Dear student question is not clear
if you are asking about permenper stop of heart beat it is cardiac arrest
it can be explained as below
Sudden cardiac arrest occurs when the heart suddenly stops beating, which stops oxygen-rich blood from reaching the brain and other organs. A person can die from SCA in minutes if it is not treated right away. ... This causes death of the heart muscle. But a heartattack does not always mean that the heart stops beating.
If you are asking about stopping of heart beat between lub dub action of heart it is due to the involuntary muscles of heart
Dear student, the above given explanation is the answer for your question. In case you meant any other meaning of stopping of heart beat, then kindly ask clearly in another section and award point to this section.