Arterial oxygen saturation and diving response during dynamic apneas in breath-hold divers

Competitive breath-hold divers try to achieve maximum times, depths, or distances underwater, thereby risking hypoxic syncope. In the present study, the cardiorespiratory responses to dynamic apnea (simultaneously initiated apneas and dynamic leg exercise) were investigated in 10 breath-hold divers. The divers performed 60 s dynamic apneas with the face in air (A) or face immersed in cold water (AFI). During apneas, the arterial oxygen saturation was reduced (A: -10%), but to a lesser extent during AFI (-6%, P < 0.01), reaching a nadir 10-15 s post-apnea. Also, changes in end-tidal O-2 and CO2 pressures (PetO2/PetCO2) were smaller during AFI than A (Delta PetO2: 8.2 vs 8.7 kPa, P < 0.01; Delta PetCO2: 3.1 vs 3.2 kPa, P < 0.05). The heart rate was lower during AFI than A (66 vs 78 bpm, P < 0.01), reflecting an augmented diving response during AFI. The maximum safe breath-hold time under the conditions of the present study was calculated to be 101 and 106 s for A and AFI, respectively, consistent with the dynamic apnea times achieved by world-class apnea divers. It is concluded that the augmented diving response during face immersion apneas is associated with a slower reduction of the pulmonary (and arterial) oxygen store, probably delaying the occurrence of a hypoxic syncope.