In Part I, we discussed Sgarbossa’s Criteria for identifying AMI in the presence of LBBB. We also talked about the “rule of appropriate T wave discordance” for bundle branch blocks and other forms of abnormal depolarization (like ventricular rhythms or paced rhythms).
You will recall that I drew a distinction between a QRS complex’s main deflection and its terminal deflection, even though they are one in the same for LBBB. I explained that it’s helpful to think in terms of the terminal deflection, because then you can apply the “rule of appropriate T wave discordance” to RBBB as well as LBBB.
And so you can!
Let’s look at an ECG.
This is from one of my “old” 12 lead ECG classes. In those days, I cropped the computer measurements and interpretive statements because I didn’t want the students to “cheat”. Nowadays, whether it’s EMS or firefighting, I’ve come to believe in reality-based training. In real life, for good or bad, you get an interpretive statement.
But, this ECG is a good example of an important concept.
So let’s look at this ECG. It’s a sinus rhythm. It has a normal axis. We know that for several reasons.
The QRS complex is smallest in lead aVL, so the perpendicular lead on the hexaxial reference system is lead II. Lead I is almost equiphasic so the perpendicular lead is aVF. The value of lead II is 60 degrees and the value of lead aVF is 90 degrees, so the frontal plane axis is somewhere between 60 and 90 degrees.
Or, to do it the “easy” way, lead I and lead aVF are both positively deflected, so we know we’re in the left inferior quadrant.
Or, because leads I, II, and III are all positive, so we know the axis is normal.
It really doesn’t matter what method you use. I use all three for every ECG.
The QRS duration is wide. When supraventricular rhythms are wide, we look at lead V1 to see if it shows RBBB or LBBB morphology. This ECG shows a terminal R wave in lead V1, which is RBBB morphology. Next we check lead I and look for a terminal S wave. We find one!
This is a simple RBBB.
You will notice that in many leads, the T wave is deflected the same direction as the QRS complex (II, III, aVR, aVF, V2, V3, V4, V5, and V6). In other leads, the T wave is deflected opposite the main deflection of the QRS complex (aVL, V1). I did not list lead I because the QRS complex is close to equiphasic.
So, how should these T waves be deflected?
The answer is, they should be deflected opposite the terminal deflection of the QRS complex, and so they are!
Look at the following image.
As you can see, when the terminal deflection of the QRS complex is negative, the T wave is positive. When the terminal deflection is positive, the T wave is negative. In other words, even if the main deflection of the QRS complex is positive, as long as the terminal deflection (or last deflection) is negative, the T wave is positive.
That’s why I’m encouraging you to always think in terms of the terminal deflection, even though for LBBB, the terminal deflection is also the main deflection.
There is method to this madness!
Although not part of Sgarbossa’s Criteria, the “rule of appropriate T wave discordance” can help you pick up on AMI in the setting of RBBB (or bifascicular block) because an inappropriately concordant ST segment and/or T wave can tip you off that something is wrong!
For example, this case from Dr. Smith’s ECG blog.
In the last lesson, we introduced Sgarbossa’s Criteria. Let’s take a look at a graphic that shows exactly what we’re looking for.
The first example shows > 1 mm of concordant ST segment elevation (and a concordant T wave). Both are abnormal for LBBB.
The second example shows > 5 mm of discordant ST segment elevation and a discordant T wave. Discordant ST segment elevation > 5 mm is abnormal for LBBB (with one very important caveat) but a discordant T wave is normal for LBBB!
In the last example, there is concordant ST segment depression in the right precordial leads, which is abnormal for LBBB, but a discordant T wave, which is normal for LBBB.
If you have a patient with signs and symptoms consistent with ACS and the ECG shows LBBB with concordant ST segment elevation, then chances are excellent that you are dealing with a STEMI.
Likewise, if you have a patient with signs and symptoms consistent with ACS and the ECG shows LBBB with concordant ST segment depression, especially in the right precordial leads, then chances are excellent that you are dealing with a STEMI.
The original criteria didn’t take into account the depth of the S wave, and as we know from other STE-mimics like LVH, the deeper the S wave, the higher the ST segment elevation. So a blanket statement that 5 mm of discordant ST segment elevation indicates acute STEMI in the setting of LBBB is not helpful in those situations where the S wave is > 50 mm deep like the example below.
Stephen Smith, M.D. of Dr. Smith’s ECG Blog has suggested that a more sensitive and specific marker is discordant ST-elevation > 0.2 the depth of the S wave (ST/QRS ratio).
Found on the Lifenet Receiving Station (LBBB with concordant ST-depression in leads V3 and V4)
62 year old male CC: Chest pain (LBBB with ST-elevation > 0.2 the QRS complex)
58 year old female CC: Chest pain – Conclusion (meets all 3 of Sgarbossa’s criteria)