Musing on DR

[cdavis]

Musings on DR


Dive reflex(DR) has been talked about a lot, but I think we don't fully comprehend all that is going on. Below are some musings/observations. I'd appreciate your thoughts, comments, corrections, etc.


It has become clear that the key to a long dive is dive reflex. The faster and harder it sets in, the longer the possible dive. Given that, how to strengthen DR and bring it on faster become a critical issues for freedivers, maybe THE critical issues. The competitive types have found a way to do that, the no warmup approach, effective but uncomfortable. The FRC types have found another way, more comfortable, but tricky and requires practice. What else is out there? What ideas can improve onset and depth of DR?



Dive reflex probably encompasses a lot of physiological changes, but the easiest one to see is bloodshift/heart rate drop. As c02 rises in the blood stream, the body begins cutting off circulation to the extremities(skeletal muscles?), shifting blood supply to the core , thereby conserving 02 for use of the brain. Heart rate falls, probably because it doesn't need to pump as hard with less blood going to the extremities. Other things, like facial immersion in cold water, also trigger bloodshift to some degree.


We notice bloodshift when it gets substantial and most discussions treat it as not happening until c02(and urge to breathe) become substantial. The implication is that blood shift doesn't start until c02 gets high enough to be uncomfortable. I think that is a bit simplistic and bloodshift and other DR impacts can begin slowly and early. You can think of it using 3 examples below.

1. Hyperventilate, blow off a lot of c02, and the body doesn't know it needs to conserve 02 so there is little or no DR until late in the dive. At that point, c02 is rising fast, 02 may be dropping and the urge to breathe, contractions, and probably blood shift come on hard and fast. Discomfort rises fast and cuts the dive short. Good thing cause 02 could be dropping fast and the diver might be in danger of BO.
   

2.No warm up. The diver carries much more c02 down with him than in 1. Urge to breath comes on quickly and gets uncomfortable early, setting off a strong DR and conserving 02. The result is a longer safer dive than 1, but very uncomfortable.

1. 3. Diving about ½ lung with minimal breathup(air volume, not minimum time). This is serial diving, but with the breathup of a no warmup approach. Like 2, the diver is taking down more c02 than 1, but also promoting DR by having much less than full lungs and doing minimal exercise in the early part of the dive.

Number 3 is the area of my experience. When you do a dive like 3, the DR seems to come on early, but more slowly than in 2. What feels like blood shift begins well before the urge to breathe gets uncomfortable. Urge to breath and contractions begin softly and increase much more smoothly than 1 or 2. Initially, time of onset of contractions is early, as in #1, but,with frequent training, time to onset gets longer and longer. Heart rate falls, but more slowly than the no warm up approach, consistent with the idea that DR is coming on slowly. This is serial diving, so the body is adjusting to it(part of DR?) and you won't get the very strong, fast DR as in 2. Never the less, you get very long dives and much more comfortable ones. Done right, blood shift is cumulative over a series of dives and that probably improves overall DR dive time. There also seems to be a training effect where DR improves with practice, although this could be other factors masquerading as DR. At any rate, with training, blood shift seems to come on earlier and earlier, with less and less urge to breathe. Comfortable dives get longer and longer.


So what is going on? It appears to me that DR is a good bit more complex than I understand or than has been discussed on these forums.


A. Anybody out there care to explain that complexity?



It would seem to me that getting the DR to kick in early, especially before much discomfort sets in, is what serial divers should be striving for. Diving ½ lung or less seems to promote that but it also would be a huge benefit to full lung divers.


B. How can we get DR to kick in stronger and earlier, especially before discomfort sets in?

 

[Apneaddict]

While tricky to "get right", I find that temperature plays a key role.

If you're "just about" to get chilly but well before you shiver... It kicks in HARD.

I can't find a way to do this reliably / repeatedly when out in open water and spearfishing, but can work for a max attempt / competition dive.

 

[manttone]

In the #3 you will probably build up slowly blood buffer levels and you will also have more red blood cells in your circulation due splenic contraction (with short intervalls they are not recovered). Furthermore, repeated dives will have a mental effect. You will be more relaxed and this will help you to block the discomfort from your mind. Those thing probably cause major differences between #2 and #3. #1 is never an option .

 

[MarcinB]

Cdavis, you are right, the dive reflex is very complex. In fact it's not a single reflex but a combination of several reflexes interacting with each other. This is why physiologists call it the dive response. One of the reasons why DR looks different during FRC dive is that lung stretch receptors are much less stimulated compared to full inhale. Activation of these receptors inhibits DR. Moreover, the extent to which we inhale air strongly affects blood volume and pressure in the pulmonary circulation. In my opinion it can also contribite to these differences.

 

[cdavis]

Manttone: I'm unfamiliar with blood buffers. My limited understanding is that they build up over the course of a diving session, making the body able to handle comfortably higher levels of Co2. Does frequent diving/training also build buffer overall buffering ability so that you start a dive session with more ability to buffer?

Marcin: What else in DR besides bloodshift is promoted by a smaller inhale( or by anything else we could do). I've spent a good deal of time experimenting with breathup, inhale volume, relaxation and exertion during the dive. At least in exhale diving (FRC) getting the longest comfortable dive(and best dive response) seems to be a balance of those things. As Apeaaddict said, hitting the window between not cold enough and too cold also makes a big difference. I wonder what else I could do to promote DR?

 

[MarcinB]

I think you have already experimented with the most important factors. The only thing that comes to my mind is facial cooling. But this works only when you submerge your face for the first time so it is useful only in the case of maximal attempts in the pool. Exposing your eyes and nostrils to water is quite effective. It is also a good idea to warm your face beforehand.