milliseconds) - (0.326 x RA in V4 in mm), where RA is R-wave amplitude and STE60 is ST elevation at 60ms after the J-point relative to the PR interval.

If the value of the formula is greater than or equal to 23.4, it is MI (Sens, spec, accuracy all around 90%); if less, then it's ER.

For the first ECG, STE60, V3 = 2.5mm, QTc = 455, and RA V4 = 17, so: (1.196 x 2.5) + (0.059 x 455) - ( 0.326 x 17) = 24.29; this is barely greater than 23.4, thus consistent with MI but also further illustrating that this ECG is very difficult.

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