Marathon runners’ risk of cardiac arrest is relatively low – equivalent to or lower than that of other athletes engaged in vigorous activity, according to a report in the Jan. 12 issue of the New England Journal of Medicine.
The number of cardiac arrests related to marathon (26.2-mile) and half-marathon (13.1-mile) races has increased in recent years, but that is largely explained by the increase in the number of people who have taken up the sport, said Dr. Jonathan H. Kim of the division of cardiology, Massachusetts General Hospital, Boston, and his associates.
"The growth of long-distance running has been accompanied by studies documenting post-race cardiac dysfunction and numerous reports of race-related cardiac arrest. These unexpected tragedies attract considerable media attention and have led to concerns regarding the health risks of this activity," the researchers wrote.
Until now, no large studies have examined the incidence, clinical profiles, and outcomes of cardiac arrests that occur during or immediately after long-distances races. The Race Associated Cardiac Arrest Event Registry (RACER) collected data to facilitate such studies. It included information on all marathon or half-marathon races held in the United States between 2000 and May 2010.
Dr. Kim and his associates in the RACER study group identified 59 marathon-related cardiac arrests (out of 10.9 million registered marathon runners) during this period and contacted survivors, as well as the next of kin of nonsurvivors, to ascertain demographic data, exercise history, personal and family medical histories, and pertinent medical records.
The overall incidence of cardiac arrest was 1 per 184,000 participants, and that of sudden death was 1 per 259,000 participants. This translates to 0.2 cardiac arrests and 0.14 sudden deaths per 100,000 runner-hours at risk, assuming an average running time of 4 hours for marathons and 2 hours for half-marathons.
"Thus, event rates among marathon and half-marathon runners are relatively low, as compared with other athletic populations, including collegiate athletes (1 death per 43,770 participants per year), triathlon participants (1 death per 52,630 participants), and previously healthy middle-aged joggers (1 death per 7,620 participants)," the researchers wrote (N. Engl. J. Med. 2012;366:130-40).
The absolute number of marathon-related cardiac arrests rose each year during the study period, but the incidence remained stable because of an annual increase in runners from fewer than 1 million in 2000 to approximately 2 million in 2010, according to the investigators.
Men were more likely than were women runners to have a cardiac arrest or sudden death (0.90 per 100,000 vs. 0.16 per 100,000), which the authors noted is "consistent with reports on other populations and reaffirms a male predisposition to exertional cardiac arrest."
The distance of a race was a key determinant of the risk of cardiac arrest, since rates were three to five times higher during marathons (1.01 per 100,000 runners) than during half-marathons (0.27 per 100,000). "A possible explanation is that longer races involve more physiological stress and thus a higher likelihood of precipitating an adverse event in a predisposed participant," the researchers wrote.
The overall case fatality rate was 71%. Sufficient information was available to determine the cause of cardiac arrest in only 31 of the 59 cases.
The most frequent cause of death was hypertrophic cardiomyopathy (8 cases) or possible hypertrophic cardiomyopathy (7 cases). This is also the primary cause of death in young competitive athletes, they pointed out.
"Notably, 9 of the 15 nonsurvivors who had cardiac hypertrophy had an additional clinical factor or postmortem finding: obstructive coronary artery disease (in 3), myocarditis (in 2), bicuspid aortic valve or coronary anomaly (in 2), accessory atrioventricular nodal bypass tract (in 1), or hyperthermia (in 1)," the researchers wrote. The race-related disorders of hyperthermia and hyponatremia, which led to the death of one runner without cardiac hypertrophy, thus are not common causes of cardiac arrest and sudden death, they added.
Among the eight survivors of cardiac arrest, ischemic heart disease was the most frequent cause of the arrest. The strongest predictor of survival of cardiac arrest was bystander-administered CPR, underscoring the importance of onsite medical services.
It was "surprising" that none of the runners with serious coronary atherosclerosis had angiographic evidence of acute plaque rupture or thrombus, because previous studies as well as expert consensus statements have suggest that exercise-induced coronary syndromes result from disruption of atherosclerotic plaque and coronary thrombosis. "In contrast, our findings suggest that demand ischemia (i.e., ischemia due to an imbalance between oxygen supply and demand) may be operative in exercise-related acute coronary events during long-distance running races," they wrote.