Once upon a time, a new Left Bundle Branch Block (LBBB) was considered to be a STEMI-equivalent requiring emergency reperfusion. Later on it was found that there were far too many false positives if everyone with a new (or presumed new) LBBB was sent to the cath lab, so this rule was removed from the ACC/AHA guidelines in 2013.^ref This means that now we need to look closer at the ECGs of patients with LBBB to find other signs to diagnose those who do have an acute infarction.

Wide complex rhythms such as LBBB can be difficult to interpret because they can cause abnormal ST segments even without any acute ischemia on top.^ref These changes are often called secondary ST changes because the real primary problem is the abnormal QRS. But what if a patient has both a LBBB and acute ischemia?

Previously it was thought to be impossible to interpret the ST segments in the presence of a LBBB. This is no longer true, as we now have validated criteria to help us interpret these ST segments as normal or abnormal. The key concept involved is being able to tell the difference between concordant and discordant ST segments.

• Concordant ST changes occur in the same direction as the QRS. So, if the QRS is positive, ST elevation is a concordant change. If the QRS is negative, ST depression is a concordant change.
• Discordant ST changes occur in the opposite direction to the QRS. If the QRS is positive, ST depression is a discordant change. If the QRS is negative, ST elevation is a discordant change.

In 1996, Sgarbossa et al. published a set of criteria to help diagnose acute myocardial infarction in the presence of a Left Bundle Branch Block. Each criteria had independent predictive value for infarction, but together they were also developed as a scoring system where a total score of 3 or more equated to a 90% specificity for acute infarction.

The 3 original criteria included:

1. Concordant ST elevation > 1mm in leads with a positive QRS complex (score 5)
2. Concordant ST depression > 1 mm in V1-V3 (score 3)
3. Excessively discordant ST elevation > 5 mm in leads with a -ve QRS complex (score 2).

In 2012, the original Sgarbossa criteria were modified by Smith et al. because the third criteria (>5mm rule) did not account for smaller QRS complexes that were followed by ST changes out of proportion to their size. (There were some other limitations to the study as well.^ref)

The Smith-modified Sgarbossa criteria are:

1. ≥ 1 lead with ≥1 mm of concordant ST elevation
2. ≥ 1 lead of V1-V3 with ≥ 1 mm of concordant ST depression
3. ≥ 1 lead with ≥ 1 mm ST elevation that is out of proportion to the size of the QRS, as defined by ≥ 25% of the depth of the preceding S-wave.

Unlike the original point scoring criteria, if any of the modified criteria are present then the cath lab should be activated.

Accurate measurements are critical in using this criteria. To be precise, the depth of the S wave should be measured in reference to the PR segment. The height of the ST segment should also be measured in reference to the PR segment and at the J point.

The modified Sgarbossa criteria have undergone external validation and found to perform better than the original criteria.^ref This means that it is time they were more widely used in everyday clinical practice.

Back in 1996, Sgarbossa et al. also studied ST changes in ventricular-paced rhythms. They found that similar criteria was useful in this setting, i.e. concordant ST elevation, concordant ST depression in V1-3, or excessive discordant ST elevation >5mm was also reasonably specific for acute infarction in patients with a ventricular pacemaker. The sensitivity of these signs is low, meaning that you can't rule out infarction based on the absence of these signs.

So far there is limited data available for using this criteria in other wide complex rhythms such as Accelerated Idioventricular Rhythm (AIVR), but there are some case studies suggesting that it might be relevant for these rhythms too. Case studies have also shown that it is possible to see evidence of coronary occlusion in Ventricular Ectopic Beats (VEBs), even if the underlying rhythm does not always show ST changes.

Right Bundle Branch Block (RBBB) doesn't need the Sgarbossa criteria because it doesn't usually have any associated ST elevation. Any ST elevation is suspicious for MI in the presence of a RBBB!

There are some exclusions and limitations of the Sgarbossa criteria. In both of the original studies, patients were excluded if they had severe hypertension, acute heart failure (pulmonary oedema), extreme tachycardia, or hyperkalemia. In these conditions the ECG can be very abnormal but the first priority should be stabilising the patient.^ref

Left Ventricular Hypertrophy (LVH) also causes secondary ST changes and in that setting these criteria have not been found to be very sensitive thus far.^ref LVH is known to cause variation in ST segments over time even without ischemia, so even comparing with old ECGs can be difficult.^ref

It is also important to note that none of these criteria have 100% sensitivity or specificity. There may still be a critical lesion even when the modified criteria are negative. They are probably most useful in stable patients who present with typical chest pain suggestive of MI.^ref Serial ECGs, careful monitoring and clinical judgement remain very important in managing these patients.^ref