In my training we were taught that the electrocardiographic diagnosis of MI was impossible in the presence of LBBB. That thinking has changed. Now we have criteria for the diagnosis of MI in LBBB which, although lacking in sensitivity, have high specificity. Here are the Sgarbossa criteria. They are somewhat arbitrary, and are best applied with a more basic understanding of what happens to the ST segments.
In bundle branch block the ST segments deviate in the opposite direction from the major portion of the QRS complex. This is known as secondary ST segment change, also known as discordant ST segment deviation. It is intuitively obvious that such ST deviations can obscure ST segment changes resulting from an ischemic process. If the ischemic process alters the ST segment in the same direction as the secondary ST change attributable to the bundle branch block, an exaggerated discordant ST deviation results. This may be difficult to “eyeball” on the ECG and generally requires some form of measurement or quantitative analysis. If the ischemic process moves the ST segment in the opposite direction from the secondary ST change, a “tug-o'-war on the stylus”, as Barney Marriott affectionately called it, results. This may merely blunt the discordant ST deviation, again necessitating measurement. Sometimes the ischemic force wins the tug-o'-war and produces ST deviation in the same direction as the QRS complex, so called concordant ST segment deviations. Those are evident at a glance and require no measurement.
Quantitative analysis of the ST segment displacement in left bundle branch block (determining whether the discordant ST segment deviation is too much or too little for the LBBB thereby representing an ischemic process) is most formally and precisely done via determination of the left ventricular gradient, a process involving vector analysis which is time consuming and beyond the skill level of many clinicians. One of the criteria in the link above employs a simple measurement to determine “too much discordance” in leads with a negative QRS (> 5mm elevation). A post by Dr. Smith on this topic provides criteria based on the ST/S wave ratio in V1-3 and discusses other important aspects of the electrocardiographic diagnosis of MI in LBBB.