ED desks accumulate artifacts from each shift of rotating physicians: coffee cups, broken calipers, and piles of papers. I like to go through the radiology reports and nursing home MARs to pull out the ECGs. Most have only have scribbles or initials, but every ECG with a left bundle branch block (LBBB) always has circled ST-segments accompanied by a pleading question mark.
The diagnosis of a LBBB begins with the identification of a wide QRS (>120ms). A deep and slurred QS complex occurs in V1, transitioning to a slurred R wave in V5 and V6. These large QRS complexes cause a bounce in the ST segment going the opposite direction (discordance). The negative QS complex causes ST-segment elevation in the anterior leads V1-V3, and ST-segment depression and T wave inversion reflects the positively deflected R wave in the lateral leads V5-V6.
LBBB develops gradually as the conduction system degrades or as left ventricular hypertrophy, scarring, or remodelling occur from ischemic heart disease, valvular disease, or other cardiomyopathy. Patients with LBBB are more likely to be older, female, have cardiovascular disease, hypertension, and congestive heart failure.
LBBB can also occur abruptly during acute coronary syndrome (ACS) if an area of ischemia involves the bundle of His or a massive lesion affects both fasicles. The LBBB, unless it is new in an AMI setting, is a marker of underlying disease rather than an indication of ischemia.
The secondary ST segment and T wave abnormalities in the anterolateral and inferior leads were considered uninterpretable for evaluating for ST elevation. A new or presumed new LBBB, therefore, was an indication for emergent coronary angiography and reperfusion.
The cardiac interventionalists began to object over time. These LBBB interlopers were rarely worth the effort because occlusion was only being diagnosed in two to four percent, and LBBB lost its STEMI-equivalent status in 2013. This reduced the false activations of the cardiac catheterization lab, but also denied reperfusion therapy to those who would benefit.
We needed to be smarter about ECG interpretation. Insight came during percutaneous transluminal coronary angioplasty (PTCA) in patients with LBBB. An occlusion occurs analgous to a myocardial infarction under these conditions. It was noticed that the underlying LBBB ST-segment deviations became exaggerated. The abnormal ST-segment deviations could occur concordant or discordant with the QRS polarity. The discordant ST-segment deviation represents an exaggeration of normal ST-segment deviation (i.e., higher than expected secondary ST elevation in the right precordial lead with a dominant S wave or higher than expected secondary ST depression in the leads with dominant R waves). The concordant ST-segment deviation is the opposite of the expected secondary ST-segment deviation (i.e., ST depression in the right precordial leads with dominant S wave or ST elevation in the leads with a dominant R wave). Other evidence of myocardial infarction in the presence of LBBB are typical coved ST elevation, prominent R waves in V1, and Q waves in V6.
This was codified in the Sgarbossa criteria to predict acute myocardial infarction in the setting of LBBB. (N Engl J Med 1996;334:481.) These criteria were found to be highly specific (98%) but not very sensitive (20%). The sensitivity of the original Sgarbossa criteria was not much better than just considering if the LBBB was new or old. To increase the sensitivity, substitute criteria have been developed by changing the absolute 5mm (500 microvolt) threshold of the concordant ST elevation to a relative measurement.
Selvester proposed setting the threshold at 10 percent of the total RS height plus the normal STEMI limits required for the given lead and patient's age. (Circulation 1982;65:342.) Smith derived a rule using 25 percent of the S-wave amplitude as the threshold. These modifications are more sensitive and have been validated. The Smith rule, using angiography proven occlusion instead of just biomarkers that may overestimate MI, has a sensitivity of 91 percent and specificity of 90 percent. (Ann Emerg Med 2012;60:766.)
Even in LBBB, we can use the ECG to guide reperfusion therapy for patients with suspected myocardial infarction. As always, hemodynamic instability or acute heart failure is a strong indicator for cardiac catheterization in AMI. For others, the original Sgarbossa criteria can be applied and if positive would be highly specific for coronary occlusion. Using the expanded criteria (with the Smith 25% threshold being the most validated), you can increase the sensitivity of detecting a STEMI. As you learn and apply these rules, realize that you will be the expert and may be ahead of your cardiologist and interventionalist. You may have to educate them.
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