Many times we confuse to confirm the presence of LBBB. Here are certain tips.
Back ground :1. Usually a big wave is created when depolarization spreads in left ventricle and small wave is created when it spreads through the right ventricle. Positive wave is created when depolarization spreads towards the lead and negative wave is created when depolarization spread away from the lead. First part of the QRS complex indicates depolarization of LV and second part of QRS complex denotes depolarization of RV.
2. In BBB the QRS complex has two parts: the first part and second part. Each part formed by each ventricle.
What happens in LBBB is left ventricle depolarizes after depolarization of right ventricle. Which is unusual.(remember the sequence RV -->LV),
Hence, first part of QRS complex denotes RV and second part denotes LV.
But lateral leads v5, v6, I, avl cannot catch depolarization of RV. So they do not have q waves. V1 to v4 may have initial small, positive 'r' wave as the depolarization speads via RV and towards the leads( RV is located anterior to the LV).
inferior leads II, III, Avf has different story, as the inferior part of heart is formed by LV. Initial small positive 'r' wave (actually negative with reference to an inferiorly located lead) is from RV.
The second part of QRS complex is formed by LV. As the conduction is delayed in the LV, the duration of deploarization wave is longer than that of RV, so that overall QRS duration is prolonged. The lateral leads v5, v6, I, Avl have only broad positive QRS wave as they do not recognize the depolarization of RV. So these are the only leads that recognize the depolarization of only LV. These are ONLY LV leads. Thats why, the QRS complex is monophasic in these leads. As these are the ONLY LV leads, the bigenning of R wave to the peak of R wave denotes the spread of depolarization from endocardium to epicardium in these leads, which is called 'intrinsicoid deflection' which is prolonged due to delay of conduction. (Intrinsicoid deflection should be calculated in these ONLY LV leads).
In v1 to v4 leads the depolarization wave spreads posteriorly(away from the leads) as the posterior part is formed by LV. So they have a deep broad negative wave. The inferior leads also have a deep, broad negative wave Which is actually a positive wave with reference to the inferior leads. So that anterior and inferior leads will have a deep, broad S wave.
A positive R wave in I, and a negative S wave in AVF causes left axis deviation. (Heart is not rotated, only the axis of depolarization is towards the left).
To summerize. Broad QRS complex >120msec and left axis deviation PLUS
1. Absent q waves in lateral leads.
2. Monophasic QRS complex in lateral leads.
3. Prolonged intrinsicoid deflection in lateral leads.
4. rS waves in anterior leads.
5. rS waves in inferior leads.
But story not ends here with depolarization. There is also a part of repolarization. Usually repolarization wave of right ventricle is not catched by the leads. Whatever repolarization changes we see on ECG are from LV. There are changes in the ST segment and T waves. But these changes are not due to defective repolarization, but are secondary to abnormal depolarization. Hence these are secondary replarization abnormalities. These ST T changes are discordant (opposite in direction) with the QRS deflection.
Greater the depolarization changes greater the ST T repolarization changes. Deeper the S wave in the V1, higher the ST T changes in V1. Taller the R wave in V6, greater the ST T depression in V6. This creates confusion in diagnosing AWMI in presence of LBBB.
In general positive R wave with positive J point elevation is specific for MI.
In LBBB, 1.positive R waves are seen in lateral leads. If these leads show a concordant J point elevation of atleast 1mm, then it is more specific for MI carrying 5 points.
Except lateral leads no other leads have a positive QRS complex in LBBB. There fore, 2. At least 1mm of J point depression in v1 to v4 concordance with QRS changes carries 3 points
Anterior lead's discordant ST T changes depend upon the depth of S wave. There fore 3. >5mm J point elevation is needed to diagnose in these leads, and it carries only 2 points. Hence this 3rd point is though more sensitive, it is less specific.
That mean, when only this is the only finding on ECG, the chances of MI are less likely.
These three constitute SGARBOSA criteria to diagnose MI in presence of LBBB. Presence of a score of >3 is diagnostic.
