Left Bundle Branch Block (Delay in Left Ventricular Depolarization Delay)

Abstract

Genesis of electrocardiographic findings in classical left bundle branch block (delay in left ventricular depolarization) is discussed with explanatory diagrams. These diagrams help in understanding the explanation for variations in electrocardiographic findings in various cardiac diseases. Causes and significance of abnormalities in the P wave, variations in mean frontal plane QRS axis, variations in the QRS complex in different leads, causes for intermittent narrow QRS complex, and effect of ventricular hypertrophy on the electrocardiogram are discussed with explanatory diagrams and representative electrocardiograms. Summary and MCQs at the end of the chapter help in quick revision and self-assessment.

Appendices

Summary

• Partial LBBB is diagnosed when the duration of the QRS complex is less than 0.12 s, but lead V₁ shows QS or rS pattern with positive T wave and leads I and V₆ show single R wave with loss of q in lead V₆.

• Progression to advanced LBBB results in progressive decrease in the height of R wave and increase in the duration of the R wave in leads I, aVL, V₅, and V₆.

• Advanced LBBB is diagnosed when in addition to findings of partial LBBB, QRS duration is more than 0.12 s, and time from onset to peak of the R wave in leads I and V₆ is 60 ms or more. ST-segment and T waves are opposite to the direction of the terminal part of QRS.

• Mean frontal plane QRS axis is usually normal. rS configuration in leads II, III, and aVF suggests left axis deviation. Leads V₅ and V₆ may show RS configuration. It suggests greater delay in depolarization over the left anterior fascicle and more severe conduction system disease. Right axis deviation suggests greater delay in depolarization over the left posterior fascicle or additional delay in conduction over the right bundle branch.

• Electrocardiographic evidence of left atrial enlargement, QRS duration of more than 155 ms, and positive voltage criteria for left ventricular hypertrophy (SV₂ + RV₆ >45 mm.) suggest underlying left ventricular hypertrophy. Severe conduction delay alone can prolong the QRS duration to more than 155 ms. Mitral regurgitation and left ventricular failure can contribute to left atrial enlargement without meaning left ventricular hypertrophy.

• Presence of LBBB alone is also associated with higher than normal morbidity and mortality from cardiovascular events in the future.

• Some persons can develop tachycardia-dependent LBBB during an exercise stress test. These persons are likely to develop LBBB at rest in future.

MCQs

Q1. Which is not a cause of LBBB?

• (a) Arrhythmogenic dysplasia

• (b) Coronary artery disease

• (c) Lenegre disease

• (d) Mahaim fibers

Q2. LBBB is usually not a manifestation of :

• (a) LVH

• (b) Calcific AS

• (c) Rheumatic heart disease

• (d) Chagas disease

Q3. Chagas disease does not produce:

• (a) Myocarditis

• (b) Atrioventricular block

• (c) Intraventricular conduction defects

• (d) Endocarditis.

Q4. Incomplete LBBB is characterized by:

• (a) Loss of septal q wave

• (b) QS configuration in leads V1 to V3

• (c) QRS duration > 0.12 s

• (d) Notch on the top of QRS complex in lead V6

Q5. Loss of the septal q wave is not seen in:

• (a) Septal infarction

• (b) LV pressure overload

• (c) Uncomplicated LBBB

• (d) LV volume overload

Q6. Resynchronization therapy is most effective when QRS duration is:

• (a) 60 ms

• (b) 100 ms

• (c) 120 ms

• (d) >120 ms

Q7. In advanced LBBB, QS configuration in lead V1 suggests :

• (a) RV infarction

• (b) Septal infarction

• (c) Anterior infarction

• (d) None

Q8. In uncomplicated LBBB, reverse progression of r from lead V1 to V4 suggests :

• (a) Septal infarction

• (b) Anterior infarction

• (c) Advanced block

• (d) None

Q9. In uncomplicated LBBB, lead V4R shows :

• (a) RS configuration

• (b) Prominent R wave

• (c) Inverted T wave

• (d) QS configuration

Q10. In uncomplicated LBBB, lead V8 shows :

• (a) RS configuration

• (b) QR configuration

• (c) Prominent R wave

• (d) Upright T wave

Q11. In LBBB, S wave in lead V6 cannot be due to :

• (a) Medial placement of lead V6

• (b) Lateral infarction

• (c) Hyperkalemia

• (d) Septal infarction

Q12. LBBB with a frontal plane QRS axis of - 60⁰ can be due to additional:

• (a) Septal infarction

• (b) Lateral infarction

• (c) LAFB

• (d) LVH

Q13. LBBB with a frontal plane QRS axis of + 130⁰ can be due to additional :

• (a) RV infarction

• (b) LPFB

• (c) RVH

• (d) None

Q14. In the presence of LBBB, premature narrow QRS complex suggests:

• (a) Tachycardia-dependent LBBB

• (b) Supraventricular ectopy

• (c) Left ventricular ectopy

• (d) Right ventricular ectopy

Q15. What is correct for LBBB?

