A single first heart sound (S1) followed by an aortic ejection click (AEC) can also mimic a split first heart sound. An AEC follows S1 by 50 milliseconds.
An aortic ejection click is caused by thickened aortic valve leaflets. This is commonly seen in a bicuspid aortic valve, a congenital abnormality where someone is born with two aortic valve leaflets instead of the normal three.
Listen carefully to the second sound of the pair. If it is an AEC it will have a shorter duration and a higher pitch than the first sound of the pair (S1).
The AEC is best heard at the Aortic valve area where splitting of S1 is not heard.