ECG Interpretation: The Evidence Behind the Pathway

It is two in the morning in a taluk hospital casualty. A 68-year-old retired schoolteacher from a village near Davanagere has had crushing central chest pain for forty minutes. He is sweating. His old ECG, from a check-up last year, already shows left bundle branch block. Tonight’s tracing shows it too, and the duty doctor says, “LBBB, so we can’t read the ST segment. Let’s wait for the troponin.” This is an illustrative case, not a real patient.

The ECG Interpretation Pathway walks you through the tracing in a fixed order and joins the findings into one read. It keeps every criterion, threshold and caution it uses, including both Sgarbossa rules and what each criterion scores. What it leaves out is how well those two rules performed when they were tested. Those figures are here. By the end you will know why “LBBB, can’t read it” is not an answer, and what the figures can and cannot tell you.

1. Why a fixed order, every time

Rate, rhythm, axis, intervals, voltage, then the ST segment and the T wave, in that order, on every tracing. A striking finding pulls the eye. On an obvious anterior injury pattern it is easy to go straight to the ST segment and miss a complete heart block or a long QT, either of which changes the next five minutes. The fixed order is what stops one finding from swallowing the rest. The QT and the electrolytes have their own article: The QT Interval and the Electrolytes.

2. Why a wide QRS hides an infarction

In left bundle branch block the ventricles are activated in an abnormal order, so the ST segment and the T wave are already displaced. Normally they point the opposite way to the main deflection of the QRS. We call that discordant. So ST elevation in a lead with a deep S wave is expected, and the usual rule of “ST elevation means infarction” cannot be applied.

“Then how do we ever diagnose infarction in LBBB?” By looking for what the bundle branch block does not explain. Sgarbossa and colleagues described three findings (Sgarbossa 1996). Two are concordant changes, which point the same way as the QRS and so cannot be put down to the block. The third is discordant ST elevation that is too large for the block.

3. The original rule and the modified rule

In the original rule each criterion carries points, and a total of 3 or more is taken as specific for infarction. The weak point is the third criterion. Discordant elevation of 5 mm is highly specific and poorly sensitive: a large discordant ST change against a small QRS can be out of proportion while never reaching 5 mm. Smith and colleagues replaced the fixed 5 mm with a ratio, the ST elevation as a share of the depth of the S wave before it (Smith 2012). Here is the table, with the study rows.

First, what do the two figures mean? Sensitivity is the share of patients with a blocked artery whom the rule calls positive. Specificity is the share of patients without one whom it calls negative. A rule with a sensitivity of 52 per cent misses about half the occlusions. A rule with a specificity of 98 per cent almost never calls a normal patient positive.

In the Smith 2012 derivation set, sensitivity rose from 52 per cent for the original weighted rule to 91 per cent, and specificity fell from 98 to 90 per cent (reference 3). The independent validation reproduced the gain without the loss (Meyers HP, Limkakeng AT, Jaffa EJ, et al. Am Heart J. 2015;170(6):1255-1264).

That is why the tool checks the modified, proportional form. A rule as insensitive as the original is not one to rely on alone when the QRS is already wide.

“So if the modified rule is negative, can I send him to the ward?” No. This is the common mistake, and it is an understandable one: a negative result feels like an answer. Even the modified rule misses one occlusion in five in the validation set. A positive finding is an indication to treat as infarction. A negative one is not licence to stand down a patient whose story fits an acute coronary event. Neither rule was derived in a paced rhythm, though both are applied there by extension.

Back to casualty

Look at the teacher’s tracing again, criterion by criterion. There is no concordant ST elevation and no concordant depression in V1 to V3. But in V3, after an S wave 20 mm deep, the ST segment is elevated by 6 mm. Under the original rule that is discordant elevation of 5 mm or more, 2 points, which is below the total of 3. Under the modified rule, 6 divided by 20 is 30 per cent, which is 25 per cent or more of the preceding S wave: positive. The duty doctor does not wait for the troponin. He treats this as an infarction and calls for reperfusion. For what follows, read Acute Coronary Syndrome: The Evidence Behind the Pathway.

Try this

A 59-year-old woman with known left bundle branch block has chest pain. In V2 the S wave is 15 mm deep and the ST segment is elevated by 3 mm. There is no concordant change anywhere. Is either rule positive? Illustrative case.

Answer. The original rule scores nothing for discordant elevation under 5 mm. The modified rule asks for 25 per cent of the S wave: 3 divided by 15 is 20 per cent, so it is negative too. Both rules are negative, and that does not exclude infarction. Repeat the tracing, compare it with an old one, send the troponin, and if her story fits, treat her as having an acute coronary syndrome.

Take-home points

• Read every ECG in the same order; the striking finding is not the only finding.
• In left bundle branch block, look for what the block does not explain: concordant change, or discordant elevation out of proportion.
• The modified Sgarbossa rule uses 25 per cent of the preceding S wave, not a fixed 5 mm, and finds far more occlusions.
• A positive rule means treat as infarction.
• A negative rule does not stand down a patient whose story fits.

Declaration: the author built and maintains the tool. It is free to use. The figures in this article are those the tool carried in its Evidence tab on 4 November 2026, each from the source the tool names. The illustrative cases follow the tool’s rules; check the tool’s result for a real patient.

References

1. Thygesen K, Alpert JS, Jaffe AS, et al. Fourth Universal Definition of Myocardial Infarction (2018). Circulation. 2018;138(20):e618-e651.
2. Sgarbossa EB, Pinski SL, Barbagelata A, et al. Electrocardiographic diagnosis of evolving acute myocardial infarction in the presence of left bundle-branch block. N Engl J Med. 1996;334(8):481-487.
3. Smith SW, Dodd KW, Henry TD, Dvorak DM, Pearce LA. 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. Ann Emerg Med. 2012;60(6):766-776.
4. de Winter RJ, Verouden NJ, Wellens HJ, Wilde AA. A new ECG sign of proximal LAD occlusion. N Engl J Med. 2008;359(19):2071-2073.
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19. Rautaharju PM, Surawicz B, Gettes LS, et al. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram, part IV. Circulation. 2009;119(10):e241-e250.
20. Xavier D, Pais P, Devereaux PJ, et al. Treatment and outcomes of acute coronary syndromes in India (CREATE): a prospective analysis of registry data. Lancet. 2008;371(9622):1435-1442.
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The validation study is cited in the tool beside the figure it supports: Meyers HP, Limkakeng AT, Jaffa EJ, et al. Am Heart J. 2015;170(6):1255-1264.