Back ground :1. Usually a big wave is created when depolarization spreads in left ventricle and small wave is created when it spreads through the right ventricle. Positive wave is created when depolarization spreads towards the lead and negative wave is created when depolarization spread away from the lead. First part of the QRS complex indicates depolarization of LV and second part of QRS complex denotes depolarization of RV.
2. In BBB the QRS complex has two parts: the first part and second part. Each part formed by each ventricle.
What happens in LBBB is left ventricle depolarizes after depolarization of right ventricle. Which is unusual.(remember the sequence RV -->LV),
Hence, first part of QRS complex denotes RV and second part denotes LV.
But lateral leads v5, v6, I, avl cannot catch depolarization of RV. So they do not have q waves. V1 to v4 may have initial small, positive 'r' wave as the depolarization speads via RV and towards the leads( RV is located anterior to the LV).
inferior leads II, III, Avf has different story, as the inferior part of heart is formed by LV. Initial small positive 'r' wave (actually negative with reference to an inferiorly located lead) is from RV.
The second part of QRS complex is formed by LV. As the conduction is delayed in the LV, the duration of deploarization wave is longer than that of RV, so that overall QRS duration is prolonged. The lateral leads v5, v6, I, Avl have only broad positive QRS wave as they do not recognize the depolarization of RV. So these are the only leads that recognize the depolarization of only LV. These are ONLY LV leads. Thats why, the QRS complex is monophasic in these leads. As these are the ONLY LV leads, the bigenning of R wave to the peak of R wave denotes the spread of depolarization from endocardium to epicardium in these leads, which is called 'intrinsicoid deflection' which is prolonged due to delay of conduction. (Intrinsicoid deflection should be calculated in these ONLY LV leads).
In v1 to v4 leads the depolarization wave spreads posteriorly(away from the leads) as the posterior part is formed by LV. So they have a deep broad negative wave. The inferior leads also have a deep, broad negative wave Which is actually a positive wave with reference to the inferior leads. So that anterior and inferior leads will have a deep, broad S wave.
A positive R wave in I, and a negative S wave in AVF causes left axis deviation. (Heart is not rotated, only the axis of depolarization is towards the left).
To summerize. Broad QRS complex >120msec and left axis deviation PLUS
1. Absent q waves in lateral leads.
2. Monophasic QRS complex in lateral leads.
3. Prolonged intrinsicoid deflection in lateral leads.
4. rS waves in anterior leads.
5. rS waves in inferior leads.
But story not ends here with depolarization. There is also a part of repolarization. Usually repolarization wave of right ventricle is not catched by the leads. Whatever repolarization changes we see on ECG are from LV. There are changes in the ST segment and T waves. But these changes are not due to defective repolarization, but are secondary to abnormal depolarization. Hence these are secondary replarization abnormalities. These ST T changes are discordant (opposite in direction) with the QRS deflection.
Greater the depolarization changes greater the ST T repolarization changes. Deeper the S wave in the V1, higher the ST T changes in V1. Taller the R wave in V6, greater the ST T depression in V6. This creates confusion in diagnosing AWMI in presence of LBBB.
In general positive R wave with positive J point elevation is specific for MI.
In LBBB, 1.positive R waves are seen in lateral leads. If these leads show a concordant J point elevation of atleast 1mm, then it is more specific for MI carrying 5 points.
Except lateral leads no other leads have a positive QRS complex in LBBB. There fore, 2. At least 1mm of J point depression in v1 to v4 concordance with QRS changes carries 3 points
Anterior lead's discordant ST T changes depend upon the depth of S wave. There fore 3. >5mm J point elevation is needed to diagnose in these leads, and it carries only 2 points. Hence this 3rd point is though more sensitive, it is less specific.
That mean, when only this is the only finding on ECG, the chances of MI are less likely.
These three constitute SGARBOSA criteria to diagnose MI in presence of LBBB. Presence of a score of >3 is diagnostic.
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