The correct answer is:

Possible acute myocardial infarction

Educational Objective:

Differentiation of LBBB from AMI

Key Point:

The “excessive discordance” Sgarbossa criterion can be difficult to discern when the J point and ST segment cannot be differentiated from the T wave, as in the above ECG. In these cases, it can be stated with certainty that “excessive discordance” in the anterior leads – and therefore an acute myocardial infarction - cannot be ruled out.

Explanation:

The first step is to diagnose LBBB.

The diagnostic criteria for complete LBBB are as follows: (1) QRS duration of 0.12 second or greater; (2) presence of a broad monophasic R wave in leads I, V5, and V6 that is usually notched or slurred; (3) absence of Q waves in leads I, V5, and V6; (4) delay of onset of the intrinsicoid deflection (the R peak time) in the leads V5 and V6; (5) concordant displacement of the ST segment and T wave in the direction opposite (discordant to) the major deflection of the QRS complex.

The above tracing meets all criteria for LBBB: there is left axis deviation and concordant secondary ST-T wave abnormalities (e.g. the ST and T wave are both displaced in the same direction). The QRS complex is prolonged to 0.18 seconds with R deflection in lead 1 and QS deflection in lead V1. Normal sinus rhythm is present, and septal Q waves are absent in leads 1, V5, and V6. This characterizes LBBB. In further support of LBBB, the anterior forces are absent or small in leads V1 to V4. The concordant ST-T waves are generally discordant (opposite) to the terminal QRS deflection. Importantly, the elevated ST segments blending into upright T waves in leads 3, aVF, and V1 to V4 are consistent with LBBB alone (the blending of ST-T components does not imply an associated MI). However, this feature hampers the process of checking the ECG for the “excessive discordance” Sgarbossa criterion.

The second step is to screen all ECGs with LBBB for the modified Sgarbossa criteria:

1. At least 1 lead with at least1 mm of ST elevation that is concordant with the QRS wave.
   
   
2. At least 1 lead of V1-V3 with at least 1 mm of ST depression that is concordant with the QRS wave.
   
   
3. Excessively discordant ST elevation (or depression) in leads with a negative QRS in proportion to the preceding S-wave (or R-wave) as determined by (1) at least 1 mm of ST elevation (or depression) AND (2) ratio of J-point deviation from baseline/S wave depth is at least 1:4 (25%).
   
   

The above ECG does NOT meet the first and second Sgarbossa criteria; however, it cannot be said with certainty, whether or not it meets the third Sgarbossa criterion of excessive discordance in leads V1 through V3 because the J point cannot be discerned with certainty.

For this reason, it cannot be stated with certainty that an acute myocardial infarction is absent (or present).

Pearl:

A 2020 study found that for sloping ST segments, either the J-point or the point of the ST segment 20 msec from the J-point are the two most accurate locations for measuring ST deviation from the baseline. The results varied slightly between males and females (see Man and Lindow publications). For the Sgarbossa criteria, ST elevation is measured at the J-point (see figure 2 from Smith et al).

Figure 2:

References:

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 [Journal] // Am Heart J. - 2010. - pp. 160:995-1003.

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 Dec 60(6): 766-776.

Chou T. Chou’s Electrocardiography in Clinical Practice, Adult and Pediatric. 4th ed. W.B. Saunders Company. 1996.

Man S, tH C, et al. Position of ST-deviation measurements relative to the J-point: Impact for ischemia detection. Journal of Electrocardiology, Volume 50, Issue 1, January–February 2017, Pages 82-89   https://doi.org/10.1016/j.jelectrocard.2016.10.012

Lindow T, Wiiala J, et al. Optimal measuring point for ST deviation in chest pain patients with possible acute coronary syndrome. Journal of Electrocardiology Volume 58, January–February 2020, Pages 165-170