Background Exertional heat stroke (EHS) is a potentially life-threatening condition to the endurance athlete. Early recognition and rapid on-site treatment with aggressive cooling measures significantly reduces mortality, improves outcome and avoids hospital transfers.

Objective Determine the incidence of EHS in triathlon competition.

Design A retrospective analysis of competitor medical records.

Setting Data was collected across two UK-based triathlons over six years (11 consecutive triathlons).

Patients (or Participants) All casualties diagnosed with EHS by the Triathlon Medical team (TMT). Diagnostic criteria of EHS included a core temperature >40°C and evidence of an altered mental state indicating neurological dysfunction.

Interventions (or Assessment of Risk Factors) Age, gender, race distance, co-morbidities and medical management required were recorded.

Main Outcome Measurements Incidence of EHS amongst triathlon competitors.

Results Of 68557 starting competitors, 429 casualties presented to the TMT. Twelve (2.8%) were diagnosed with EHS, giving an incidence of 1.74/10000 starters. Eight EHS casualties (67%) collapsed during the run phase, with three collapsing after the finish line. One EHS casualty presented during the swim phase in cardiac arrest. This patient was successfully resuscitated with CPR before ice-bath cooling for EHS. Four (36%) EHS casualties were discharged directly from the TMT and the remaining transferred to hospital.

Conclusions EHS is a significant risk in mass-participation triathlon events. EHS can be successfully managed on-site with aggressive cooling measures which may reduce hospital transfers. The majority of EHS cases occur during or after the run phase and therefore we recommend event organisers focus both medical resources and prevention strategies at this phase.