This condition occurs from a lack of oxygen in the blood.
There is an interesting connection between hypoxemia and hyperventilation. As the body exercises, cells in the body excrete carbon dioxide which the body, by breathing hard, (up to 20 breaths per minute) is able to expel. The body has adequate oxygen but must work harder to dispel the carbon dioxide being produced by the cells. As a person experiences fear, there is a tendency for the body to hyperventilate naturally. This places more oxygen into the system, and after a time, can cause dizziness. A hyperventilating pregnant woman, risks of hypocapnoea and subsequent acidosis when this occurs in the third trimester. Breathing into a bag (artificially inducing hypoxemia) will reduce the amount of oxygen in the air being breathed. Less oxygen in the blood, may actually tire the body enough for the person in fear to relax given that ther hard breathing is potentially hazardous to their health. In the case of the pregnant, third trimester woman, the elevated acidosis can cause fetal complication.

ŠJamie Longstreet 1996-2004