Panic attack symptoms can include shortness of breath or hyperventilation, heart palpitations or racing heart, trembling or shaking, nausea or upset stomach, and even the fear of dying, losing control, or going crazy. A panic attack is an intense wave of fear characterized by its unexpectedness and debilitating, immobilizing intensity. Panic attacks often strike out of the blue, without any warning, and sometimes with no clear trigger. They may even occur when you’re relaxed or asleep.