Mechanical Chest Compression Devices, are we looking at the new MAST suit?
By Professor Hugh Grantham
Similarities between the implementation and withdrawal of the military antishock trousers (MAST suit) and the implementation of mechanical chest compression devices are made. The MAST suit has a specific place in the history of prehospital care and emergency care rising in popularity and being withdrawn over a period of approximately 30 years.
Both devices demonstrated effectiveness against the surrogate markers of raising blood pressure for the MAST suit and improving perfusion under CPR for the mechanical chest compression devices. Unfortunately, both have provided disappointing results against the ultimate outcome marker of survival.
The possibility that the chest compression devices will follow the pattern of implementation and withdrawal seen with the MAST suit is real.
In the past many pieces of medical equipment have come and gone as knowledge and understanding has grown. A classic example of this was the military anti shock trousers (MAST suit), which reached almost universal acceptance before the dangers associated with its use were fully appreciated and it was withdrawn. This article draws parallels between the process surrounding implementation of the mast suit and the use of mechanical chest compression devices. There are lessons from the implementation of the MAST suit that may apply to the mechanical chest compression device.
Pneumatic antishock garments known as MAST suits (military anti shock trousers, or medical anti shock trousers) became popular in the late 1970s, the basis for their use was the ability to raise BP (Hoffman 1980). A MAST suit was once so common that it needed no description; today many modern clinicians have not seen it. The suit was a piece of medical equipment used to raise BP in shocked patients by providing pneumatic compression to the legs and abdomen via three compartments inflated with an air pump. When inflated, pressure on the legs and abdomen was thought to both increased venous return from the legs and abdomen and increase peripheral resistance.
The MAST suit was extrapolated from a concept used in military flying where a pressurised flight suit prevented blackouts caused by a drop in blood pressure under high G forces found in military planes manoeuvring at speed. Early trials demonstrated that application of compression to the legs and abdomen did increased BP in a similar manner to the military application where it prevented a drop in BP. By the late 1970s (Wayne 1978) the favourable results in terms of ability to increase BP in shocked patients were being reported with recommendations that its use should be expanded. During the following decade this piece of equipment became standard in most emergency departments and developed ambulance services. At this stage it was noted that rapid deflation could be associated with a rapid deterioration in perfusion and clear procedures governing controlled deflation in a safe environment were in common use. During the 1980’s and1990s (Mattox, Bickell et al. 1986, MATTOX, BICKELL et al. 1989) (Bickell, Pepe et al. 1987) the increased mortality/morbidity associated with the use of the MAST suit was reported with recommendations that its use should be limited. Despite observations of increase in mortality it continued to be carried by many ambulance services for many years. Initially recommendations were to limit its use and eventually by the end of the 1990’s it’s withdrawal was recommended.
Mechanical chest compression devices are not new and have been proposed and built for many years. Currently they are becoming more popular with both the ambulance services and cardiac catheterisation units (Grogaard, Wik et al. 2007, Wagner, Terkelsen et al. 2010). In animal and human studies the use of mechanical chest compression device has been shown to deliver superior aortic arterial pressures under resuscitation (Perkins, Brace et al. 2010) and superior CO2 production (Ward, Menegazzi et al. 1993). As the aim of resuscitation is to supply oxygen to both the heart and brain an improved CPR technique, delivering better pressures seems desirable. Based on a proven ability to provide consistent and better perfusion pressures large studies of mechanical chest compression devices in the field have been undertaken. Based on the superior performance of these devices when compared to traditional manual CPR an improved survival rate was hypothesised.
Given the well-established correlation between perfusion and survival the results of these studies were surprising. (Couper, Smyth et al. 2015, Perkins, Lall et al. 2015) In one study the CIRC trial with 4753 patients (Wik, Olsen et al. 2014) survival rates were actually lower 11% vs 9.4%, although not statistically significant. The LINK study (Rubertsson, Lindgren et al. 2014) with 2593 randomised cases did not demonstrate any benefit but did note 23 adverse events associated with use of a mechanical device. Whilst in the PARAMEDIC trial with 4471 patients (Perkins, Lall et al. 2015) there appeared to be no difference overall but a lower survival rate was noted in the VF/VT subgroup LUCAS study VF VT group.
A similarity exists between the implementation of both these pieces of equipment. In both situations they performed well against an objective assessment that was felt to be crucial. In the case of the mast suit it did raise BP and in the case of the mechanical compression device it does produce higher perfusion pressures. It is important to reflect that while performing well with these surrogate outcomes both pieces of equipment were disappointing when measured against the ultimate outcome of saving life.
The reason for the difference in results in the MAST suit studies was attributed to problems with deflation, increased rate of bleeding and loss of haemoglobin and clotting factors, and possibly a respiratory component from the intra-abdominal pressure. Despite the fact that they were undeniably producing worse results over all there is still some conjecture as to the mechanism behind these results.
The failure of the chest compression devices to live up to their expected potential is also not explained. Possibilities include technical issues with application detracting from quality resuscitation. Interestingly the investigators were well trained and highly motivated so whilst a technical issue might be expected during the run-in period it would be concerning if this was the long-term cause. Some of the studies involve large numbers making a learning curve an unlikely explanation. Another area for investigation includes continuous compressions although it is noted that some of the study protocols did include breaks in compression (Rubertsson, Lindgren et al. 2014). Once again the exact cause for the difference in predicted and observed results remains unclear.
Assessing any intervention against a surrogate outcome will always produce useful information and may encourage further research to confirm that the good results for surrogate outcomes can in fact be translated into good results for the ultimate outcome. In the case of the MAST suit it had become popularly accepted before results of its effect on mortality/morbidity were objectively reported. The mast suit remained in service for many years after these results were published. Anecdotal evidence as to the effectiveness of the MAST suit in individual cases was often provided when considering the implications of the adverse outcomes in the studies. Secondary benefits of the MAST suit as a splint in open book fractures of the pelvis were also introduced into the discussion delaying the withdrawal of the equipment.
Unlike the sequence of events with the implementation of the MAST suit the disappointing mechanical chest compression results have been published before these devices have been widely implemented. Considerations of the occupational health and safety risks involved with mobile CPR and the ability to perform chest compressions whilst under cardiac catheterisation are being focused on. These are undoubtedly important but should they be used to justify a procedure, which at best seems to provide no clinical benefit in the current emergency and cardiology systems? Should a reliable repeatable procedure for performing angioplasty whilst under resuscitation with compressions be developed and found to be effective the cost benefit might potentially be different. However until such time as such a system is developed and proved concerns raised in this paper will remain valid.
It is suggested that secondary outcomes and surrogate markers are relevant to the debate but should not over rule the primary outcome when investigating the effectiveness/appropriateness of clinical procedures and equipment.
In the story of the MAST suit the primary outcome of survival was not focused on once the equipment had been shown to raise BP. It was only some years afterwards that the survival outcome was considered and even then it took many years before the equipment was finally withdrawn. The MAST suit story represents a potential preventable mortality/morbidity impact as well as a significant investment in resources and training that might have been better utilised elsewhere.
The similarities between the implementation of these two pieces of equipment create a question as to whether the mechanical chest compression device may also eventually be withdrawn.
The logical recommendation is that large-scale studies measured against the primary outcome of survival should be conducted before implementation is considered. Interestingly these have already been conducted and yet implementation is still being considered.
The story of the MAST suit is a good illustration of the need to consider the primary outcome when evaluating procedures and equipment. The possibility that the mechanical chest compression devices may have similar issues to the MAST suit needs to be considered.
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