Hypoxic blackout in dynamic apnea: A case report

Blackout (BO) in breath-hold diving is attributed to cerebral hypoxia, yet direct observations are rare. We continuously recorded arterial oxygen saturation (SpO₂) and heart rate (HR) in 11 trained freedivers (5 females) performing two dynamic apneas (75 m, 100 m) using a waterproof forehead oximeter. One diver experienced BO at the end of a 100 m dive (SpO₂ 51%), recovering within 5 s. Group SpO₂ fell from 98±1% to 77±9% (75 m) and 68±9% (100 m; range 51–83%), while mean HR declined from 83±12 to 43±8 and 40±4bpm, respectively. No arrhythmias were detected. Within-diver SpO₂ nadirs were consistent between distances (r=0.93), whereas HR nadirs were not (r=0.40). This case confirms BO can occur at SpO₂ values around 50%, even in the absence of arrhythmia. The BO diver consistently showed the lowest SpO₂, indicating profound hypoxemia as the most likely contributing factor. Findings support individualized risk screening based on early desaturation patterns in submaximal dives. • Continuous underwater SpO₂–HR monitoring captured a real blackout event. • Blackout occurred at 51% SpO₂ without any detectable arrhythmia. • SpO₂ nadirs were highly consistent within divers across two distances. • Blackout diver showed fastest and deepest desaturation at both distances. • Early desaturation patterns may help identify individual blackout risk.