EMCrit Podcast 48 – PhD in EKGs Part II: Left Bundle Branch Block

Left Bundle Branch Block (LBBB) doesn’t = STEMI!

A few months ago, we had Dr. Stephen Smith on the podcast to discuss a variety of EKG issues. Dr. Smith has an EKG blog that is required reading for every ED and ICU doc. This is Part II and I think it discusses an incredibly important issue: right now major medical societies including the AHA and ACEP are asking us to fibrinolyse or PCI patients with new or presumed new LBBB. However, your interventionalists will tell you that this strategy is a ridiculous waste given how few acute occlusions will actually be found. Why this discrepancy?


Dr. Smith actually created a post specifically for this podcast; here is the full text:

A 45 year old male with no history of cardiac disease presented with new onset pulmonary edema.  He was intubated prehospital.  BP before and after intubation was 110 systolic, with HR of 120.

There is sinus tach with LBBB.  There is no concordant ST elevation.  V4 has 2 mm of discordant ST elevation (at the J-point, relative to the PR segment) following a 5 mm S-wave.  The ST/S ratio is 0.40 in this lead. Lead II has proportionally excessively discordant ST depression, with 1.25 mm STD and only 4.0 mm R-wave, for a ratio of 0.31.  This is also a sign if ischemia (reciprocal inferior ST depression).              Also, look at V3: complexes vary slightly: 2nd complex has approx 2.5-3.0 mm STE following a 14 mm S-wave; complex 4 has 2-2.5 mm STE following a 10.5 mm S-wave.   So these approach an ST/S ratio of 0.20, but it is not definite.

In a study of 19 patients with LAD occlusion, vs. 129 controls with ischemic symptoms and LBBB, at least one complex in V1-V4 with at least 2mm of STE and an ST/S ratio > 0.20 was highly specific for LAD occlusion (1).   Here is the reference for the abstract on proportionally excessively discordant ST depression (2).

Cases with excessive discordance of at least 5mm [Sgarbossa criteria 3] that did not have proportional discordance, did not have LAD occlusion.  The mean highest ST/S ratio for those without occlusion was 0.10 (95% CI: 0.09-0.11); the mean highest ST/S ratio for those with occlusion was 0.44 (95% CI: 0.19-1.05)

Because of this study, I believe the following rule is as good for diagnosis of STEMI in the setting of LBBB as standard interpretation of STEMI in the absence of BBB (and that it is more sensitive and specific than the Sgarbossa rule):

Smith modified Sgarbossa rule:

1) at least one lead with concordant STE (Sgarbossa criterion 1) or
2) at least one lead of V1-V3 with concordant ST depression (Sgarbossa criterion 2) or
3) proportionally excessively discordant ST elevation in V1-V4, as defined by an ST/S ratio of equal to or more than 0.20 and at least 2 mm of STE. (this replaces Sgarbossa criterion 3 which uses an absolute of 5mm)

It is important to remember that this is not sensitive for “MI” which is diagnosed by biomarkers. The lack of sensitivity of the Sgarbossa rule in previous studies is because the ECG is always (even without BBB) insensitive for MI.  It is, however, much more sensitive for occlusion.

Because of proportionally excessive discordance in lead V4, (and, of course, clinical instability), the patient was taken for immediate angiography, which confirmed a 100% mid-LAD occlusion.

For a case with more than 5 mm of ST elevation in V1-V4, but without excessive proportional discordance, see this post:

Tom Bouthillet has done a great job of describing my ratio rule here:

To learn more about the meaning of New LBBB, look here:

Caution: these data have not been published in a peer review journal, and the ACC/AHA still (though I believe wrongly, and this recommendation is rarely followed) recommends reperfusion for patients with ischemic symptoms and new LBBB, even without any specific findings of STEMI.


1. Dodd KW. Aramburo L. Broberg E. Smith SW. For Diagnosis of Acute Anterior Myocardial Infarction Due to Left Anterior Descending Artery Occlusion in Left Bundle Branch Block, High ST/S Ratio Is More Accurate than Convex ST Segment Morphology (Abstract 583). Academic Emergency Medicine 17(s1):S196; May 2010.

2. Dodd KW. Aramburo L. Henry TD. Smith SW. Ratio of Discordant ST Segment Elevation or Depression to QRS Complex Amplitude is an Accurate Diagnostic Criterion of Acute Myocardial Infarction in the Presence of Left Bundle Branch Block (Abstract 551). Circulation October 2008;118 (18 Supplement):S578.

Additional References

(1) Jain S, et al. Utility of left bundle branch block as a diagnostic criterion for acute myocardial infarction Am J Cardiol 2011;107(8):1111-6.
(2) Poon K, et al. Abstract 4317: Does a New or Presumed New Left Bundle Branch Block Have Equivalent Mortality to an Acute ST-Elevation Myocardial Infarction? Circulation 120: S935.
(3) Kontos MC, et al. Outcomes in patients with chronicity of left bundle-branch block with possible acute myocardial infarction Am Heart J 2011;161(4): 698-704.
(4) Chang AM, et al. Lack of association between left bundle-branch block and acute myocardial infarction in symptomatic ED patients Am J Emerg Med 2009;27(8):916-21.

If you want a .doc or .pdf of these abstracts, email: dr.smiths.ecg.blog@gmail.com


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  1. J Kim says

    New LBBB triage suggested algorithm article by Sgarbossa herself, incorporating Smith’s ST-S rule: Cai et al. American Heart Journal sep. 2013.

    Of note, the final Smith’s published ST-S rule uses 0.25, not 0.20 as suggested in this podcast, as cut-off.


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