This week we are briefly looking at an interesting paper that suggests that HEMS services have much to offer in the management of traumatic cardiac arrest (TCA).
We know that outcomes from TCA are poor, though arguably similar to the outcomes in medical cardiac arrest, but there is the possibility of very positive outcomes in a population which is often young and with great potential. In the prehospital population traumatic cardiac arrest presents significant difficulties relating to the availability of people, equipment and skills. In addition, TCA is not that common and may be widely geographically distributed, whilst also being a time critical condition. Unsurprisingly then, helicopter emergency medical services (HEMS) teams have developed systems and processes to try to get the right team to the patient as fast as possible with the hope that the patients will benefit, but do they really benefit and do HEMS interventions make a difference?
This month we have a paper in the Resuscitation journal that addresses this question for a UK based HEMS service. The abstract is below, but as always please read the full paper and make up your own mind as to its quality and message.
What kind of paper is this?
This is a database analysis of a retrospective cohort of patients treated by the Kent, Surrey and Sussex air ambulance in the UK. Data is collected routinely on all patients treated by the service.
Who was studied
The authors have interrogated the database between 2013 and 2018, searching for any cases where a traumatic cardiac arrest was recorded. It’s important to note that the KSS air ambulance service covers quite a large (in UK terms) geographical area which is predominantly rural. This is in marked contrast to some other services, such as London HEMS, which serve a predominantly urban population. This leads to significant differences in case mix, with penetrating trauma forming a much smaller proportion of patients as described in urban or military reports.
What were they looking at?
This paper is written by the HEMS service itself and so focuses on what they do. Essentially they are primarily looking at the processes and procedures of the HEMS service in response to the call to attend TCA patients. The focus is therefore on what they have done in terms of things like RSI, thoracostomy, thoracotomies, use of blood etc.
They have also tried to look at whether these interventions make a difference to patients in terms of ROSC or survival. This has been done by analysing the data using a logistic regression model to see if any of the HEMS interventions are associated with ROSC.
Tell me about the patients
Over the 5-year period the authors identified 263 patients with TCA, so roughly one a week which demonstrates the relative rarity of the event for the service and for any individual within it. Patients were predominantly male (75%) and mostly the victims of blunt trauma (86%). Read the paper for a full description of the patients but it is largely as you would expect. Patients were severely injured with significantly deranged physiology.
What are the main results?
This paper is largely about process and it’s clear that the majority (88%) of these patients receive complex resuscitation interventions that can only be delivered by a HEMS team in the prehospital setting. In addition, all patients had other interventions delivered by ground based paramedic teams (e.g. intubation without drugs).
So in terms of process these patients were deemed to require significant interventions to try to achieve ROSC.
51 patients had a sustained ROSC, and of those just 7 survived to hospital discharge. That’s a lower proportion than in other studies, but this may be because of the smaller number of penetrating cases in this cohort and also the significant geographical distances involved (the assumption being that outcome is improved in patients with penetrating trauma and short response times).
So does HEMS make a difference?
The authors make the case, and I think the data supports this, that a HEMS service has a number of procedures that can be delivered to patients following traumatic cardiac arrest, but that’s not as important as considering whether it makes a difference to outcome. It’s less than clear in this study. If we look at the number of patients who obtain ROSC then there appears to be an association between some interventions and ROSC. These are BVM, RSI, thoracostomies and blood products, although it is unclear whether these were performed pre- or post-ROSC.
This is important as the matter becomes much less clear when we look at the patients who survive to hospital discharge, which I believe is a far more important outcome than ROSC. Amongst those 7 patients it is clear that all achieved ROSC before the arrival of the HEMS team. To be clear, 28 patients had ROSC before the arrival of HEMS and all hospital survivors had ROSC before HEMS arrival, though all of them subsequently had a HEMS intervention post ROSC. What we don’t know is anything about the functional outcome of the patients who survived. What we really want to see is something like a Glasgow Outcome Score. We might be able to determine the value of HEMS interventions in hospital survivors if there was more detail, but in the paper the data is limited (though they may well have made a difference).
The authors rightly point out that the diagnosis of cardiac arrest is difficult in trauma patients and they relied on the ground crew to determine this. It is possible that those who achieved ROSC before HEMS may have been in low-flow states as opposed to absent circulation.
Ultimately this study shows that some HEMS procedures are associated with ROSC, but fails to demonstrate a benefit in terms of patient survival. It also demonstrates that survival to hospital discharge only occurred if ROSC was achieved before the arrival of the HEMS team. Neither of these facts tell us whether HEMS makes a difference to the final outcome in this very seriously injured group of patients. Some interventions have an association with ROSC, but association does nor equal causality. Additionally the number of hospital survivors is too few, and the details too limited to draw meaningful conclusions.
What appears to be the case, but something that is not highlighted in the paper is that the most important factor in determining whether a patient will survive to discharge is if they regain a circulation before HEMS arrive. My basic calculations suggest that the odds ratio of pre arrival ROSC as a factor to survival is essentially infinite (as no survivors in one arm), with a p value in the region of 0.012. This was calculated using statpages online calculator (which is pretty nifty by the way). The table below has survival as the condition, and presence or absence of a circulation pre- or post-HEMS as the test.
It is reasonable to argue that ROSC is an essential component on the way to future survival and positive long term morbidity and mortality benefits, but as we have seen in other critical care prehospital studies (such as PARAMEDIC2) an increased survival may be at the expense of increased morbidity. Unfortunately, the low incidence of TCA in the UK means that further evidence, or any form of trial is likely impossible. We should also be mindful that this is the data from one service and we should be cautious in extrapolating to other services which may have very different patient and logistical characteristics.
Although my personal belief is that there is likely to be a benefit in a very small number of patients, the data presented here does not confirm this. Whilst it is possible that HEMS are influential in the small number of survivors, it is also possible that much time, expense and effort is being delivered by the service for marginal, or perhaps even futile gains.
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- TCA cardiac arrest stats: http://www.stemlynsblog.org/jc-uk-traumatic-cardiac-arrest-stats-st-emlyns/
- Epidemiology and aetiology of traumatic cardiac arrest in England and Wales — A retrospective database analysis EdBarnardab DavidYatesc AntoinetteEdwardsc MarisolFragoso-Iñiguezc TomJenksc Jason E.Smithbd https://doi.org/10.1016/j.resuscitation.2016.11.001
- Traumatic Cardiac Arrest: Who Are the Survivors? DavidLockey, KateCrewdson,GarethDavies, https://doi.org/10.1016/j.annemergmed.2006.03.015
- St Emlyn’s resources on Cardiac Arrest http://www.stemlynsblog.org/tag/cardiac-arrest/