According the Sgarbossa's original criteria, 5 mm of discordant ST-segment elevation is required to identify AMI in the presence of LBBB.
Why 5 mm when normally we require only 1 or 2 mm of ST-elevation?
Because in the setting of left bundle branch block or paced rhythm, it's normal for the ST-segment and T-wave to be defected opposite the main deflection of the QRS complex!
That's why it's necessary to consider the depth of the QRS complex when examining the amount of discordant ST-segment elevation. The deeper the S-wave, the greater the secondary ST-T wave abnormality in the opposite direction!
In the original article I wrote on the topic, I showed this example 12-lead ECG to show why the 5 mm criterion is problematic.
As you can see, this 12-lead ECG shows sinus rhythm with left bundle branch block and > 5 mm of discordant (opposite the QRS complex) ST-elevation in leads V1, V2, and V3 (the right precordial leads). The T-wave are huge!
The problem is, this patient was not experiencing acute myocardial infarction. The ST-segments are elevated > 5 mm because the S-waves are extremely deep (off the bottom of the ECG paper for leads V2 and V3).
Had we used the modified criterion of discordant ST-elevation that is = or > to 0.25 the QRS complex (credit to Dr. Smith), we would have seen that in lead V1 the S-wave is 50 mm deep. Thus, we would require at least 12.5 mm of ST-segment elevation to consider this finding positive for acute STEMI.
There's another way the modified criterion can help you!
Consider this 12-lead ECG that shows a ventricular paced rhythm. It's been in my collection for many years, and I regret that I no longer recall where it came from.
This ECG does not meet Sgarbossa's criteria for diagnosing AMI in the presence of LBBB. With the exception of lead V6, the paced QRS complexes show appropriate T-wave discordance, and none of the ST-segments are elevated to 5 mm or more.
But wait! The ST-segments are elevated far greater than 0.25 the depth of the QRS complex in leads II, III, and aVF! This patient is experiencing acute inferior STEMI!
The intrinsic QRS complex in the right precordial leads also shows an > R/S ratio in lead V1 and V2 and ST-segment depression suggesting posterior extension, which clinches the diagnosis.
So remember, when using Sgarbossa's criteria, huge QRS complexes can cause false positive and tiny QRS complexes can cause false negatives, unless you use the modified rule that considers ST-segment elevation as a percentage of the QRS complex!
See also:
Identifying AMI in the presence of LBBB
Sgarbossa's criteria - new graph
"New" LBBB - What's the big deal?
STEMI best seen in PVC (Dr. Smith's ECG Blog)
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