Kǒnghuāngzhèng fāzuò shí huì xīntiào jiāsù, hūxī kùnnán.
When a panic attack hits, the heart races and breathing becomes difficult.
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恐慌症 is the clinical diagnosis (panic disorder). A single panic episode is 恐慌发作. Don't confuse with 焦虑症 (generalized anxiety disorder) - 恐慌症 refers specifically to recurrent, discrete attacks.