Signs and symptoms of a panic attackThe signs and symptoms of a panic attack develop abruptly and usually reach their peak within 10 minutes. Most panic attacks end within 20 to 30 minutes, and they rarely last more than an hour.
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http://helpguide.org/mental/panic_disorder_anxiety...
SYMPTOMSPeople with panic disorder have feelings of terror that strike suddenly and repeatedly with no warning. They can't predict when an attack will occur, and many develop intense anxiety between episodes,
http://psychcentral.com/disorders/sx28.htm