Bradycardia & dive response

[blackwater]

In one of the popular dive manuals, I find the following paragraph:

Neither the pinnipeds nor the cetaceans ever suffer from decompression sickness (see section Two, The Physiology of Diving) because upon diving, the lungs collapse and air is pushed into nonabsorptive areas of the body, primarily the trachea. To facilitate the beginning of this diving response (called Bradycardia), the nostrils have moved....

My reading of this paragraph focuses on the breathing portion of a dive response, but Bradycardia is the slowing of the heart, which may also be part of a dive response, but is not the dive response discussed in the paragraph.

am I all wet or what?
GT

 

[pufferfish]

Sounds like they are discussing the diving response in diving mammals such as the seals, sea lions and walruses, and the whales, dolphins, and porpoises.

The diving response can differ in different mammals but many of the physiologic responses are similar such as slowing of the heart (bradycardia), collapse of the lungs (minimize absorption of nitrogen), slowing of metabolism (minimize oxygen consumption), and contraction of the spleen (increase red blood cell mass to carry more oxygen). These can all occur more or less depending on how well a mammal is adapted to diving. A well adapted human breathhold diver in cool water for example exhibits bradycardia, splenic contraction,shunting of blood to brain and heart, and metabolic slowing much like a seal.

Too bad we can't collapse our lungs though like the whales and avoid DCI. Penguins even have evolved to do 'safety stops' where instead of coming straight up they do a stepped accent to reduce probality of DCI.

Here is a cool link to a group looking at the human diving response.

Human diving response

 

[akforsyth]

Sounds like a vagus nerve response that humans get. I have slowed my HR down to about 4-6 beets per minute with this. Unfortunately, the reason this happens is because I am extremely sensitive to it and am afflicted with a condition called vasovagal syncope (aka I faint when pressure is pinpointed on my auricular artery and vagus nerve). Other organisms may have evolved such a mechanism to extend their time under water, and just like humans, the systems do not act independant of one another. The breathing response may trigger the HR response...who knows...kind interesting though...time for a little more research...

 

[John Reinertson]

There is more to the difference in DCS between humans and cetaceans than the lung collapsing.
Remember, the cetacean is a breath-hold diver. We, on the other hand, keep breathing at depth and will run 80 cubic feet of air thru our lungs on a dive. (OK, 75 when we surface with a reserve) That's a whole lot more air than even the biggest whale can run thru in a single breath. If a Whale absorbed 100 % of the nitrogen from a single breath, it would have a hard time achieving saturation. It could be done (on multiple dives) and that's where nonabsorbing spaces come in, plus some other factors that aren't completely understood.

The most proficient record-class breath-hold divers achieve nearly complete lung collapse at great depth. The dead space in the respiratory tree is at least 150 ML, (not counting nose and sinuses) in humans.
A 5,000 milliliter lung capacity will compress to about 425 ML at 400 fsw. If you take away ten percent of the inspired air as being absorbed at depth, you're down to very near complete collapse of the lungs in a deep free-diving human.

The greatest difference has to do with the fact that we keep breathing at depth, and thus absorb more nitrogen over the duration of a dive.

Keep blowing bubbles

John

 

[pufferfish]

Tonight with the wind chill the temperature is to bottom out at minus thirty which is too damn cold. Even normally fun activities like skating, skiing, or ice diving are NOT recommended today.

What better to do on a cold winters night than contemplate why whales and free divers do not get DCI. I see on Dr Bove's site he has some nitrogen modelling on whales and DCI. I'm not great with numbers but my calculation tells me a hundred foot, hundred ton sperm whale has a lung capacity of about 4600 litres. That is a heck of a lot of air or nitrogen to consider prior to a dive to 400 feet for an hour.

So tell me if the best free divers have now past the theoretical limits for thoracic squeeze and it is speculated that abdominal contents and blood enter the thoracic cavity to prevent this, at what depth will DCI become a concern for these divers? Or do we not know enough about the diving response at this depth in humans to predict?

On the subject of cold and various adaptive responses have a look at this. Not recommeded to try this one is your backyard tonight !

The wood frog (or Rana sylvatica) which is commonly found across Canada has what's called freeze tolerance. In winter, the wood frog hibernates on land, usually with their only shelter being a pile of leaves. Because this leaves it exposed to the cold, frost penetrates their skin and they freeze. Ice forms around its internal organs. Its blood stops flowing. Breathing stops. The heart stops beating and muscles stop moving. The wood frog's body functions return to normal when it thaws.


DCI and Whales

It is time to seek out my 'cobija humana' as they say in the Sierra Madres when it gets real cold

 

[blackwater]

yes, the paragraph is about dive response in aquatic animals.
Several things happen as part of the dive response (in animals and humans), but reading these sentences, it seemed to me that they were saying the pulmanary (sp?) changes were called Bradycardia, and I know that bradycardia is a slowing of the heart, so that just seemed wrong.

Apon re-reading the whole paragraph (several times), I realize the first sentence touches on the heart changes, so "the beginning of this response" was appearantly refering to the first sentance of the paragraph, not the previous sentence in the paragraph as I enterpreted it. I now feel the sentance is unclear rather than wrong.

thanks for the input
GT

p.s. Of course I'm not sure how moving nostrils facilitates a slower heart beat, so maybe I'm back to thinking that the perenthetical phrase is misplaced.