EMS Myth #1: Medical Anti-Shock Trousers (MAST) autotransfuse a significant amount of blood and save lives.

Paramedics in the 1970s and 1980s often used Medical Anti-Shock Trousers (MAST), also called the Pneumatic Anti-Shock Garment (PASG), for all forms of trauma. It was the standard of care. On many occasions, I came to believe that I had seen patients pulled from the jaws of death after MAST application. In EMS circles, we told stories about doctors or nurses removing or cutting off MAST in the emergency department, only to have the patient become immediately hypotensive and die. EMS people were not the only true believers in MAST. They were often a common component of trauma resuscitation rooms and operating rooms. Invariably, we would have to retrieve the MAST from the OR, as they remained on the patient until the surgical lesion was repaired. We knew the MAST worked. We had seen it work. But, did the MAST really work?

MAST History
The concept of the MAST was first described in 1903 by famed surgeon George W. Crile as a "pneumatic rubber suit" to decrease postural hypotension in neurosurgical patients.^1,2 During World War II, Crile's suit was used to prevent blackout in pilots who were subjected to high G forces while flying combat aircraft. The National Aeronautics and Space Administration (NASA) claimed responsibility for developing the medical anti-shock trousers at their Ames Research Center in the 1960s.³ MAST were introduced into medical practice during the war in Vietnam and called "Military Anti-Shock Trousers."⁴ The value of MAST in the military setting was documented when soldiers with massive trauma, previously considered fatal, were able to survive a 45-minute helicopter ride to a definitive care hospital.⁵ MAST were introduced into civilian EMS in the 1970s.⁶
It was postulated that the MAST reversed hypotension by three different mechanisms: 1) Increasing peripheral vascular resistance; 2) tamponading of intra-abdominal bleeding; and 3) autotransfusion of blood from the lower extremities and abdomen to the head and upper trunk.
Most authorities supported the theory that MAST provided a significant autotransfusion. McSwain estimated the amount of blood autotransfused to be 750–1,000 mL.⁷ In another paper, McSwain estimated that approximately 20% of a patient's blood volume was autotransfused into the heart, brain and lungs following application of MAST.⁸ Dillman also estimated the amount of blood autotransfused to be approximately 20% of the total blood volume (approximately 1,200 mL in an 85-kg man).⁹ Based upon these reports, the EMS textbooks of the era picked up the information on MAST, and it was incorporated into day-to-day EMS teaching. The first paramedic textbook stated: "The pressure applied to the legs squeezes at least 2 units of blood out of these extremities, where it is less critically needed, and into the systemic circulation. The net effect is as if the patient were given a 2-unit transfusion of blood; in a sense, then, it is an AUTOTRANSFUSION, since the patient is transfusing himself with blood from his extremities. (Remember, though, that the converse is also true. When the MAST is deflated, blood returns to the legs, and it is as if the patient suddenly lost 2 units of blood. Thus, the MAST is never deflated until adequate volume replacement has been achieved.)"¹⁰ The first edition of Basic Trauma Life Support stated the following: "No one has proven how MAS trousers work, but the most likely mechanism is an increase in peripheral vascular resistance by way of circumferential compression. The important thing is that they do work to improve blood pressure and cerebral circulation in the hemorrhagic or spinal shock victim."¹¹ Likewise, the first edition of Pre-Hospital Trauma Life Support stated, "If the patient is hypotensive or there is suspicion of bleeding within the abdomen, the pneumatic anti-shock garment (PASG) should then be placed on the patient and inflated until an adequate blood pressure is obtained. The early use of the PASG will assist in reducing rapid intra-abdominal bleeding."¹²

Applying the Scientific Method
Later, researchers applied the scientific method to study the effects and effectiveness of the MAST and found that the actual benefits were far less than originally thought. Researchers at Valley Medical Center in Fresno, CA, evaluated the effects of the MAST on healthy volunteers. After removing one liter of blood from the volunteers, the MAST were applied. The amount of blood auto-transfused from the lower extremities and abdomen to the head and upper trunk was measured using sequential radioisotope scans. They found that application of the MAST resulted in an auto-transfusion of less than 5% of the patient's total blood volume. This was approximately 300 mL in an 85 kg man.¹³ This amount was much less than initial estimates that ranged from 750–1,200 mL. A similar study measured the amount of blood auto-transfused following MAST application to dogs who were suffering hemorrhagic shock following phlebotomy. Again, the amount of blood auto-transfused was approximately 5% of the total blood volume.¹⁴ Based on these studies, statements about the auto-transfusion capabilities of the MAST were dropped. Instead, teaching was changed and stated only that MAST increased peripheral vascular resistance.
Researchers then began to look at patient outcomes following application of the MAST. The initial study that questioned the benefit of the MAST was conducted in Houston, TX, in 1989 using the Houston Fire Department EMS system. During a 2½-year period, 201 consecutive patients presenting with penetrating anterior abdominal injuries and an initial prehospital systolic blood pressure of 90 mm Hg or less were entered into the study. All prehospital care was provided by the Houston Fire Department and all patients were delivered to the same regional trauma facility (Ben Taub Hospital). The patients were randomized into control and MAST groups by an alternate-day assignment of MAST use. The resulting study groups were found to be well matched for survival probability indices, prehospital response and transport times, and the volume of IV fluids received. The results demonstrated no significant difference in the survival rates of the control and MAST treatment groups. Based on these data, researchers concluded that, contrary to previous claims, the MAST provides no significant advantage in improving survival in urban prehospital management of penetrating abdominal injuries.¹⁵
Another prospective, randomized study investigated 291 traumatic shock patients greater than 15 years of age with blunt or penetrating trauma and a systolic blood pressure of 90 mm Hg or less with clinical signs of hypotension. The patients were randomly assigned to a MAST or non-MAST group. The researchers found that there were no significant differences in hospital stay or mortality between MAST and non-MAST patients. Similarly, in the subset of patients with blunt trauma, MAST were not found to be beneficial.¹⁶ In a prestigious Cochrane Review, researchers performed a meta-analysis of the two studies described above and found that the duration of Intensive Care Unit (ICU) stay was 1.7 days longer in the MAST-treated group. They concluded that there was no evidence to suggest that MAST/PASG reduce mortality, length of hospitalization or length of ICU stay in trauma patients. In fact, they found, MAST may actually increase these. They concluded that the data do not support the continued use of MAST/PASG in trauma patients.¹⁷

