The second heart sound comprises of aortic (A2) and pulmonary (P2) component. In

LBBB, the aortic closure is delayed because the left ventricle contracts later. This then causes reversed splitting (A2P2 → P2A2) if the second heart sound.

LBBB and left heart strain in HCM and aortic stenosis can cause reversal of A2P2 second heart sounds. Also, in type B wolf parkinson white syndrome, early activation of the right ventricle through an accessory pathway can cause P2 to close prematurely. Patent ductus arteriosus is another cause.