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Free divers' heart rates can drop as low as 11 beats per minute
Yasemin Saplakoglu 1.7.21

The world's best free divers can survive brain oxygen levels lower than those found in seals, according to a new study.
Free-divers (without breathing gear) can hold their breath for >4 minutes, and descend to ocean depths of >100 m.
But this endurance feat takes a toll on the body's ability to pump oxygen through the blood & to the brain (if not enough oxygen goes to the brain, free divers are at risk of losing consciousness).
Erika Schagatay (prof animal physiol. Mid-Sweden Univ):
"Before now, understanding the effects on these exceptional divers' brains & cardio-vascular systems during such deep dives, and just how far these humans push their bodies, was not possible:
all research was done during simulated dives in the lab.
The diver can reach a point where hypoxic (low-oxygen) blackout occurs, and the diver then needs to be rescued.
One of the main aims of the research is to warn the diver & safety personnel of an imminent blackout."

To understand how this extreme feat affects the human body, Schagatay cs adapted a bio-medical device, previously developed by the Dutch company Artinis Medical Systems, to withstand extreme ocean pressures.

The bio-medical device (typically used to measure brain function) fires 2 different wavelengths of light from LEDs onto the divers' foreheads to measure heart rate & oxygen levels in the blood & in the brain, according to a video about the research. The device worked at depths of at least 107 m, according to the statement.

They found that the free-divers who reached those depths had brain oxygen levels that dropped to levels lower than those found in seals, some dropped as low as 25 %.
Chris McKnight (research fellow Univ St/Andrews' Sea Mammal Research Unit):
that's "equivalent to some of the lowest values measured at the top of Mount Everest."
Brain oxygen levels are typically c 98 %,
if they drop below 50 %, a person is almost certain to lose consciousness.
McKnight:
They also found that divers' heart rates dipped as low as 11 beats/minute.
As divers descend, their heart rates begin to decrease to help preserve blood-oxygen levels, according to the video.

The divers' heart rates were as low as those of diving seals, whales & dolphins.
These marine creatures are some of the world's best athletes, e.g.
elephant-seals can hold their breath for 2 hrs underwater to hunt for food (The Conversation).
McKnight:
"Beyond the exceptional physiological responses that free-divers display, and the extremes they can tolerate, they may be a very informative physiological group.
Their physiological reactions are so unique, and the conditions they're exposed to are not easily replicated, so they offer a unique way of understanding how the body responds to low blood oxygen, low brain oxygenation & severe cardio-vascular suppression."

The findings can thus also inform researchers on how to protect the hearts & brains of patients who undergo surgical procedures or experience cardiac events.

Originally published on Live Science.