Conclusion
Based on available data, in 1997 the National Association of EMS Physicians issued a position paper on use of MAST/PASG in modern EMS.¹⁸ The association concluded that MAST are definitely beneficial in ruptured abdominal aortic aneurysm and possibly beneficial in hypotension due to pelvic fracture, anaphylactic shock refractory to standard therapy, otherwise uncontrollable lower extremity hemorrhage and severe traumatic hypotension (palpable pulse, no blood pressure).¹⁹ Even considering these possibilities, any benefit from application of the MAST may be accomplished through rapid transport to a trauma center. Many EMS services have kept MAST for use in possible pelvic and lower extremity fractures. Patients with femur fractures are best treated with traction splints, while patients with pelvic fractures can be treated with a long backboard or similar device. Furthermore, the MAST are expensive (approximately $500 per pair) and take up valuable storage space on the ambulance. MAST are a relic of our past and belong in EMS museums, not on modern ambulances or rescue vehicles.

References
1. Crile GW. Blood Pressure in Surgery: An Experimental and Clinical Research. Philadelphia, PA: JB Lippincott Company, 1903.
2. Crile GW. The Cartwright Prize Essay for 1903. Philadelphia, PA: JB Lippincott Company, 1903.
3. National Aeronautics and Space Administration. 1996 Space Technology Hall of Fame. Innovation 4(2), 1996.
4. Schwab CW, Gore D. MAST: medical antishock trousers. Surgery Annual 15:41–59, 1983.
5. Cutlet BS, Daggett WM. Application of the "G-Suit" to the control of hemorrhage in massive trauma. Ann Surg 173:511–514, 1972.
6. Kaplan BC, Civetti JM, Nagel EL, et al. The military anti-shock trouser in civilian prehospital care. J Trauma 13(10):843–848, 1973.
7. McSwain NE. Pneumatic trousers in the management of shock. J Trauma 17(9):719–724, 1977.
8. McSwain NE. MAST pneumatic trousers: A mechanical device to support blood pressure. Medical Instrumentation 11(6):334–336, Nov–Dec 1977.
9. Dillman PA. The biophysical response to shock trousers. J Emerg Nurs 3(6):21–25, 1977.
10. Caroline NL. Emergency Care in the Streets. Boston, MA: Little, Brown and Company, 1979, p. 86.
11. Campbell JE. Basic Trauma Life Support: Advanced Prehospital Care. Bowie, MD: Brady Communications Company, 1985, p. 54.
12. Butman AE, Paturas JL, McSwain NE, Dineen JP. Pre-Hospital Trauma Life Support. Akron, OH: Emergency Training, 1986, p. 98.
13. Bivins HG, Knopp R, Tiernan C, et al. Blood volume displacement with inflation of antishock trousers. Ann Emerg Med 11(8):409–412, 1982.
14. Lee HR, Blank WF, Massion WH, et al. Venous return in hemorrhagic shock after application of military anti-shock trousers. Am J Emerg Med 1(1):7–11, 1983.
15. Bickell WH, Pepe PE, Bailey ML, et al. Randomized trial of pneumatic antishock garments in the prehospital management of penetrating abdominal injuries. Ann Emerg Med 16(6):653–658, 1987.
16. Chang FC, Harrison PB, Beech RR, Helmar SD. PASG: Does it help in the management of traumatic shock? J Trauma 39(3):453–456, 1995.
17. Dickinson K, Roberts I. Medical anti-shock trousers (pneumatic anti-shock garments) for circulatory support in patients with trauma. Cochrane Review, The Cochrane Library, 2002, p. 4.
18. O'Connor RE, Domeier R. Use of the Pneumatic AntiShock Garment (PASG): NAEMSP Position Paper Prehosp Emerg Care 1(1):32–35, 1997.
19. Chapleau W. PASG: Bad wrap or bad rap? Emerg Med Serv 31(1):75–76, 2002.

Bryan Bledsoe, DO, FACEP, EMT-P, is an emergency physician, author and former paramedic whose writings include: Paramedic Care: Principles and Practice and Paramedic Emergency Care.