Breath-hold diving (BHD) is becoming increasingly popular. 2 recent articles published in DHMJ shed more light on the physiology of apnea diving.
Breath hold divers typically conduct repetitive dives at moderate depths with short surface intervals. A rule of thumb is that recovery intervals should be twice as long as the single dive. Mulder et al. investigated whether this recommendation is scientifically tenable. They had six amateur breath hold divers perform three dives to 11 m of freshwater with surface intervals of 2 minutes and 30 seconds. Using an underwater pulseoximeter, they determined peripheral oxygen saturation (SpO2) and heart rate (Hf). This resulted in a median dive time across all dives of 81.5 seconds. Baseline Hf was median 76/' and decreased to median 40.5 - 48.5/' during the dives. Peripheral pre-dive SpO2 was 99.5% (median). The behavior of the SpO2 immediately after the dives is interesting. It decreased successively. If the lowest SpO2 after the 1st dive was 97%, it decreased to 83.5% after the 2nd and 82.5% after the 3rd dive to hypoxic values. It recovered within 20 seconds after each dive.
Oxygen debt already from the 2nd repetitive dive
This result is remarkable. The fact that a drop in SpO2 of this magnitude already occurs during the first 2 repetitive dives gives food for thought and means that safe diving is not guaranteed in this mode. It seems that during the repetitive dives an oxygen debt is built up which cannot be repaid during the surface intervals.
In practical terms, this means that the recommendation for the length of surface intervalls during BHD needs to be reconsidered. For full recovery and to ensure safety during more extensive dive series, longer surface breaks should be taken. How long these should be chosen in the individual case remains open for the time being.
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