Google tells me "Heart Stopper" and if it is at all correct (and it often is not) then the context would be necessary to determine the correct meaning. Something that is a shock to your senses. In English it would be hyphenated. A beautiful person can be a heart stopper in a positive way. "She was a real heart-stopper!" But so can a traumatic event. "When he slipped off the edge I thought he was going to fall. That was a real heart-stopper."

physical or psychological? If physical, could be 「心肌梗塞」 - the translation I googled is "Myocardial Infarction". If psychological, in other words in Chinese, should be 「揪心」, then the translation would be "(something, like: anxiety) grips the heart".