Inflatable trauma pants, also known as trauma air pants, "MAST pants" ("Military Anti-Shock Trousers") or a "pneumatic anti-shock garment", are high-waisted inflatable pants that reach from the base of the ribcage to the ankle. When inflated, trauma pants pressurize the legs and abdomen, forcing about two units of blood into the upper torso and head.
During World War II, German dive-bomber pilots wore pressurized leg garments to minimize their "blackout" time during steep dives. In the Vietnam War, similar pants were used to help transport wounded soldiers. The term, "Military Anti-Shock Trousers" (MAST) comes from this U.S. military use.
Trauma pants are used to help prevent hypovolemic shock from loss of blood or blood volume. Blood volume can be depleted by excessive loss of other bodily fluids caused by severe burns, extreme diarrhea or vomiting. "Preventing shock is easier than trying to treat it once it happens". (Medlineplus) Trauma pants may also help stop internal bleeding by applying indirect pressure, and in immobilizing patients with a fractured femur or pelvis.
Use of trauma pants in emergency medical treatment is somewhat controversial. Difficulties arise in treating patients in trauma pants once they arrive at trauma care facilities: they get in the way, and taking them off causes a sharp reduction in blood pressure. Thus, once applied and inflated, they should NEVER be removed until the patient is at a definitive care facility. Also, they cannot be used on pregnant women, and are not indicated for many kinds of wounds. For these and other reasons, trauma pants are not favored in cities where transport times to trauma centers are short.
Sources:
Graeser, Tom, RN, EMT-P; "Military Anti-Shock Trousers",
http://www.med.umich.edu/survival_flight/Education/mast.html
Story describing use of trauma pants after MVA:
www.earth-netone.com/BryceZabel/lifeflightchp1.pdf
Medlineplus: Hypovolemic shock:
http://www.nlm.nih.gov/medlineplus/ency/article/000167.htm | 
