This document discusses left bundle branch block (LBBB) and how it affects electrocardiogram (ECG) patterns, making the diagnosis of myocardial infarction (MI) more complicated. Key points: - LBBB alters the normal sequence of ventricular activation, masking typical Q wave changes seen with MI. - ST-T wave changes are usually discordant in LBBB (elevated in leads with negative Q waves, depressed in leads with positive Q waves). - "Hard signs" of MI in LBBB include new Q waves, ST elevation ≥0.1mV in direction of QRS, or ST/S ratio ≥25%. - Cabrera's sign (notching of

1. MI IN LBBB

2. Normal LV electrical vector….

4. Vectors in LBBB….

5. LBBB…
• Initial septal activation with LBBB typically occurs on the right (rather than on the
left) septal surface, leading to right to left (rather than left to right) activation of
the septum, so that normal septal q waves are absent.
• Left ventricular activation then typically begins on the left septal surface, with a
delay of 40 milliseconds or longer caused by slow transseptal spread from the
right ventricular side of the septum.
• The subsequent activation of the ventricular free wall is highly variable,
depending on the type, location, and extent of the underlying cardiac disease.
• Most commonly, the region of block is located anteriorly, and the lateral and
posterolateral portions of the left ventricle are activated by wave fronts moving
around the apex and across the inferior wall in a U-shaped pattern.

6. • Irregular spread predominantly through working muscle fibers rather than the
specialized conduction system results in notching and slurring of the wide QRS
complex.
• Overall activation may then require more than 180 milliseconds, depending on the
functional status of the distal left bundle and Purkinje systems and on the speed of
propagation through working cardiac muscle; activation of portions of the left
ventricle may not occur until well beyond the end of the QRS complex.

7. ST T CHANGES in LBBB…
• The discordant ST-T wave pattern is a reflection of the altered pattern of ventricular activation.
• With LBBB, the right ventricle is activated and recovers earlier than the left, so recovery vectors are
directed toward the right and away from the left ventricle.
• Hence positive ST-T waves will be registered in leads over the right ventricle that show S waves and
negative ones over the left ventricle with prominent R waves.
• ST-T wave changes are prominent with LBBB. In most cases, the ST segment and T wave are
discordant with the QRS complex. That is,
the ST segment is depressed and the T wave is inverted in
leads with positive QRS waves (e.g., leads I, aVL, V5, and V6),
and the ST segment is elevated and the T wave is upright in leads with
predominantly negative QRS complexes (e.g., leads V1 and V2).
• These ST-T wave changes are referred to secondary ST-T abnormalities because they are generated
by abnormalities in conduction

8. ST T changes in LBBB…
• ST-segment elevation with tall, positive T waves frequently is seen in the right
precordial leads with uncomplicated LBBB.
• Secondary T wave inversions are characteristically seen in the lateral precordial
leads.
• However, the appearance of ST-segment elevations in the lateral leads or ST-
segment depressions or deep T wave inversions in leads V1 to V3 strongly
suggests underlying ischemi.
• More marked ST-segment elevations (>0.5 mV) in leads with QS or rS waves also
may be caused by acute ischemia.
• Use of the ratio of the amplitude of ST-segment elevation to S wave magnitude,
determined in any relevant lead, has been proposed, with a value of less than 0.25
reported as having greater accuracy than that of the original criterion.

10. Electromechanical dissociation.
• Septal and lateral wall moves in dys synchrony.
• More than 60 msec of dyssynchrony is commom and as much as 120
to 150 msec out of phase contractions can occur between lateral and
septal wall.

11. •MI IN LBBB

12. The diagnosis of myocardial infarction may be
obscured
• LBBB can mask or mimic MI.
• emergence of abnormal Q waves with infarction is dependent on a normal initial
sequence of ventricular activation, which is absent with LBBB.
• In addition, ECG patterns of LBBB, including low R wave amplitude in the
midprecordial leads and ST-T wave changes, can mimic anterior infarct patterns.

13. • The diagnosis of infarction in the presence of LBBB is considerably more
complicated and confusing, because LBBB alters the early and the late phases of
ventricular depolarization and produces secondary ST-T changes.
• These changes may mask and/or mimic myocardial infarction findings.
• Infarction of the left ventricular free (or lateral) wall ordinarily results in abnormal
Q waves in the midprecordial to lateral precordial leads (and selected limb leads).
• However, the initial septal depolarization forces with LBBB are directed from
right to left. These leftward forces produce an initial R wave in the midprecordial
to lateraprecordial leads, usually masking the loss of electrical potential (Q waves)
caused by the infarction.
• Thereforeacute or chronic left ventricular free wall infarction by itself will not
produce diagnostic Q waves in the presence of LBBB

14. Hard signs of MI in LBBB…
• The presence of QR complexes in leads I, V5, or V6 or in II, III, and avf
with LBBB strongly suggests underlying infarction.
• Chronic infarction also is suggested by notching of the ascending part of a wide S
wave in the midprecordial leads or the ascending of a wide R wave in lead I, aVL,
V5, or V6.

16. • In 1996, Sgarbossa et al. (13) published an analysis from the GUSTO-1 (Global
Utilization of Streptokinase and Tissue Plasminogen Activator for Occluded
Coronary Arteries-1) trial evaluating multiple different ECG criteria that had been
suggested as being potentially useful for the diagnosis of AMI in the setting of
LBBB.

18. SMITH modification of Sgarbossa criteria…
• A new criterion from Smith et al. significantly increases the sensitivity (up
to 90%)
• ST elevation or depression in the opposite direction as the QRS with a ST/S
ratio of −0.25 or less
(ie that the ST deviation is bigger than 25% of the S wave amplitude)
In summary
• All those criteria can be put together like this:
• ST deviation
• – ≥0.1 mV in the same direction as the QRS or
• – ST/S ratio ≥ 25%

19. • Cabrera's sign
• Cabrera’s sign is used to diagnose an acute myocardial infarction in
the setting of a left bundle branch block and consists of notching at
40 milliseconds in the upslope of the S wave in lead V3 and V4. This
has a poor sensitivity of 27% for myocardial infarction

22. • Mohammad kunju 65/m
• Known case of CAD, DCM, HTN
• MI 4 years back.
• ED III for 4 months.
• Elective PCI to prox LAD 3 months back.
• Presented with chest pain..
• ECG was taken.

26. Refrences..
• Braunwalds text book of Heart Diseases 10th edition.
• Leo scamroth ,An introduction to Electrocardiography 8th edi.
• Evolving Considerations in the Management of Patients With Left Bundle Branch
Block and Suspected Myocardial Infarction JACC Vol. 60, No. 2, 2012 Neeland et
al. 99 July 10, 2012:96–10
• 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.
Smith et al Ann Emer Med 2012 Dec;60(6):766-76