Left bundle branch block (LBBB) is a cardiac conduction abnormality seen on the electrocardiogram

Electrocardiogram

Electrocardiography is a transthoracic interpretation of the electrical activity of the heart over a period of time, as detected by electrodes attached to the outer surface of the skin and recorded by a device external to the body...

 (ECG). In this condition, activation of the left ventricle

Left ventricle

The left ventricle is one of four chambers in the human heart. It receives oxygenated blood from the left atrium via the mitral valve, and pumps it into the aorta via the aortic valve.-Shape:...

 is delayed, which results in the left ventricle contracting later than the right ventricle

Right ventricle

The right ventricle is one of four chambers in the human heart. It receives deoxygenated blood from the right atrium via the tricuspid valve, and pumps it into the pulmonary artery via the pulmonary valve and pulmonary trunk....

.
The criteria to diagnose a left bundle branch block on the electrocardiogram:

• The heart rhythm must be supraventricular in origin
• The QRS duration must be ≥ 120 ms
• There should be a QS or rS complex in lead V1
• There should be a RsR' wave in lead V6.

The T wave should be deflected opposite the terminal deflection of the QRS complex.