There has been a lot of discussion lately about identifying AMI in the presence of LBBB (see Dr. Bearemy’s “My Emergency Medicine Blog” here and a recent thread on the EKG Club). I’ve also been receiving a lot of emails offlist, so I think a full discussion is in order.
In my recent post Who benefits the most from reperfusion therapy? I posted a graph that demonstrates how patients with new bundle branch block benefit the most from reperfusion therapy.
*** Important Update ***
Recent evidence suggests that new (meaning previously undetected) LBBB patients do not “rule-in” for AMI at any greater rate than any other group of patients! That’s why it’s so important for health care practitioners to understand Sgarbossa’s criteria! Those are the patients who need immediate reperfusion therapy in the cardiac cath lab!
*** End Update ***
The problem is that in many prehospital 12 lead programs (and regional STEMI systems), patients with LBBB or a QRS duration > 0.12 sec (120 ms) are excluded! In other words, patients with wide QRS are taken to the local community hospital without interventional capability. Or, the cath lab is not activated while EMS is still in the field.
Why would you exclude the very patients who stand to benefit the most from prompt, expertly performed PCI at a cardiac center?
It’s too difficult to figure out whether or not the BBB is new! The ECG diagnosis of STEMI can be difficult in the setting of BBB.
In False Positive Cardiac Cath Lab Activations I reviewed Larson, Menssen, Sharkey et all, False-Positive” Cardiac Catheterization Laboratory Activation Among Patients With Suspected ST-Segment Elevation Myocardial Infarction, JAMA 2007;298(23):2754-2760.
Patients with new or presumably new left bundle-branch block had an inordinately high prevalence of false positive catheterization laboratory activation (almost half did not have a culprit artery). Patients with a previous myocardial infarction or previous coronary bypass surgery had a significantly higher prevalence of no culprit artery, likely because of abnormal baseline ECG results.
This is obviously a big problem, and subjecting all patients with LBBB and signs and symptoms of ACS to an emergent cath or the risks associated with thrombolytic therapy is not the answer, as some authors have suggested.
If only there was some kind of algorithm that could help distinguish between patients with LBBB and acute STEMI from patients with LBBB who are not experiencing acute STEMI.
But there is such an algorithm! It’s been around for over 10 years!
The GUSTO investigators Sgarbossa et al., Electrocardiographic Diagnosis of Evolving Acute Myocardial Infarction in the Presence of Left Bundle-Branch Block. N Eng J Med 1996; 334(8):481-487 published an algorithm which has come to be known as “Sgarbossa’s Criteria”.
The criteria seems complicated but it’s really not. Like anything else, it’s a tool. A very important tool for a critical subset of patients.
The original paper contains a flow chart from which the patient receives a score. I’m not going to publish the flow chart, because it’s not something you need to memorize.
Here is the criteria. A patient is presumed to be experiencing an evolving AMI if any of the following are present.
- ST segment elevation = or > 1 mm that is concordant with the QRS complex.
- ST segment depression = or > 1 mm in leads V1, V2, or V3.
- ST segment elevation = or > 5 mm that is discordant with the QRS complex.
It is the last criterion that has caused the most controversy and requires qualification.
However, before we address the third criterion, we have to dispose of a common misunderstanding.
What do we mean by concordant and discordant? The short answer is, concordant means “the same direction” and discordant means “the opposite direction”.
The rule of appropriate T wave discordance
In the presence of abnormal ventricular depolarization (left bundle branch block, right bundle branch block, paced rhythm, ventricular rhythms) the T wave should be deflected opposite the terminal deflection of the QRS complex (appropriate T wave discordance).
What is the terminal deflection?
The terminal deflection is the last deflection, or wave, of a QRS complex.
Please take the time to learn this! It is extremely important!
Take a look at the following image.
You will notice that each of these QRS complexes is labeled according to the waves are present. If the wave is large, it gets a capital letter. If the wave is comparatively small, it gets a lowercase letter.
I could talk about this image for a long time, but for now, I just want you to notice that an Rs complex is positively deflected while an rS complex is negatively deflected, even though both of them contain only an R and an S wave. But the terminal deflection of each is negative, because they both end in an S wave!
Why is this important?
When teaching Sgarbossa’s Criteria, students always get confused as to whether or not the ST segments and T waves should be deflected opposite the main deflection of the QRS complex or opposite the terminal deflection.
Well, guess what?
With LBBB, the terminal deflection is the main deflection!
So why are we splitting hairs?
Because if you learn to think in terms of the terminal deflection, you can use the rule of appropriate T wave discordance for RBBB, too!
Let’s start by looking at a patient with a normal LBBB.
I have no idea why the GE-Marquette 12SL interpretive algorithm is giving the “data quality prohibits interpretation” message for this ECG. There’s a little bit of artifact in the inferior leads, but it’s not that bad!
This is a normal looking LBBB. We know the frontal plane axis is around 0 degrees, because the QRS complex is isoelectric in lead aVF. Therefore, the perpendicular lead in the hexaxial reference system is lead I. Since lead I is positively deflected, we can place the frontal plane axis at 0 degrees. A physiological left axis deviation (0 to -30) is normal for left bundle branch block.
To put it another way, a negative QRS complex in lead III is normal for LBBB, but it should be upright and monomorphic in lead I.
Now, let’s look at the QRS complexes and the T waves.
You will notice that in every lead, the T wave is deflected opposite the QRS complex! This is “appropriate T wave discordance” in the presence of left bundle branch block.
To help illustrate this point, consider the following graphic.
The blue arrow shows the direction of the terminal deflection of the QRS complex (which is also the main deflection in the setting of LBBB). The red arrows shows the direction of the ST segment and the T wave.
This is what we mean by “appropriate T wave (and ST segment) discordance” with LBBB. Note that with RBBB, the T wave should be discordant, but the ST segment should remain isoelectric. This is why RBBB is usually not listed as a STE-mimic.
With LBBB, there is also a discordant shift of the ST segment, which is why it’s one of the most common STE-mimics! ST segment elevation in the right precordial leads (V1-V3) is a normal finding for LBBB!
In Part II, we’ll look at the “rule of appropriate T wave discordance” as it applies to RBBB and talk more about Sgarbossa’s Criteria.
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)