Inclusion criteria

The included videos had to display the victim during one or more of these instances: preactivity leading up to the cardiac arrest, visualisation of the victim during the presumed cardiac arrest, or characteristics of the medical intervention. In all included events, further online research was conducted to confirm the findings and outcome of the SCA based on reports from medical personnel, coaches, players, family or the surviving victim. These reports were found as additional video interviews or text information added to the videotaped events, in news articles or Wikipedia.org (since most of the victims were professional athletes with personal Wikipedia articles).

Exclusion criteria

Two videos of SCA were excluded because they did not display the victims during preactivity leading to SCA, throughout loss of consciousness or during the resuscitation.

Patient involvement

In this observational study, no patients were involved at any point to obtain the study material.

Parameters

The videos were analysed and categorised according to the following parameters: gender, race, age group, type of sport, professional or amateur sport, timing of collapse relative to competition, level of intensity just prior to collapse, position and facial features after collapse, presence of brief seizure-like activity or agonal respiration after collapse, type of intervention, timing for intervention (≤1 min=quick, >1 min=delayed, or no intervention) and survival as final outcome.

Physical intensity evaluation

The intensity of activity just prior to the SCA was divided into three different groups. Low intensity was defined as standing, walking or slow jogging. Moderate intensity was defined as faster jogging to running, however, not sprinting. High intensity was defined as sprinting, running, followed by jumping, doing rapid start and stop movements within the run of play or wrestling.

When the sportsman rapidly changed from one intensity phase to another, and then subsequently had an SCA, a cut-off time had to be set to define which intensity phase to categorise the SCA. After careful considerations of the videos, a 5 s cut-off time was chosen as this was the most feasible and demarcated best a distinct change in physical workload. For instance, if an athlete sprinted down the field, turned and starting walking and suffered SCA within less than 5 s of sprinting, this was still categorised as high intensity. If the athlete was standing, walking or slow jogging for more than 5 s after high-intensity activity, the SCA event was categorised as low intensity. In a particular group of events, a recurring observation was that SCA occurred during low-intensity activity following high-intensity activity. This subgroup of the low-intensity group was further defined to be in an ‘early recovery phase’ after high-intensity activity.

Associated signs of SCA

The victim’s body position, facial expressions, breathing patterns and movements were evaluated during the SCA. Known associated signs of SCA are seizures-like movements, agonal respiration (periodic abdominal or chest movements, abnormally deep breathing, and gasping and grunting), prone landing and eyes wide open/rolled backwards after assumed loss of consciousness.18–20 In three events, information about these associated signs was collected through statements from video interviews of two medical doctors and one school teacher actively involved in the resuscitation.

Statistics

Fisher’s exact test was used to assess the effect of treatment on survival, while Χ² test was used to assess event rates across pre-event activity levels.

Intensity level prior to SCA

A novel finding of this study was that SCA predominantly occurred during low-intensity activity, and not high-intensity activity as might be expected. It is not clear which arrhythmic mechanisms are causing SCA during these periods. However, there are theoretically several options related to the interplay of the sympathetic and parasympathetic nervous system. Exercise also affects release and regulation of intracellular and extracellular electrolytes in muscle cells, most importantly potassium, which could be influential in the mechanism of exercise-induced cardiac arrest.22

A common understanding is that SCA has a higher chance to occur during exercise compared with rest,23 as high adrenergic cardiac stimuli have the propensity to induce cardiac arrhythmias if a pathologic substrate is present. Increased sympathetic activation, elevated blood pressure and heart rate, and an altered metabolic state expected during prolonged high-intensity training may lower the arrhythmic threshold and may lead to SCA. Indeed, adrenergic cardiac stimuli have been proven to lower the threshold for cardiac tachyarrhythmias, while vagal stimuli are believed to be cardioprotective.24–26 In the current study, most cases of exercise-related SCA occurred during low-intensity activity. Additionally, nine cases of SCA occurred in a low-intensity phase preceded by a high-intensity phase. This might indicate that an athlete in such an early recovery phase after vigorous activity is at increased risk of SCA. Furthermore, it may be a partial explanation for the frequent SCA in a start/stop sport like soccer.14 The present data support this theory as eight of these nine cases occurred in swift ball games, soccer being predominant.

Within the first minute of recovery, especially after higher heart rates, vagal cardiac stimuli are increased to reduce the heart rate.27–29 Autonomic imbalance may occur which may prolong cardiac repolarisation and the refractory period, increasing the chance of a malignant arrhythmia.30

Presumably, parasympathetic stimuli to the heart may play a more significant role in the pathophysiological mechanism of SCA in athletes than previously believed. In a study of 357 athlete SCDs, 39% died during rest.31 These athletes were more likely to have normal hearts at postmortem examination compared with SCDs during exertion (54% vs 34%).31 The importance of vagal stimuli to SCA has also been raised in three large population-based studies of SCD in which a high proportion (13%–38%) of SCD occurred during sleep.13 32 33 By autopsy, SCD at night was also more often associated with structurally normal hearts.13 32 33

Furthermore, the present cases of SCD may occur in hearts with underlying ion channel abnormalities known sometimes to be triggered by increased vagal stimuli during exercise.28 33 34 Makimoto et al demonstrated that in 34/91 patients with Brugada syndrome, early recovery phase after treadmill exercise induced ST-segment augmentation in V1–V3, characteristic for Brugada syndrome.28 The typical ECG findings of Brugada syndrome were exacerbated, most likely, by increased vagal heart stimulus to slow down the heart rate. The same 34 patients showed a significantly higher risk of ventricular arrhythmias in the follow-up period of 75 months.28 Accordingly, undiagnosed ion channel abnormalities may be one explanation why SCA in athletes occurs in the early recovery phase. The frequent but varying degree of sudden unexplained death at autopsies (median 27%, range 6%–40%) in sports-related and general population studies on SCD1 6 10 13 32 33 may support this hypothesis.

