here's a patient with a left bundle branch block if I told you this patient is having chest pain you might have a difficult time picking up ST segment changes that is compared to someone with a normal EKG but thankfully the scar Bosa criteria can be helpful in specifically teasing out EKGs that may look more worrisome than they should be on EKGs with left bundle branch blocks it is normal to have discordance in a left bundle branch block the sequence of repolarization is altered and in this case the ST segment and T wave vectors are in the opposite direction to the QRS complex this is what we call discordance and in the setting of a left bundle branch block this is normal so we call this appropriate discordance this is important to realize because these changes can mask the ST segment depression and T wave inversions that are caused by myocardial ischemia this is where the scar bolus of criteria comes in it's a tool that you could use to help you read through EKGs with left bundle branch blocks in patients your concern from myocardial ischemia it's composed of three criteria on a scale from zero to five the first criteria is concordant St elevation greater than one millimeter and leads with a positive QRS complex if you see this on an EKG you assign a score of five looking at this in more detail here you see your EKG lead with a positive QRS complex in this case you have your ST segment going in the same direction this is what we call concordance when both the QRS complex and the ST segment elevation are going in the same direction if there are concordant st elevations greater than one millimeter you assign a score of five the next criteria is concordant st depressions greater than one millimeter in leads v1 through VN looking at this in more detail here you see a negative QRS complex and concordant ST depression because both the QRS and ST segments are going in the same direction when you see this in v1 to v3 you assign a score of three the final criteria is the presence of excessively discordant St elevations greater than five millimeters in leads with a negative QRS complex in this case the QRS complex and ST segment are going in the opposite directions this is called discordant remember in the setting of a left bundle branch block this is normal however if the discordance is greater than five millimeters you would consider this to be too much and therefore suggestive of ischemia so when you see excessive discordance you assign a score of two now that we've gone through the scoring system what you need to know is that a scar bolsa criteria or score of greater than or equal to three is highly specific up to 98% but much less sensitive about 20% in other words because concordance assigns you more points when you see an EKG with concordance this is bad and more suggestive of ischemia compared to excessive discordance alone and although the scarborough criteria is highly specific and much less sensitive you can increase sensitivity by comparing changes with previous EKGs or observing dynamic changes with serial EKGs here's an example of a patient who's having chest pain on their EKG you could see concordant ST elevation in leads to v5 and v6 and concordant ST depressions in lead v1 v2 and v3 in this case this patient has a scar Bosa criteria of greater than 3 and therefore myocardial ischemia even in the setting of a left bundle branch block should be considered you