The military antishock trousers (MAST) garment is a one-piece layered device made of polyvinyl fabric that encircles the legs and lower abdomen and can be inflated to apply external pressure to enclosed body parts. The legs are enclosed separately from the inguinal crease to the ankle; the feet are exposed. The abdominal component encloses the body from the lower rib cage to the pelvis, with the perineal area exposed. Three compartments are fastened with Velcro (Velcro USA, Inc.). Most versions of the garment allow separate inflation and deflation of the three compartments. The MAST is inflated with a foot pump, and some units are equipped with inflation pressure monitoring gauges. Internal pressures of the suit are limited by pressure relief valves (set at 104 mmHg) and the ability of the Velcro fasteners to withstand stress.
The MAST is a old concept but was rediscovered in Vietnam when a commercially available "G-suit" inflated to a pressure of 30 to 40 cmH 2O was used during the transport of seriously injured soldiers from combat areas to the hospital.14 The authors of this study felt that the device was beneficial because four soldiers survived while using it who otherwise would have died, a judgment based on the physician's previous experience with similar casualties. Since then, considerable study, thought, and opinion have been published concerning the utility of the MAST in civilian EMS. —I6 Currently, the MAST has fallen into disfavor because there is little evidence that it improves survival rates for the condition for which it is most commonly used: hemorrhagic shock after trauma. In fact, there is evidence from the Houston EMS system that the MAST may be detrimental in cases of cardiac and thoracic trauma with short transport times.17 It is not clear whether MAST may be useful in other situations or in disease entities other than trauma. For instance, there is theoretical and anecdotal evidence that the MAST may be useful in bluntly traumatized patients with long transport times, in ruptured abdominal aortic aneurysms (AAAs), or in pelvic fractures. A position paper from the National Association of EMS physicians attempts to bring a reasoned approach to the use of MAST by categorizing the possible indications into classes I, II-A, II-B, and III, based on possible efficacy or lack thereof.15 Incidentally, the only class I intervention (usually indicated, useful, and effective) for MAST in that paper is ruptured AAA. It may be reasonable to keep MAST on ambulances for now, especially for rural services, to use it primarily for blunt trauma with long transport times and to avoid its use with penetrating thoracic injury.
The physiologic effects of the MAST are often misunderstood.16 The MAST increases blood pressure primarily by increasing peripheral vascular resistance (afterload). The MAST does not reliably mobilize pooled blood (autotransfuse) from the legs or abdomen, especially in hypovolemic trauma patients. The MAST does decrease bleeding from vessels under the compartments by applying external pressure that decreases the pressure gradient for continued hemorrhage.
In practice, the MAST is applied as the patient is being immobilized on the long spine board. After physical assessment of the abdomen, pelvis, and legs, the three compartments are closed over the patient. If the patient is hypotensive, the garment is inflated sequentially, first the legs and then the abdomen. Inflation is done in stages until the desired response by the patient is seen—usually a systolic blood pressure greater than 90 mmHg—and the MAST is not inflated further. While the compartments have pressure relief valves set at 104 mmHg, the Velcro fasteners usually give way at pressures far less, usually around 60 mmHg; thus, it is difficult to achieve high MAST compartment pressures.
Once a patient has been placed in a MAST and the compartments have been inflated, their cardiovascular system responds and adapts to the increased afterload. If that external pressure is suddenly relieved, systolic blood pressure can fall precipitously, especially in a hypovolemic patient. Therefore, MAST deflation in the emergency department should be done slowly, with blood pressure monitoring and IV fluid administration in case of sudden hypotension.
Since the MAST increases afterload, pulmonary edema is the one absolute contraindication to its use. Contraindications for use of the abdominal compartment of the MAST (and relative contraindications to the use of the MAST in general) are pregnancy, the presence of impaled objects, evisceration of the abdominal contents, and thoracic and diaphragmatic injuries.
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