Left bundle branch block (LBBB) is the consequence of anatomical or functional dysfunction in the left bundle branch, causing the impulse to be blocked. Depolarization of the left ventricle will be carried out by impulses spreading from the right ventricle. The impulse will spread (through the left ventricle) partly or entirely outside of the conduction system, which is slow and therefore the QRS duration becomes prolonged. The hallmark of left bundle branch block is QRS duration ≥0,12 seconds, deep and broad S-wave in V1/V2 and broad clumsy R-wave in V5/V6. Refer to Figure 1.


Figure 1. Normal ECG, left bundle branch block and right bundle branch block at paper speed 50 mm/s.

Figure 1. Normal ECG, left bundle branch block and right bundle branch block at paper speed 50 mm/s.

ECG criteria

  • QRS duration ≥0,12 seconds.
  • Leads V1-V2: deep and broad S-wave. The small r-wave is missing or smaller than normal. If it is missing, a QS complex appears in V1 and occasionally V2, but rarely V3. The S-wave in V1 may be notched and resemble the letter “W”.
  • Leads V5-V6: Broad, clumsy, completely positive and often notched R-wave.
  • Leads I and aVL: Similar to V5 and V6.
  • ST-T changes: Left sided leads (V5, V6, I and aVL) shows T-wave inversions and ST segment depressions. V1–V3 shows ST-segment elevation and positive T-waves. The ST-segment elevation rarely exceeds 5 mm.



Electrophysiology of left bundle branch block

Ventricular depolarization normally starts in the interventricular septum, which obtains Purkinje fibers from the left bundle branch. Thus depolarization of the septum starts in its left aspects and heads towards its right aspect (refer to Chapter 1). Depolarization of septum yields the small r-waves seen in V1 and V2, and the small q-waves seen in V5 and V6 (“septal q-waves”). In left bundle branch block, depolarization of septum instead occurs via impulses spreading from the right ventricle. Thus, the small r-wave in V1–V2 and small q-wave in V5–V6 is either diminished or disappears. Depolarization continues (slowly) towards the left ventricular free wall, and the vector is continuosly directed leftward. This causes a wide S-wave in V1–V2 (it is referred to as QS complex if the r-wave is absent) and broad and clumsy R-wave in V5–V6. The R-wave may be notched at the apex.

Since left ventricular depolarization is abnormal, the repolarization will also be abnormal and secondary ST-T changes are always present. In left bundle branch block it is expected that ST segment depressions and T-wave inversions exist in left sided leads (V5, V6, I and aVL). Simultaneously, V1–V3 should display ST segment elevation and large R-waves.

The electrical axis may be unaltered or deviate to the left or (rarely) to the right. Left axis deviation suggests a pronounced left bundle branch block.

Clinical significance

Left bundle branch block is always pathological. It affects left ventricular contractility and pumping function. Consequently, left bundle branch block confers adverse cardiovascular outcomes. Left bundle branch blcok is associated with hypertension, ventricular hypertrophy, valvular heart disease, myocarditis, ischemic heart disease, heart failure and cardiomyopathies. The Framingham Heart Study showed that acquired left bundle branch block was associated with seven times as great a risk of heart failure, two times as great a risk of coronary artery disease and significantly higher risk of developing right ventricular hypertrophy. Left bundle branch block is rare in young individuals and appears to affect their prognosis little.

Diagnosis of ischemia/infarction

Left bundle branch block in acute and chronic ischemia has been discussed in a separate article.

Left ventricular hypertrophy and left bundle branch block

Left ventricular hypertrophy is characterized by increased left ventricular mass. The increased muscle mass may lead to prolonged de- and repolarization, and thus slightly increased QRS duration, but not 0.12 seconds or longer. Moreover, the QRS morphology in left ventricular hypertrophy may also resemble that of left bundle branch block (particularly incomplete left bundle branch block). However, it is usually easy to separate the two. In hypertrophy, the septal q-waves (V5, v6, aVL and I) are preserved (or even amplified), the QRS complex has very large amplitude. In left bundle branch block, the QRS duration is at least 0.12 seconds. Of course, these conditions may accompany each other.

Incomplete left bundle branch block

Incomplete left bundle branch block is less common than the complete variant. Conduction is preserved in the left bundle branch, but it is diminished as compared to a normal bundle branch. Thus, the initial depolarization of the left ventricle occurs via impulses spreading from the right ventricle, but after a while the impulse passes the block in the left bundle branch and executes the remained of ventricular depolarization normally. Hence, the initial QRS complex resembles left bundle branch block but QRS duration is <0.12 seconds. Incomplete left bundle branch blocks tend to progress to complete bundle branch blocks.

  • QRS duration >0,10 but < 0,12 seconds.
  • R-wave peak time ≥0,06 seconds in i V5, V6.
  • Absence of normal septal q-wave in V5, V6, I and aVL.
  • Notched ascending limb of R-wave in V5, V6, aVL and I.
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