The biological model of stress is largely based on research conducted by Dr. Hans Selye, who studied responses to stress in humans and animals and discovered similar patterns in both. The stress response Dr. Selye observed is sometimes called "fight or flight," and involves a quickened heart rate and rate of breathing. More blood flows to the muscles, mental acuity increases, and chemicals which act as painkillers flow through the bloodstream. This is sometimes called the "alarm reaction" and does not last long. Selye found that, in extended periods of stress, the alarm reaction soon gives way to the a phase he called "adaptation," in which these responses are modified. According to the biological model of stress, panic attacks are a heightened form of alarm reaction which occur in subjects with anxiety disorders.
There are several cognitive models of stress, but perhaps the most influential, as well as the earliest, has been the model developed by Dr. Richard Lazarus. Lazarus describes two stages of appraisal during the cognitive response to stress. The first occurs when the subject recognizes that an event or situation is threatening. The second begins when they start to evaluate their ability to tackle this threat, and comes to a decision about how to approach it. In this model, a panic attack occurs when the subject experiences the threat acutely and fails to make the transition from the first stage of appraisal to the second.
Further information about the two models and their originators from the National Center for Biotechnology Information is attached below.