Making Sense of Sgarbossa’s Criteria – Chest Pain and Left Bundle Branch Block – Part 1
Left Bundle Branch Block
Left bundle branch block presents a dilemma for many clinicians in the evaluation of chest pain (or other signs and symptoms of ACS) and it’s easy to see why.
- There is a pervasive myth that it’s impossible to diagnose acute STEMI in the presence of left bundle branch block
- Left bundle branch block causes a secondary ST/T-wave abnormality that makes the diagnosis of acute STEMI more difficult
- “New” left bundle branch block was long considered to be a STEMI equivalent (many still believe this to be the case)
- Historically there has been a high rate of “false positive” cardiac cath lab activations based on the presence of left bundle branch block
- The diagnostic criteria to identify acute myocardial infarction in the presence of left bundle branch block (Sgarbossa’s criteria) was originally based on a rise and fall of cardiac biomarkers, not angiography (in other words it combined STEMI and NSTEMI)
For these reasons, when it comes to field activation of the cardiac cath lab, most EMS protocols exclude patients with left bundle branch block or paced rhythm.
“You can’t diagnose STEMI in the presence of left bundle branch block”
Let’s start with the myth that has been around for many years and just won’t go away. The idea that you can’t diagnose acute STEMI in the presence of left bundle branch block.
The myth is well rooted in the medical literature. Here are two randomly selected statements found in a Google Scholar search.
“It is well recognized that the electrocardiographic diagnosis of myocardial infarction in the presence of complete left bundle branch block is in most instances difficult and frequently impossible.” Int J Cardiol. 1983;2(5-6):521-9
“It is common knowledge that the ECG diagnosis of completed myocardial infarction in the presence of left bundle branch block (LBBB) is extremely difficult and often impossible.” – Ann Emerg Med. 1995 Jul;26(1)69-82
Secondary ST/T-wave Abnormality in Left Bundle Branch Block
So what is it about left bundle branch block that makes identifying an acute heart attack so difficult?
Left bundle branch block causes an abnormal depolarization and when you have an abnormal depolarization you also have an abnormal repolarization which causes a so-called secondary ST/T-wave abnormality.
A primary ST/T-wave abnormality would be caused by ischemia or acute injury.
Bundle branch blocks, paced rhythms, left ventricular hypertrophy, ventricular rhythms, and pre-excitation from Wolff-Parkinson-White syndrome all cause abnormal depolarizations, so to some degree they all display secondary ST/T-wave abnormalities. That’s why all of them can be STEMI mimics.
Let’s look at the secondary ST/T-wave abnormality associated with left bundle branch block.
In the presence of left bundle branch block, for the most part, both the ST-segment and T-wave are deflected opposite the majority of the QRS complex. This is referred to as the rule of appropriate ST-segment and T-wave discordance.
Example A. shows left bundle branch block with a positively deflected QRS complex. You will note that the ST-segment is depressed (opposite the majority of the QRS complex) and the T-wave is inverted (opposite the majority of the QRS complex).
This is a normal finding for left bundle branch block. It is a secondary ST/T-wave abnormality. It does not indicate ischemia.
Example B. shows a left bundle branch block with a negatively deflected QRS complex. You will note that the ST-segment is elevated (opposite the majority of the QRS complex) and the T-wave is positive (opposite the majority of the QRS complex).
Again, this is a normal finding for left bundle branch block. It is a secondary ST/T-wave abnormality and does not indicate acute injury.
You will recall that I said that this rule applies “for the most part”. Let me show you why.
This 12-lead ECG shows left bundle branch block. To review, the rules for left bundle branch block are:
- Superventricular rhythm
- QRS duration ≥ 120 ms (but < 200 ms)
- rS or QS complex in lead V1
- Monomorphic R-wave in lead I (some will say both lead I and lead V6)
In this case there is a left axis deviation and a persistent S-wave in lead V6. A purist might call this a “non-specific intraventricular conduction defect” as opposed to left bundle branch block. I allow a persistent S-wave in lead V6 as a normal variant. This can also be influenced by precordial lead placement.
Let’s put that nitpicking aside for a moment.
You might point to lead V5 and say, “The ST-segment is shifted in the same direction as the majority of the QRS complex!” That makes it concordant (in the same direction as the majority of the QRS complex) as opposed to discordant (opposite the majority of the QRS complex).
