Population at risk

SCD is an unexpected death, usually due to cardiac disease, in someone perceived to be well within 4 hours of the presumed fatal event. Globally SCD is estimated to be responsible for 12%–15% of all global mortality: some 15 million deaths annually, with roughly 80% of these caused by ventricular tachyarrhythmia.28 Across populations, the rate of SCD is estimated between 53 and 90 per 1 00 000.29 This number is heavily skewed by the exponential increase in cardiac death in those older than 35 years30 31 when ischaemic heart disease becomes the most common cause of cardiac arrest and SCD.

The UK Office of National Statistics estimates the rate in those <35 years in the UK at 1.8/100 000, though this is likely to be an underestimate.32 Much of the data regarding SCD and survival rates in younger populations comes from populations of competitive athletes, primarily in the USA and Europe.30 33–36 In the general population, males constitute 90% of SCDs. The likelihood of SCD in athletic populations is more than twice that of non-athletes, with 80% of deaths occurring during, or immediately following, strenuous physical exertion.36 37

While it is tempting to consider the military population as a young population similar to the well-characterised cohorts of competitive athletes and extrapolate data from these populations, the reality is far more nuanced. The average age of servicepersons is in the mid-30s with 90% white and 90% male. The US military data on sudden death in recruits demonstrated the highest mortality rates of all published populations at 13 per 1 00 000 recruits per year.37 Indeed, death either due to a confirmed or suspected cardiac cause accounts for 11% of all deaths in the UK Armed Forces38 with the greatest risk borne by those in the 35–55 age group.39 Distorting the risk to an undefinable extent are the large numbers of non-military contractors who work on and around military bases. Their age ranges and risk factors are not known, but as a cohort they pose a significant risk. Allegorically, at US College American football games, while the emphasis and motivation behind AED programs centres on saving athletes’ lives, deaths among coaches, spectators and officials accounted for 77% of SCDs.40

In the majority of both civilian and military SCD at <35 years, the terminal event is also the first symptomatic manifestation of an inherited or congenital primary arrhythmic condition,41 such as Wolf-Parkinson-White syndrome, cardiomyopathy or channelopathy.33 36 37 42–45 This means the only ways to prevent these deaths is to identify susceptible individuals through effective screening of the asymptomatic and investigation and treatment of the mildly symptomatic. Premortem identification of asymptomatic pathology will never be 100% successful, so measures to the enable immediate resuscitation of individuals that suffer a cardiac arrest is the next most important step in preventing SCD.

Utility of AEDs in young populations

Very few studies have the power to appraise the outcomes of AED use in young people. A survey of 154 schools in the USA yielded just one AED use, and this on a sports coach rather than a student.46 These findings were echoed in a 2006 review of nine cases in which the survival rate was just 11% despite early CPR and defibrillation.47 Similarly in 2005, no survival benefit among young athletes was demonstrated, but significant survival advantage was given to those older members of the population in the same location.34 Contrastingly, in a separate study of US high schools who possessd AEDs, 32 cardiac arrests were reported, 14 of which were in students. Of these, an AED was employed in resuscitating 83% of students and remarkably nine of the 14 survived to hospital discharge. All patients were athletes.48 This suggests a significant role for AEDs in areas where exercise is to be carried out, and it may be that an under-reporting of the time to defibrillation in the 2006 paper yielded the poor outcome rate. Eckart’s demonstration of the temporal association with exertion suggests less value for young people in areas where activities are sedentary.

Acceptance of AED use

The real-world usage of AEDs has been appraised in three broad settings. The first is in the community where minimally trained members of the public use the AED in presumed cardiac arrest settings.9 13 18 49–52 The second is the use of non-medical personnel such as police, fire personnel,7 sports coaches, athletes and administrators33 40 53 and teachers48 who are frequently first responders. This group had higher levels of training than the first, but no formal medical background. The third group are medically trained paramedics and doctors working in environments where traditional defibrillators are not available.54

Even in the first category there were no reported cases of harm, including when the sole responder was a layperson.49 The AEDs were frequently taken to sites of suspected cardiac arrest and used when required. Ease and speed of use of the devices was elucidated in a small simulated study which compared minimally trained 11-year olds with experienced paramedics55 and found the children to be as effective and safe and only marginally slower (90 s vs 67 s) than the paramedics.