At minimum, findings from the current study do not support the traditional premise that adrenergic stimuli are the direct and singular mechanism of SCA.

Resuscitation

As expected, athletes who received quick resuscitation after SCA had a higher survival rate. The survival rate in the quick resuscitation group is comparable to other studies in which prompt resuscitation and early defibrillation were achieved in exercising athletes with SCA.3 Drezner et al reported 89% survival in high school student-athletes with a witnessed SCA during physical activity.3 Marijon et al  35 also demonstrated a nearly threefold higher survival rate in athletes compared with non-athletes. This was attributed to victims of sports-induced SCA being younger, with less underlying cardiovascular disease. The events were more often witnessed, allowing for CPR to be initiated faster. The rhythm was more often shockable.

Signs associated with SCA

The frequent observation of associated signs of SCA (seizure-like movements, agonal respiration, prone landing, eyes wide open/rolled backwards after assumed loss of consciousness) provides critical information to improve recognition of SCA in athletes. Drezner et al observed seizure-like movements in 50% of the student athletes with SCA.3 Panhuyzen-Goedkoop et al reported on six professional soccer players with SCA, and all six presented with eyes wide open and a fixed gaze.21 A Japanese study of mobile emergency response teams at marathons reported that 89% of runners with SCA demonstrated agonal respirations.36 The current study found agonal respirations in 34% of the cases, while seizure-like activity and abnormal gaze were seen in 20% and 23% of the cases, respectively.

Victims presenting with these signs of SCA may lead to delayed recognition of SCA and hence postpone resuscitation.37–39 In the current study, the group with associated signs of SCA was less likely to receive resuscitation and more often delayed, compared with the group without associated signs. This highlights that these associated signs can distract a responder and the collapse be misinterpreted for something other than SCA. More education to recognise SCA in an athlete, including the associated signs, is needed and may improve survival.

Evaluating the videos for the associated signs was challenging, however, as it required good quality imaging and direct view of the victims. In the 14 videos where the typical signs of SCA were not observed, it was often due to inadequate visualisation of the event, not necessarily lack of associated signs. The presented estimates, thus, should be taken as a lower estimate. Superior image quality, longer recording time and additional recording angles would allow enhanced characterisation of the associated signs of SCA in athletes.

Medical intervention at professional sports events

Most of the videos were taken at professional sports events. Based on visual observation of the medical intervention, in some events the medical personnel were not properly prepared for the SCA. Collapsed and unresponsive victims, in whom SCA was later confirmed, were sometimes placed in the lateral decubitus position, CPR was delayed several minutes and in some cases a defibrillator was not present at the stadium. It is feasible to improve survival rates with proper and quick initiation of CPR and use of an automated external defibrillator. With more education and practical training, the number of quickly resuscitated athletes can improve, reducing mortality and morbidity from SCA in sports.

Male athletes over-represented

In accordance with a body of literature on SCA in sports,1 6 7 23 40 41 there was a large over-representation of male victims. There are several reasons why male athletes are over-represented in video-recorded cases of SCA. Male sports often have a larger television audience, hence are more often recorded. Furthermore, male sex alone could be an independent risk factor for exercise-induced SCA.23 40 41

Spontaneous recovery after presumed SCA

Four videos showed spontaneous recovery after collapse from presumed SCA. Spontaneous recovery after cardiac arrest is considered rare. In these victims, the cause of collapse and brief loss of consciousness may have been from self-terminating ventricular tachycardia. Two of these victims died of SCD within a short time after the initial SCA; one victim the same day, the other victim 3 months after the initial event. These two examples illustrate the importance of extreme caution and a comprehensive cardiac evaluation in athletes with a non-traumatic collapse during exercise as an elevated risk of cardiac arrhythmia may still be present.

Study limitations

The identified cases are limited to video-filmed sports, creating an inherent bias towards events from the most popular and professional sports. Surely, many cases of SCA in athletes were missed. While the investigators used all available online resources to confirm the cases included were from SCA, an absolute guarantee that all victims entered SCA immediately after their collapse cannot be given. Other reports and interview statements from people involved with the resuscitation were used to confirm outcome of the events, and to acquire information regarding some parameters if the video recordings did not display sufficient information. Furthermore, the evaluation of physical intensity level and the possibility of vagal stimuli is hypothesised, but not confirmed, by physiologic measures. As the athletes were altering between different intensity levels, a cut-off period was necessary to define the intensity level that predominated just prior to SCA. Based on a thorough observation of all the videos and taken into account that a majority of the videos were short, a 5 s period was set as the cut-off to demarcate a change from one level of intensity to another. However, the level of autonomic nervous stimulation will certainly overlap between the intensity levels. Lastly, the degree of psychological stress the athletes were under, which also influences the autonomic balance, could not be assessed.