Left bundle branch block (LBBB) has a curious but important relationship with STEMI . LBBB inflicts a dramatic change in qrs morphology with a diagonally opposite polarity of ventricular activation . This masks the initial qrs vector and makes it a difficult task to diagnose acute MI in this setting. The ST segment which is of primary importance in STEMI is lifted up due to altered repolarization .
LBBB can be associated with STEMI in the following ways
- Acute necrotic LBBB with massive myocardial damage – Impending shock
- Chronic LBBB with acute STEMI
- Transient ischemic LBBB during STEMI
- Rate dependent LBBB (Usually tachycardic , rarely bradycardic )
- STEMI in pacemaker rhythms
While every one of the above can be experienced , the most common diagnostic conundrum occurs , when a patient comes with acute chest pain and LBBB . There has been many criterias suggested to diagnose STEMI in the presence of LBBB.
The criteria proposed by Sgarbosa (A GUSTO off shoot ) in 1996 caught our imagination .One prime reason for this is , it came from the prestigious NEJM and Duke university combine. Suddenly this became the de- facto standard to diagnose STEMI
In the past 15 years , our experience in one of largest coronary care units in India , we have found this criteria to have little utility value in STEMI and LBBB . Most of the time a correct diagnosis was made by simple clinical guessing .
Next to clinical assessment, we found cardiac enzymes (Troponin and CPK ) were reliable in diagnosing STEMI with LBBB.
Surprisingly ,echocardiography was as unreliable as ECG .( The paradoxical septal motion invariably confuses the already confused cardiology fellow who usually does the emergency echo !)
Even as our CCU documentation was far from satisfactory , now this article from Mayo exactly reflect our observation.
Sorry Sgarbosa . . . the criteria was based on sound observation and a good electrical principle . . . still LBBB is able to beat it convincingly ! ( Very low sensitivity !)
Dr.S.Venkatesan MD 
The problem is that STEMI and AMI are not the same thing. STEMI is a subset of AMI and Sgarbossa’s criteria is good at identifying patients with an acute thrombotic lesion in an epicardial coronary artery. It’s true that discordant ST-elevation > 5 mm has a low specificity but the modified criterion proposed by Smith et. al from Hennepin County Medical Center takes into account the depth of the S-wave (ST/QRS ratio > 0.2) which improved both the sensitivity and specificity of discordant ST-elevation. This problem has been solved.