The second category showed a similar pattern, with usage rates varying by location and linked to levels of regional training.48 Some outcome trials demonstrated extraordinary outcomes, with higher levels of shockable rhythms, return of circulation and survival to hospital discharge with non-medical volunteer-delivered resuscitation versus EMS-delivered care.56 This reflected the lower response times in the former cohort. In this category, notable was the variability of the regularity and frequency of training. Due to the infrequency of the SCA events, it is unclear whether this leads to a worse outcome, though we know that infrequent refresher sessions leads to worse skill retention.57

In hospital settings, the preferred systems are manual defibrillators owing to greater control. Patients resuscitated in this environment with manual external defibrillators experienced shorter preshock and perishock pauses in CPR54 which are independently associated with return of spontaneous circulation.58 59 While this more rapid decision-making was associated with a greater error rate, the benefit of reduced pauses seems likely to outweigh the risk of detrimental rhythm change associated with shocking a heart in a normal rhythm.

Types of AED

AEDs are selected for their ability to remove the human component from the decision-making process. In doing so, they allow for those attending an arrest to deliver the best available care irrespective of previous experience. In situations where the environmental complexity is high, such as a combat environment,60 healthcare providers were faster to shock delivery with an AED due to their simplicity relative to a manual defibrillator. A study comparing automated versus manual defibrillation in paramedics of variable training grades in more than 2000 out-of-hospital cardiac arrests demonstrated no significant difference in the preshock pause between automated and manual defibrillation.61

If users are untrained members of the public, intuitive use of AEDs, rather than training in their use, becomes vital, as it is possible the users may be entirely unfamiliar with the devices. A number of usability studies among laypeople compared different AEDs, measuring factors, such as time to defibrillate, pad positioning, safety, quality of CPR delivery and participants’ subjective assessment of usability.50 51 62–64 The variability between devices in all parameters is wide and heavily influenced by human factors. The presence of detailed spoken instructions yielded far better technique and higher rates of shock delivery, while variation in inbuilt BLS algorithms led to unacceptable variation in no flow fraction, perishock pause and time to first shock.65 These studies were published over the period of a decade, and while they help weigh the relative importance of various attributes of AEDs and support informed procurement decisions, they do not enable up to date advice regarding specific device selection.

Location

Expert consensus is that placement of AEDs should be available in areas where sports matches are played, where footfall is highest and colocated with trained personnel.36 42 43 53 The standards set by international sporting authorities suggest that anywhere teams are training or playing competitive matches should be equipped with pitch side AEDs.53 A 2-year prospective study in US high schools suggested that athletics facilities were the most common location for SCD and significantly rates of use in both training and competitive matches.33

Crucially, if the number of AEDs is limited, AEDs should be centrally located and the response system designed so that it could be brought to the location of the collapsed person and a shock delivered within 3–5 min.40

Training

The timely delivery of a shock is constrained by a number of factors (Table 1).

While intuitive AEDs allow faster time to shock and reduced error rates, speeds with even the least ergonomic machine can be greatly improved with training,66 though a shock can be effectively delivered with no training at all.63 These studies were performed in mannequins in a classroom environment in which the participants have essentially been told a cardiac arrest is underway and been given an unfamiliar device. This deviates from reality in two critical ways:

1. Cardiac arrest is not obvious to most lay people.48 More than half of young people have sporadic myoclonic activity which can be confused for a seizure, or agonal gasps easily mistaken for normal ventilation. Training to regard the collapse of any young person as a potential cardiac arrest would make people more likely to apply the AED pads immediately and so deliver the shock earlier.

2. Knowing what an AED is, where it is located and physically retrieving it, must all happen prior to the situation arrived at in the classroom. These elements are prerequisites and realistically would be unlikely to happen without at least one person with relevant training.