On most occasions we would say that’s abnormal. Is it abnormal in this case? Probably not. But why?
In left bundle branch block there should be a transition in the precordial leads. In other words, the QRS complex should start out negatively deflected in lead V1 and end up positively deflected in lead V6.
In this example the transition is delayed until lead V6. In other words, the QRS complex is still negative in lead V5 and is mostly equiphasic in lead V6 (slightly positive). One imagines that if lead V7 were recorded it would show a positive QRS complex. Maybe even a monomorphic R-wave. But that is beside the point.
You will note that the ST/T complex in lead V6 is biphasic. Why should this be so?
It’s because the QRS complex is biphasic. The first part of the ST/T complex is deflected opposite the first part of the QRS complex and the second part of the ST/T complex is deflected opposite the second part of the QRS complex.
Even though the majority of the QRS complex is negative in lead V5, lead V5 is the lead above the transition, and I have noticed over and over again that you cannot trust the ST/T complex in the lead above the transition.
I have never read this in any article or textbook but it’s true. I believe this is one of many reasons that people find left bundle branch block (and bifascicular block) to be so confusing when it comes to analyzing ST-segments and T-waves.
Can you see why the presence of a baseline abnormality like left bundle branch block makes the diagnosis of acute STEMI more difficult? The normal rules that we use to diagnose acute STEMI do not apply.
(As a side-note, the “normal rules” we use to identify acute STEMI have their own set of issues but that is outside the scope of this tutorial.)
Making Sense of Sgarbossa’s Criteria – Chest Pain and Left Bundle Branch Block – Part 1
Cai Q, Mehta N, Sgarbossa E et al. The left bundle-branch block puzzle in the 2013 ST-elevation myocardial infarction guideline: From falsely declaring emergency to denying reperfusion in a high-risk population. Are the Sgarbossa Criteria ready for prime time?. American Heart Journal. 2013;166(3):409-413. doi:10.1016/j.ahj.2013.03.032.
Chang A, Shofer F, Tabas J, Magid D, McCusker C, Hollander J. Lack of association between left bundle-branch block and acute myocardial infarction in symptomatic ED patients. The American Journal of Emergency Medicine. 2009;27(8):916-921. doi:10.1016/j.ajem.2008.07.007.
Garcia T, Holtz N. 12 Lead ECG: The Art of Interpretation. Boston, Ma: Jones and Bartlett; 2001.
Larson D, Menssen K, Sharkey S et al. “False-Positive” Cardiac Catheterization Laboratory Activation Among Patients With Suspected ST-Segment Elevation Myocardial Infarction. JAMA. 2007;298(23):2754. doi:10.1001/jama.298.23.2754.
Meyers H, Limkakeng A, Jaffa E et al. Validation of the modified Sgarbossa criteria for acute coronary occlusion in the setting of left bundle branch block: A retrospective case-control study. American Heart Journal. 2015;170(6):1255-1264. doi:10.1016/j.ahj.2015.09.005.
O’Gara P, Kushner F, Ascheim D et al. 2013 ACCF/AHA Guideline for the Management of ST-Elevation Myocardial Infarction: A Report of the American College of Cardiology Foundation/American Heart Association Task Force on Practice Guidelines. Circulation. 2012;127(4):e362-e425. doi:10.1161/cir.0b013e3182742cf6.
Smith S, Dodd K, Henry T, Dvorak D, Pearce L. Diagnosis of ST-Elevation Myocardial Infarction in the Presence of Left Bundle Branch Block With the ST-Elevation to S-Wave Ratio in a Modified Sgarbossa Rule. Annals of Emergency Medicine. 2012;60(6):766-776. doi:10.1016/j.annemergmed.2012.07.119.
Surawicz B, Knilans T, Chou T. Chou’s Electrocardiography In Clinical Practice. Philadelphia: Saunders; 2001.
Sgarbossa E, Pinski S, Barbagelata A et al. Electrocardiographic Diagnosis of Evolving Acute Myocardial Infarction in the Presence of Left Bundle-Branch Block. New England Journal of Medicine. 1996;334(8):481-487. doi:10.1056/nejm199602223340801.
Stephen Smith, M.D. (@SmithECGBlog)