“Once you see LBBB on an ECG, you cannot comment further on the ST segments.”
Actually, if you see LBBB on an ECG, you should look very carefully at the ST segments.
The whole ‘LBBB means I can’t look for evidence of MI’ stems back to the days before angioplasty and thrombolysis, when ECGs were mainly used for identifying old infarcts. This meant hunting for Q waves. It is true that Q waves are obscured by LBBB. However, the ST changes that occur when there is total occlusion of a coronary vessel are pretty much as visible in LBBB as they are in hearts with a normal conduction system. They just take a little getting used to, like a new car.
The rules I like to apply are the Smith modified Sgarbossa rules:
1) at least one lead with concordant STE (Sgarbossa criterion 1) or
2) at least one lead of V1-V3 with concordant ST depression (Sgarbossa criterion 2) or
3) proportionally excessively discordant ST elevation in V1-V4, as defined by an ST/S ratio of equal to or more than 0.20 and at least 2 mm of STE. (this replaces the non Smith modified Sgarbossa criterion 3 which uses an absolute of 5mm)
These are explained simply at http://ems12lead.com/tag/inappropriate-concordance/ The gist of it is that in ‘normal’ LBBB, you have the QRS complex and ST segments going in opposite directions. If there is MI on top, you may have ST segments and QRS complexes in the same direction.
It’s worth pointing out that these rules are specific, but not very sensitive, so their absence does not mean much but their presence means an occlusion is very likely, and so a trip to the catheter lab is justified.
Only 2-4% of ACS patients with LBBB that are not confirmed as old actually have a coronary occlusion that is amenable to PCI. Some interventionists do not accept cardiac history + new LBBB as an indication for urgent PCI, and there is evidence to support this, even if the AHA/ACC has very cautious guidelines regarding LBBB. Whilst as junior doctors we should of course treat all LBBB that are not known to be old as potential MIs, we should also hunt for evidence of occlusion on the ECG. Passing this information on to the catheter lab when referring patients would help convince them to do a PCI urgently.