3.11

LBBB with signs of myocardial infarction: why LBBB masks infarction and how the Sgarbossa criteria help recognise it.

Code 3.11 — left bundle branch block with signs of myocardial infarction (Novacode class 3.1 subcategory). Patients with a suspected myocardial infarction in the setting of LBBB present a unique diagnostic challenge: LBBB itself changes the ST segment and T wave, so the usual ECG signs of infarction are hidden. This code describes LBBB together with features that suggest ischaemic injury, assessed with the Sgarbossa criteria.

Left bundle branch blockMyocardial infarctionSgarbossa criteria

Concordant ST elevation≥ 1 mm — 5 points

ST depression V1–V3≥ 1 mm — 3 points

Discordant ST elevation≥ 5 mm — 2 points

Score ≥ 3high specificity for MI

Why LBBB complicates diagnosis

In LBBB the ST segment normally moves in the direction opposite to the main QRS deflection ("appropriate discordance"): ST elevation in V1–V3 where the QRS is negative, and ST depression in V5–V6 where it is positive. An infarction can therefore be missed, or a normal LBBB pattern mistaken for infarction. Sgarbossa and colleagues identified three findings that indicate acute infarction despite LBBB.

Figure 1. Left: LBBB with appropriate discordance — ST and T opposite to the QRS. Right: concordant ST elevation (ST in the same direction as the positive QRS) — a Sgarbossa criterion of acute infarction.

Sgarbossa criteria

A score of 3 or more is highly specific for acute myocardial infarction, although sensitivity is limited. Later modifications use the ST/S ratio for the third criterion.
Finding	Points
ST elevation ≥ 1 mm concordant with (in the same direction as) the QRS in any lead	5
ST depression ≥ 1 mm in V1, V2 or V3	3
ST elevation ≥ 5 mm discordant with the QRS	2

Clinical significance

People with LBBB and an acute infarction often have extensive damage and a higher risk of complications; delays in diagnosis worsen the outcome. Therefore, in a person with LBBB and symptoms of an acute coronary syndrome, the ECG is interpreted with these criteria, compared with previous recordings, and supported by blood troponin tests and, when indicated, urgent coronary angiography.

Seek emergency care immediately

if you have known or new LBBB and develop chest pain or pressure, pain spreading to the arm, jaw or back, breathlessness, cold sweat, nausea or fainting. Call emergency services — do not drive yourself.

References

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. doi:10.1056/NEJM199602223340801
Thygesen K, Alpert JS, Jaffe AS, et al. Fourth universal definition of myocardial infarction (2018). Eur Heart J. 2019;40(3):237–269. doi:10.1093/eurheartj/ehy462
Surawicz B, Childers R, Deal BJ, Gettes LS. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram: part III: intraventricular conduction disturbances. Circulation. 2009;119(10):e235–e240. doi:10.1161/CIRCULATIONAHA.108.191095

This article is for general information and does not replace a consultation with a doctor. ECG findings must be interpreted by a physician together with symptoms and other tests.

Published on 14 May 2019

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