Abstract Body:
Background :
Sudden cardiac arrests (SCA) during endurance events remain rare but tragic. We hypothesized that an acceleration in the final kilometer of a race may lead to a higher risk of SCA.

Methods:
We integrated data from two independent sources. The first source was a cardiac arrest registry of 55000 cases, which prospectively collected all out-of-hospital cardiac arrests in individuals over 18 in one city and its inner suburbs between October 2011 and April 2024. The second source included participation and performance records from two major running events from the same city (the half-marathon and a 20 km race) during the same period. Participant acceleration was measured by comparing the speed over the last kilometer with the average speed of the preceding 5 km, utilizing detailed timing data from the half-marathon. SCA cases were precisely mapped along the race routes. We calculated the mortality incidence rate ratio in the last kilometer of a half marathon by comparing the death rate per kilometer in this segment to the rest of the race. A 95% confidence interval was derived using a log-transformation and normal approximation.

Results:
Out of over 714 856 race participations, 12 SCA cases were identified, with 9 (75%) occurring during the last km and two (16.7%) additional just before the last kilometer, setting the risk of experiencing SCA in the final kilometer over 60 (95% Confidence Interval : [16 - 222]) times higher than during earlier segments of the race (Figure 1). Analysis of acceleration revealed that 87% of half marathon finishers increased their pace in the final kilometer.

Conclusion:
The study shows 60 times higher SCA risk in the last kilometer compared to other segments of endurance races. This risk, correlated with runner acceleration, suggests that a physiological strain during this phase may significantly contribute to the higher SCA risk. Emergency units should stay close to the finish line in these events to be able to maximise survival.