Bradycardia and the mammalian diving reflex

[Drew Sailbum]

I was discussing a freediving accident in which my friend Rick died. I had always assumed it was due to shallow water blackout. (Hyperventilation resulting in insufficient CO2 levels during ascent leading to loss of conciousness). But one of the other divers there insisted that loss of conciousness occurred at depth.

Rick was in his early thirties, and very athletic. Autopsy revealed no apparent heart disease, and listed cause of death as drowning.

The only other mechanism I could think of was slowing heart rate from the mammalian diving reflex. I don't know what Rick's resting heart rate was, but I would guess it would be low since he was so athletic.

I'm looking for a little speculation about how much the mamalian diving reflex can lower the heart rate. Could it be enough for a diver to lose conciousness?

 

[devjr]

This usually occurs when the nostrils are exposed to water. I don't see how it applies to a diver who wears a mask. Some freedivers learn to slow the heartrate through meditation or some other trick.

Your friend may have fainted from low sugar, irregular heart beat(athlete's heart), "epilepsy", thyroid disorder, or something else not related to diving reflex.

Athletes sometimes have irregular beats but I don't know if this could be inferred on autopsy, perhaps not.

Sad to lose a young diver in any manner but freedivers are out there on the edge. It's where they live, and sometimes die.

 

[rcohn]

The receptors that trigger the reflex are actua;lly found aroundthe eyes and eyebrows,

From:http://www.envir.lu.se/zoofysiol/Dyk/summary.html

Where are the receptors located?
In previous studies, it had been established that apnea in combination with face immersion in cold water results in a diving response with approximately twice the magnitude of that obtained from apnea alone. The neural input for this augmented response had been discussed, some investigators assigning a role to water receptors in the area around the nostrils. No such receptors have been found. However, in our studies it was found that skin cold receptors are responsible for the augmentation of the response during immersion. By studying the diving response during apnea when individual parts of the face were stimulated by plastic bags containing cold water , the location of the cold receptors involved in the triggering of the response in humans could be revealed.
It was found that stimulation of the forehead or eye region during apnea induced a diving response significantly greater than the response resulting from apnea alone. Taken together, the effects from chilling these two areas was equivalent to the effect from chilling the entire face. Chilling of other facial areas did not increase the response from that obtained during apnea alone. This indicates that the area innervated by the ophthalmic branch of the trigeminus nerve, e.g. the forehead and eye region, provides the most important sensory input for the augmented diving response elicited by face immersion. This location of the cold receptors involved implies that a mask covering the forehead and eyes, as is often used in modern breath-hold diving, will reduce the diving response towards that elicited by apnea alone. A suitable breath-hold diving mask should thus cover as little as possible of the forehead, just as the goggles used by traditional divers.

The most likely reason for blackout is simply a low O2 level from too long an apnea. This happens in static apnea contests and training frequently with no change in depth.

However, in ELITE freedivers there are significant cardiac arrhythmias occurring. I heard a talk by an Italian physician describing studies on several top freedivers. Heart rates fell into the 20s and they considered the arrhythmias they observed to be rather frightening.

So while it is possible that a serious arrhythmia might occur, it is more likely that he remained at depth too long for his current physical state and blacked out from lack of O2. Remember your ability to hold your breath varies with workload and other variables on a day to day or even dive to dive basis. A dive that was easy one day might kill you the next. Freedivers need to be sensitive to how they feel during each dive.

Ralph