What is it?

MAST pants, officially known as military anti--shock trousers (also known as pneumatic anti-shock garmets, PSAG), have been around for a long time yet many EMT's have never even seen this piece of equipment much less applied them in the field. Kevin Lateef has a great paper on the pros and cons of this device. Critical Review of the Medical Anti-Shock Trousers

The history of the Military Anit-Shock trousers date back to 1903 when a surgeon by the name of George Crile described a pneumatic suite for brain surgery patients to control low blood pressure. This suite was adapted during World War II to prevent blackout of pilots during high G-Force maneuvers. This device later became a g-suite.

The device was then introduced in the Vietman War. It's value was documented when soldiers, with massive trauma and bleeding, were able to survive a 30 to 60 minute helicopter ride to a trauma facility. Previously such soldiers were considered a fatal casualty.

In the 1970's Military Anit-Shock trousers were introduced to the civilian emergency medical services. In 1977 the Committee on Trauma (a section of the American College of Surgeons) recommended that Military Anit-Shock trousers be an essential device on all ambulances.

As with many topics in EMS, much of the "Data' for the use the device was anecdotal. Starting in 1997, nearly 20 years after its recommendation, some clinical data was presented on the benefits of Military Anit-Shock trousers use. The National Association of EMS Physicians noted that Military Anit-Shock trousers were beneficial in cases where the patient had a ruptured abdominal aortic aneurysm. The NAEMSP also saw some benefit in the prehospital use of MAST in cases where there was low blood pressure in pelvic fractures, lower leg uncontrolled bleeding, anaphylactic shock that did not respond to standard treatments and when there was no palpable pulse. Many EMS systems kept the Military Anit-Shock trousers on ambulances only for unstable pelvic fractures and bilateral femur fractures.

The question is, Did MAST ever really work?

What are Military Anit-Shock trousers and how is it applied? Here is an Intruction Manual that shows how to apply the device.
There are basically three methods of applying the MAST:
The trouser method, the log-roll method and the diaper method.

Each method is pretty much self-explanatory. Keep in mind that a potential Military Anit-Shock trousers patient may also have cervical spine injury. Here is a video on how the United States Army applies the MAST How are the Military Anit-Shock trousers supposed to work. There are three mechanisms that are thought to be correct.

1. Increasing peripheral vascular resistance
2. Tamponade effect on intra-abdominal bleeding
3. Auto-transfusion of blood from the lower extremities

The concept of auto-transfusion was thought to decrease the potential spaces where blood could pool and thus allow the blood to reach the vital organs that required it. Applying the garment also was supposed to increase the peripheral resistance to blood flow and increase venous return to the heart. In a normal patient (one not in shock), this seems to be true. The ventricles of the heart (both right and left side) see significant increases in blood flow when the MAST is applied. Based on the studies in normal patients, it was estimated that 20 % of the patient's blood volume could be auto-transfused (approximately 750 to 1000 mL).

How does one test this theory? In 1982 in Fresno, California, volunteers had 1 liter of blood removed and then the Military Anit-Shock trousers applied. Radio isotopes were used to measure the blood flow from the legs to the abdomen and head. The tests revealed that only about 5% of the total blood volume was auto-transfused. This was about 300 mL in an average adult male. This is considerably less than was originally estimated.

This study has resulted in a general retraction of the theory that Military Anit-Shock trousers result in auto-transfusion of blood. It still has some use as a pressure bandage for blunt trauma to abdomen and as an air splint for pelvic fractures.

In brief, the MAST does not work to help prevent Shock

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