Read Bledsoe’s April Column: MAST Again: Medical, Not Military Anti-Shock Trousers
 The MAST |
In March of 2003, I began an eight-part series entitled EMS Mythology that was published in Emergency Medical Services magazine. The first installment was on the fallacy of the medical/military anti-shock trouser (MAST), also called the pneumatic anti-shock garment (PASG). Today, the MAST is rarely seen and even more rarely used. The National Association of EMS Physicians (NAEMSP) has a position statement on the MAST which I feel needs to be revised in order to downplay the role of the MAST.1
Recently, I was forwarded an email from one of the various EMS lists, where the writer was lamenting the premature demise of the MAST based upon a “single, flawed Houston study.” Perhaps it is time we revisit this MAST issue and see what the science really says.
The term MAST is an acronym for “medical anti-shock trouser” and not “military anti-shock trouser,” as is commonly believed. The MAST, as we know it today, was a derivative of the G suit developed by noted surgeon George Crile.2,3 In actuality, the MAST was developed by NASA. There was pressure by Congress for NASA to ensure that some of the billions of dollars spent on the space program also benefited the public.4 It was later used by the military, but its origins were with NASA.
Like many things in EMS, the MAST was thrust upon EMS with little or no scientific evidence supporting its use. It seemed like a good idea and was widely embraced. In 1977, N.E. McSwain reported the following regarding a series of patients:
| “Forty-seven patients were brought to the Emergency Department with a good blood pressure which probably would not have existed without the use of MAST Trousers. Most important, not one patient who survived the operation and immediate postoperative period developed adult respiratory distress syndrome or acute renal failure. It is our belief that prevention of prolonged hypotension in the prehospital period was a major factor in achieving this result.”5 |
He also published an additional paper about a series of 70 patients, whereupon he wrote:
| “This report consists of the results of treatment of approximately 70 patients whose blood pressures were significantly reduced before application of the trousers. These patients, for the most part, were delivered to the hospital in relatively good clinical condition. Complications with this device have been few. There is a question of pulmonary compromise because of the increased intra-abdominal pressure. Hard data on the number of lives saved are difficult to obtain because of the complexity of the human system in determining which lives would or would not have been saved without long-term studies. However, it can be shown that there is a significant increase in blood pressure with the utilization of such devices. This, then, infers that lives probably were saved in specific patients.”6 |
In a 1983 review article, C.W. Schwab and D. Gore wrote, “MAST are a simple, safe and sound device with applicability in all shock states; they provide a number of benefits to the hypotensive patient.”7 McSwain estimated that approximately 20 percent of a patient’s blood volume was autotransfused into the heart, brain, and lungs following application of the MAST.6 P.A Dillman also estimated the amount of blood autotransfused to be approximately 20 percent of the total blood volume (approximately 1,200 mL in an 85 kg man).8 This mechanism was then picked up and promoted by the textbooks of the era.9,10,11
Multiple researchers have documented problems with the MAST. And the “flawed Houston paper” was actually methodologically sound and its results were later repeated by F.C. Chang. H.G. Bivins evaluated the effects of the MAST on healthy volunteers. After removing one liter of blood from the volunteers, the MAST was applied. The amount of blood autotransfused from the lower extremities and abdomen to the head and upper trunk was measured using sequential radioisotope scans. He found that application of the MAST resulted in an autotransfusion of less than 5 percent of the patient’s total blood volume. This was approximately 300 mL in an 85 kg man.12 This amount was much less than initial estimates that ranged from 750-1,200 mL.
A similar study measured the amount of blood autotransfused following MAST application to dogs who were suffering hemorrhagic shock following phlebotomy. Again, the amount of blood autotransfused was approximately 5 percent of the total blood volume.13 Based on these studies, statements about the autotransfusion capabilities of the MAST were dropped. Instead, teaching was changed and stated only that MAST increased peripheral vascular resistance.
