heart skip a beat
1. The heart leaves out or seems to leave out a beat; the heart beats hard or leaps from excitement or strong feeling. — Often considered trite.
When Paul saw the bear standing in front of him, his heart skipped a beat.
2. To be startled or excited from surprise, joy, or fright.
When Linda was told that she had won, her heart missed a beat.
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Source: A Dictionary of American Idioms