Correct Answer:

The catheterization laboratory should be activated. This is an equivalent of ST elevation myocardial infarction (STEMI) with modified Sgarbossa criteria.

Educational Objective:

Describe the applicability of the modified Sgarbossa criteria in the context of a paced rhythm.

Key Point:

A paced rhythm typically presents as a pacer-induced LBBB. The Sgarbossa criteria still apply in the same way as they do with a non-paced LBBB. (Paced or native) LBBB with concordant (same direction) QRS and ST depression or elevation strongly suggest acute ischemia.

Explanation:

Acute myocardial infarction in cases of patients with either paced or native LBBB can be diagnosed with the Sgarbossa criteria. An LBBB must show at least 1 of 3 possible Sgarbossa criteria to justify emergent (rather than urgent) activation of the catheterization team:

The original three criteria used to diagnose infarction in patients with LBBB:

• Concordant ST elevation > 1mm in leads with a positive QRS complex (score 5)
• Concordant ST depression > 1 mm in V1-V3 (score 3)
• Excessively discordant ST elevation > 5 mm in leads with a -ve QRS complex (score 2).

 The now more commonly used,modified Sgarbossa criteria:

• ≥ 1 lead with ≥1 mm of concordant ST elevation
• ≥ 1 lead of V1-V3 with ≥ 1 mm of concordant ST depression
• ≥ 1 lead anywhere with ≥ 1 mm STE and proportionally excessive discordant STE, as defined by ≥ 25% of the depth of the preceding S-wave.

The above patient’s ECG shows the second modified criterion: concordant ST depression in V2-5 (suggestive of a posterior MI, which was indeed confirmed during PCI)

Clinicians must exercise caution: the presence of concordance in combination with LBBB is meaningful for indicating the presence of coronary ischemia. However, the absence of concordance is not meaningful for indicating the absence of ischemia. Unfortunately, Sgarbossa criteria are absent in 80% of patients with LBBB and ischemia.

It is anticipated that the third criterion will be dropped or replaced in the next set of STEMI guidelines with a relative ratio, specifically a ratio between the ST change/S wave of the same complex of less than –0.25. In other words, if the ST change is more than one-fourth of the size of the preceding S wave measured from the baseline, then there is a STEMI-equivalent event occurring. Further study will be needed to clarify this dynamic guideline area.

Another caution: the mere fact that a LBBB in a patient without a pacemaker (e.g. NOT the above patient) is new or age indeterminate was once considered cause for emergent catheterization laboratory activation; however, this is no longer recommended.

ECG findings with LBBB and the normally associated discordance (QRS points in opposite direction of T wave) are shown in Figure 2 (no signs of ischemia).

Figure 2.

ECG findings with LBBB and myocardial infarction with Sgarbossa criteria concordant ST depression in leads V2 and V3 are shown in Figure 3.

Figure 3.

Readers desiring a more detailed ECG review and summary can follow this link.

References:

ECG Courtesy of Mike Cadogan

Rokos IC, French WJ, Mattu A, et al. Appropriate cardiac cath lab activation: optimizing electrocardiogram interpretation and clinical decision-making for acute ST-elevation myocardial infarction. Am Heart J. 2010;160(6):995-1003, 1003.e1-e8.

Smith SW, Dodd KW, Henry TD, et al. Diagnosis of ST-elevation myocardial infarction in the presence of left bundle branch block with ST-elevation to S-wave ratio in modified Sgarbossa criteria. Ann Emerg Med. 2012;60(6):766-776.