• (a) Mimics LVH in leads V1,V2

• (b) Mimics LVH in leads V5,V6

• (c) Masks LVH in leads V1,V2

• (d) Mimics LVH in limb leads

Q16. In the presence of LBBB, presence of left ventricular hypertrophy correlates with:

• (a) QRS Voltage in V6

• (b) QRS duration in V6

• (c) Mean frontal plane QRS axis

• (d) Intrinsicoid deflection in lead V6

Q17. Exercise-induced LBBB is associated with :

• (a) Short-term risk of development of atrioventricular block

• (b) Short-term risk of acute myocardial infarction

• (c) Risk of future development of LBBB at rest

• (d) Short-term risk of LVF

Q18. In LBBB, QRS duration does not correlate with:

• (a) LV dysfunction

• (b) LV asynchrony

• (c) LVH

• (d) Long-term prognosis

Q19. Echocardiographic findings in LBBB include:

• (a) Early systolic posterior motion of IVS

• (b) Absence of systolic thickening of IVS

• (c) Absence of systolic thickening of posterolateral wall of LV

• (d) Diastolic dysfunction of LV

Q20. In LBBB, response to resynchronization therapy correlates best with :

• (a) QRS voltage

• (b) QRS duration

• (c) LV dysfunction

• (d) LV dyssynchrony

Q21. LBBB produces:

• (a) Lateral displacement of the left ventricular apex

• (b) S3 in mitral area

• (c) Soft aortic component of the second sound

• (d) Paradoxical split of the second sound

Q22. Which noninvasive test can be useful in a patient with LBBB complaining of effort angina?

• (a) Treadmill stress test

• (b) Echocardiography

• (c) Stress SPECT

• (d) Cardiac MRI

Q23. QRS duration less than 0.12 s, rS configuration with positive T wave in leads V₁, V₂, and R wave in lead I, V₅, V₆ suggests:

• (a) Left anterior hemiblock

• (b) Incomplete LBBB

• (c) Complete LBBB

• (d) Left posterior hemiblock

Q24. Absence of q wave in leads V_5, V₆ can suggest:

• (a) Septal infarction

• (b) Incomplete LBBB

• (c) Normal variant

• (d) None

Q25. Progression of incomplete to advanced LBBB results in progressive:

• (a) Increase in the height of R wave in lead V₅

• (b) Decrease in the height of R wave in lead V₅

• (c) Appearance of S wave in lead V₆

• (d) Appearance of q wave in leads I and aVL

Q26. QRS duration of 0.14 s, QS pattern with positive T wave in leads V₁ to V₃, RR’ pattern with downsloping ST segment, and negative T wave in leads I, aVL, and V_5, V₆ suggests:

• (a) Incomplete LBBB

• (b) Advanced LBBB with septal infarction

• (c) Uncomplicated advanced LBBB

• (d) Advanced LBBB with lateral infarction

Q27. In uncomplicated advanced LBBB, mean frontal plane QRS axis shows:

• (a) Normal axis

• (b) Left axis deviation

• (c) Right axis deviation

• (d) Right upper quadrant axis

Q28. Advanced LBBB with rS configuration in leads II, III, aVF suggests greater delay in conduction over

• (a) Left anterior fascicle

• (b) Left posterior fascicle

• (c) Right bundle branch

• (d) All

Q29. Advanced LBBB with rS configuration in lead I and prominent qR in leads II, III, aVF suggest:

• (a) Greater delay in conduction over the left anterior fascicle

• (b) Greater delay in conduction over the left posterior fascicle

• (c) Additional right bundle branch block

• (d) Additional RVH

Q30. Which ECG findings is specific of diagnosis of LVH in the presence of advanced LBBB ?

• (a) ECG evidence of left atrial enlargement

• (b) QRS duration of 0.16 s or more

• (c) Left axis deviation

• (d) None

Q31. Which of the following is associated with higher than normal cardiovascular mortality and morbidity in the future?

• (a) Advanced RBBB without structural heart disease

• (b) Advanced LBBB without structural heart disease

• (c) Left anterosuperior hemiblock

• (d) All

Answers

(1) a, (2) c, (3) d, (4) a, (5) d, (6) d, (7) d, (8) d, (9) d, (10) c, (11) d, (12) c, (13) b, (14) c, (15) a, (16) a, (17) c, (18) c, (19) a, (20) d, (21) d, (22) c, (23) b, (24) a, b, c, (25) b, (26) c, (27) a, (28) a, (29) b, d, (30) d, (31) b.