Mast: military anti-shock trousers

As the name implies, the military was instrumental in the development of anti-shock trousers. Military Anti-Shock Trousers (MAST) were extensively used during the conflict in Vietnam. They are also marketed under the name Medical Anti-Shock Trousers. MAST is an inflatable garment that surrounds the legs and torso. They have three separate compartments that can be inflated. They are capable of sustaining internal pressure of approximately 100 torr. The primary use is to help slow the progress of shock, but they are also used for other purposes as well.

At first, it was thought that MAST helped reverse the signs of shock by squeezing blood out of the lower extremities into the central circulation. The theory was that the MAST worked to reverse shock in different ways. The first was by stopping any bleeding in the lower extremities (like applying direct pressure to a cut on an arm). The second was by increasing peripheral vascular resistance the lower body (squeezes more blood out but lets less blood enter the lower extremity). The third was the belief that the body was able to better perfuse the upper body while MAST were in place. Current studies have shown that MAST work in different but similar ways than originally believed. One of these is by increasing blood flow to the brain and other vital organs.

Once in place and inflated, the MAST should not be deflated in the field. If for some reason the MAST has to be deflated in the field, this should be done slowly and methodically with the vital signs taken after each deflation and prior to deflating further. During the deflation, the blood pressure should be checked every two minutes. A drop of 5 mm Hg or more indicates the need to stop the deflation and have 100 to 200 ml of fluid by IV until the blood pressure is stabilized. The process is continued until fully deflated.

Rapid loss in pressure or deflation of the MAST can be dangerous. Sudden deflation is physiologically equivalent to losing two units of blood in a few seconds time. Changes in temperature can also cause partial deflation. Going from summer heat to the air-conditioned ER can cause partial deflation of the garment. The same is true for helicopter use. The pressure in the garment increases as altitude is gained and decreases as altitude is lost. The pressure should be carefully monitored whenever there are any temperature or altitude changes.

MAST can also be used to splint fractures in the extremities. Traction splints can also be used in conjunction with MAST for femur fractures. MAST also work well for hip and pelvic fractures. MAST are designed so that x-rays can be obtained with the garment in place. In cases where the patient has already lost a significant amount of volume, oftentimes inflating the MAST prior to attempting to start IV will increase circulation enough to make veins more apparent and easier to cannulate. The same goes for a patient in cardiopulmonary arrest. The use of MAST in this situation can easily make the difference in a successful or unsuccessful cannulation. MAST can also be placed, without inflating, as a precautionary matter. If the patient deteriorates, all that is needed is to inflate the garment as it is already in place.

There are some times when the MAST are contraindicated. The only absolute contraindication is pulmonary edema (fluid in the lungs). Other contraindications are pregnancy, impaled objects, evisceration (abdominal contents outside the body secondary to injury), significant head trauma, bleeding in the chest, and severe uncontrolled bleeding above the MAST. Common sense should prevail from situation to situation. In pregnancy, for example, it is permissible to inflate only the leg compartments.