The cardinal symptoms of nonphobic panic are respiratory: shortness of breath, chest discomfort, palpitations and choking or suffocation sensations. Normally, carbon dioxide (CO2), the waste product of respiration, is exhaled from the lungs. In the event of suffocation, the theory suggests that levels of CO2 in the blood, and particularly the brain, rise. Neurons in the solitary nucleus of the brain stem, which are constantly sampling the blood in the brain for levels of carbon dioxide, become activated about two minutes after an actual or misperceived drop in CO2. This causes a deep sigh, and sensations of smothering (respiratory distress: "Hey, you're not breathing big fella!").
About one and one half minutes following activation of this phase of the alarm, signals are sent to the locus coeruleus. This nucleus is the "on-switch" for almost all of the norepinephrine-containing neurons of the brain. As a result of the activation of this nucleus via the solitary nucleus, the second wave of symptoms of panic occur, involving the non-respiratory systems listed above (fight/flight response: "You better act now if you want to live!") as well as compensation mechanisms in the respiratory system itself (hyperventilation: "Pick up the breathing pace, big guy!"). Thus the panic itself is a time limited episode of short (about 4 minutes) duration.