As a result, training programmes aim to focus on those people most likely to be present at the time of a cardiac arrest.40 42 Fédération Internationale de Football Association, for example, defines these people as paramedics, coaches, athletic trainers and referees each of whom receive a complete educational package.53 The UK military currently confines this more narrowly to, ‘Defence Medical Services employed healthcare staff who have contact with patients…within their area of responsibility’.

Increasing the frequency of training appears to improve the ‘effectiveness, efficiency and organisation of the response’.40 A study reviewing resuscitation skillsets at 7 and 12 months following initial training showed that self-reported confidence was higher at 7 months and skill fade was significantly less. Time to shock improved by 17 s after refresher courses.57 Consensus in high school athletics, American football and football appears to be that an annual refresher should be the minimum requirement for those trained to deliver CPR or use AEDs.33 40 42 48 53

While face-to-face training was important for the very physical practice of CPR, AED skills and understanding could be adequately delivered by computer based self-instruction.67 The delivery of CPR itself was found to be poor despite repeated training in these high school students which implies that even limited AED instruction to those personnel currently receiving BLS training would have a high success rate.66

Rationalising current British military practice

British military practice with respect to SCD is evolving, and the recently formed Defence Resuscitation Committee has a central role in providing triservice, multispeciality, multiplatform subject matter expert input into this process.

Basic life support competency is mandated as an annual refresher session for all service personnel, and this should continue. Given the importance of defibrillation in those with tachyarrhythmias, and the apparent ease with which people retain the ability to defibrillate, the inclusion of either practical or simulated AED training to as many personnel as practicable should be considered. BLS and AED training is currently mandated annually for relevant primary care personnel, but not for those most likely to be present during the initial stages of a cardiac arrest during physical training—an initial focus on providing AED training to unit Physical Training staff might rectify this.

Larsen and Hansen’s models suggest a linear relationship between delay to shock and survival from an initial success rate of between 60% and 70% with a 5%–10% reduction in survival each minute.5 This has led to a reasonable target of 3–5 min to deliver a shock as a necessary compromise between the constraints of the real world and the very real benefit of a rapid response.68 69 The defence standards policy suggests a response time of 8 min to get someone capable of delivering a shock to a site, based on standard UK ambulance service response targets. This target is not ambitious enough, and measures are being actively taken to revise this across Defence. Where AEDs are held, they should be accessible, with a local plan to enable their immediate deployment to the scene of a cardiac arrest, with concurrent activation of the emergency medical services.

Defence recommendations suggest AEDs should be held at Defence Medical Services (DMS) primary and secondary care facilities and ambulances as these might best represent the confluence of need and concentration of expertise. It concedes there may be a benefit of positioning additional AEDs, ‘at other locations, for example the guardroom or gymnasium, and providing non-DMS personnel with appropriate training’. Given the likelihood of cardiac arrest in young personnel happening during strenuous exercise,37 53 it would seem prudent that the primary placement of additional devices should indeed be gymnasiums. Furthermore, it could be argued that physical training instructors could be good targets for formal training, who would also be able to ensure an AED’s presence with any formally organised training activity such as sports matches and training sessions. This is likely to reduce the response time to closer to the more widely accepted goal of 3–5 min.

Scanning the horizon, rapid, lightweight AED deployment has been achieved using contemporary drone technology, and we should monitor developments in this space with regards to a potential solution for providing and maintaining AED cover over large areas such as military training centres.64 Unmanned aerial vehicles (UAV), such as the US Special Operations Command CQ-10B ‘Snowgoose’70 are already in service as for resupplying remote ground units. A recent design group at the University of Surrey worked up a ‘MedEvac’ UAV, capable of vertical take-off and landing, with patient-carrying capacity, although the high specifications resulted in a projected unit cost of £2.6 million, admittedly as the device is designed for tactical medical evacuation rather than AED delivery.

Future work

This document can form the basis of more up to date and ambitious guidance for AED placement and personnel training in line with current civilian practice. Auditing our current practice both against current and future guidance seems a prudent first step towards delivering world class care. Further discussion is required to cover the substantial remote area training risk while acknowledging the burden placed on already heavily laden combat medical technicians.