Later, researchers began to look at patient outcomes following application of the MAST. The initial study that brought serious doubts to the benefit of the MAST was a 1989 study conducted in Houston, TX, using the Houston Fire Department EMS system. During a 2.5-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 conclude that, contrary to previous claims, the MAST provides no significant advantage in improving survival in the urban prehospital management of penetrating abdominal injuries.14
And then there is the paper people always seem to forget. Chang and colleagues completed a randomized controlled trial of 291 traumatic shock patients, greater than 15 years of age with blunt or penetrating trauma and a systolic blood pressure of 90mm Hg or less with clinical signs of hypotension. The patients were randomly assigned to a MAST or non-MAST group. They 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 was not found to be beneficial.15
In a prestigious Cochrane Review, researchers performed a meta-analysis of the two studies described above. The researchers 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 reduces 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 the MAST/PASG in trauma patients.16
That pretty much sums up the existing perspectives on MAST. I checked the current draft of the new EMS curricula and there is no mention of MAST/PASG. Likewise, in the current edition of the ITLS text, the only mention of MAST is in a historical perspective.17 The PHTLS text has taken a more affirmative stance in their current edition — not sure the reason.18 Thus, there is a great deal of evidence against the MAST and little evidence in support of it. Certainly, there is a lot more than a “single flawed Houston study.”
References
1. O’Connor RE, Domeier R. Use of the Pneumatic AntiShock Garment (PASG): NAEMSP Position Paper Prehospital Emergency Care 1997;1(1):32-35
2. Crile GW. Blood Pressure in Surgery: An Experimental and Clinical Research. Philadelphia, PA: JB Lippincott Company, 1903
3. Crile GW. The Cartwright Prize Essay for 1903. Philadelphia, PA: JB Lippincott Company, 1903
4. National Aeronautics and Space Administration. 1996 Space Technology Hall of Fame. Innovation 1996;4(2):
5. McSwain NE. Pneumatic trousers in the management of shock. Journal of Trauma 1977;17(9):719-724
6. McSwain NE. MAST pneumatic trousers: a mechanical device to support blood pressure. Medical Instrumentation 1977 Nov-Dec;11(6):334-6
7. Schwab CW, Gore D. MAST: medical antishock trousers. Surgery Annual 1983;15:41-59
8. Dillman PA. The bio-physical response to shock trousers. Journal of Emergency Nursing 1977;3(6):21-25.
9. Caroline NL. Emergency Care in the Streets. Boston, MA: Little, Brown and Company, 1979 (p.86)
10. Campbell JE. Basic Trauma Life Support: Advanced Prehospital Care. Bowie, MD: Brady Communications Company, 1985 (p.54)
11. Butman AE, Paturas JL, McSwain NE, Dineen JP. Pre-Hospital Trauma Life Support. Akron, OH: Emergency Training, 1986 (p. 98)
12. Bivins HG, Knopp R, Tiernan C, dos Santos PA, Kallsen G. Annals of Emergency Medicine 1982;11(8):409-12
13. Lee HR, Blank WF, Massion WH, Downs P, Wilder RJ. Venous return in hemorrhagic shock after application of military anti-shock trousers. American Journal of Emergency Medicine 1983;1(1):7-11
14. Bickell WH, Pepe PE, Bailey ML, Wyatt CH, Mattox KL. Randomized trial of pneumatic antishock garments in the prehospital management of penetrating abdominal injuries. Annals of Emergency Medicine 1987;16(6):653-658
15. Chang FC, Harrison PB, Beech RR, Helmar SD. PASG: does it help in the management of traumatic shock? Journal of Trauma 1995;39(3):453-456
16. 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;4
17. Campbell JE et al. International Trauma Life Suppor for Prehospital Care Providers, Sixth Edition. Upper Saddle River, NJ: Pearson/Brady, 2008
18. Salamone JP, Pons PT. Prehospital Trauma Life Support, Sixth Edition. St. Louis, MO: Mosby, 2007
