This is Part I of the conclusion to 80 year old male CC: Chest pain.
As we have discussed on numerous previous occasions, the expected relationship between the QRS complex and the ST-segment and T-wave in the setting of left bundle branch should be one of discordance.
This is sometimes referred to as the rule of appropriate T-wave discordance.
That means that in the setting of left bundle branch block, negatively deflected QRS complexes can be expected to show ST-elevation and upright T-waves.
Positively deflected QRS complexes can be expected to show ST-depression and inverted T-waves.
That's why left bundle branch block is an anterior STEMI mimic.
It is normal for the ST-segments to be deflected opposite the S-waves in the right precordial leads (V1-V3).
However, there is a limit to how much discordance is appropriate.
Sgarbossa's criteria requires at least 5 mm of discordant ST-elevation in order to be significant.
However, this criterion is problematic because it does not take into account the rule of proportionality.
That's why it's the weakest of Sgarbossa's critiera.
Discordant ST-elevation of 5 mm (as a stand-alone finding) only indicates a 50% probability of AMI according to Sgarbossa's original scoring algorithm.
This ECG from a previous case post demonstrates the dilemma.
The ST-elevation in leads V1-V3 is well over 5 mm but the S-waves are so deep that they are running off the bottom of the ECG paper.
This patient was not experiencing acute STEMI.
Stephen Smith, M.D. (of Dr. Smith's ECG Blog) uses a modified criterion which considers the ST/QRS ratio.
He has found that when the ST-segment is deviated more than 0.2 the QRS complex it is both a sensitive and specific marker for acute STEMI in the setting of left bundle branch block (and probably also paced rhythm).
(Note: This has since been revised to 0.25 the QRS (download PDF here). However, I still think it terms of allowing 1 mm of ST-elevation for every 5 mm of S-wave depth. When I do this, I round up. In other words, if the S-wave is 18 mm deep, I round up to 20, which means that I would allow up to 4 mm of ST-elevation in that lead.)
Let's examine each of these QRS complexes separately.
We'll start with the positively deflected QRS complex marked 'A'.
As you can see, the R-wave measures 10 mm. The J-point (relative to the PR segment) is depressed 3 mm. Therefore, the ST/QRS ratio is 0.3 (which is higher than 0.2). Hence, this finding would strongly suggest acute STEMI.
Now let's look at the negatively deflected QRS complex marked 'B'.
In this example the S-wave measures 10.5 mm. The J-point (relative to the PR segment) measures 3.5 mm. Therefore, the ST/QRS ratio is 0.33 (which is higher than 0.2). Hence, this finding, would strongly suggest acute STEMI.
In Part II we'll apply Dr. Smith's decision rule to the our recent case study.