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Hypoxic blackout in dynamic apnea: A case report
Eric Mulder1, Isak Löfquist2, Felix Schagatay3
1Department of Health Sciences, Mid Sweden University, Sweden; Swedish Winter Sport Research Centre, Mid Sweden University, Östersund, Sweden.
The Journal of Physiological Sciences : JPS
|January 30, 2026
Summary
Blackout in breath-hold diving can occur at 50% arterial oxygen saturation (SpO₂), even without arrhythmias. Early desaturation patterns in submaximal dives may indicate individual risk for blackout.
Area of Science:
- Physiology
- Sports Science
- Diving Medicine
Background:
- Blackout (BO) in breath-hold diving is linked to cerebral hypoxia.
- Direct observation of BO events and associated physiological changes is limited.
- Understanding the physiological triggers of BO is crucial for diver safety.
Purpose of the Study:
- To investigate arterial oxygen saturation (SpO₂) and heart rate (HR) during dynamic apnea.
- To document physiological responses leading to blackout in trained freedivers.
- To identify potential predictors of blackout in breath-hold diving.
Main Methods:
- Continuous SpO₂ and HR monitoring using a waterproof oximeter in 11 trained freedivers.
- Performance of two dynamic apnea distances: 75 meters and 100 meters.
- Case observation of one diver experiencing blackout during a 100-meter dive.
Main Results:
- Group SpO₂ decreased significantly with increasing dive distance, reaching a mean of 68% ± 9% at 100m.
- Mean HR declined substantially during both dive distances, with no detected arrhythmias.
- One diver experienced blackout at 51% SpO₂, with rapid recovery.
- SpO₂ nadirs were consistent within divers across distances, unlike HR nadirs.
Conclusions:
- Blackout can occur at SpO₂ levels around 50%, primarily driven by profound hypoxemia.
- Absence of arrhythmias does not preclude blackout in breath-hold diving.
- Individualized risk assessment using early desaturation patterns during submaximal dives is recommended.
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