Low SAC rate and hypothermia

[SeaHorse81]

Being a somewhat diminutive female with enough diving experience to not be a total newbie, I have a very low SAC rate. I occasionally dive with someone who consumes air at a similar rate, but have encountered a rare few who actually consume less. I'm hoping I'm not a CO2 retainer because I take really long, deep breaths while on a regulator.

I also have an unnatural tendency toward hypothermia, requiring a 7 mil suit to just manage to get by diving in water temps where some others need no exposure protection at all (i.e., Caribbean waters).

I never considered the possibility of a connection between the two until yesterday, when an instructor who was aware of my issues with cold said, upon learning my SAC rate, "No wonder you have so much trouble with hypothermia." When I asked him what he meant, he said that there wasn't enough oxygen getting into the system to "fuel the furnace."

As yesterday's course was conducted in conditions I would never choose for casual diving (70 degree water, low 70s air temp, no sun) I got the joy of one mild/moderate hypothermic episode for each of the two dives I did. This is not unusual, sadly, and happens despite every effort on my part to mitigate it. (Yes, I know I could dive dry -- that's another discussion.)

I noted something yesterday that I know I have experienced every time I've been hyporthermic, but never thought much about: I breathe very, very heavily when I'm super cold, as if I'd just run a mile. The heavy breathing starts well before the shivers and body tremors, and continues until I've gotten enough core temp back for the tremors to just about finish up. In light of the instructor's comment, I could see an argument that the system is demanding high oxygen input as it attempts to generate heat.

Does anyone know more about this? Does anyone else experience it?

 

[Thalassamania]

It is not unusual for smaller people to chill much faster than larger ones, as an oversimplification: mass goes up as the cube of a change in height while surface area goes up at the square, so as you get shorter (for the same basic body shape) the ratio between heat transfer from your surface area and thermal mass will result in more rapid chilling. Shivering is an attempt by your body to warm up, thus you burn more calories which takes more oxygen. Nothing unusual there.

 

[DMDavid]

Interesting! Very interesting!
When I was much younger and slimmer I needed a wetsuit to dive the local springs.

Now that I carry and extra 60 lbs of natural blubber I no longer feel the need for a wetsuit in the (68f) springs.


FB-Florida Scuba Diver

 

[Thalassamania]

Mass varies as the cube, strength and surface area vary as the square. Now you understand why many dinosaurs were so large, why no mammals are smaller than a shrew and why terrestrial arthropods are not so big that they rule the universe (Starship Troopers type "bugs" require a low gravity high oxygen planet).

 

[knotical]

Although there might be a correlation between SAC and thermal tolerance, I don't accept the oxygen-as-fuel hypothesis. I will speculate that with a low SAC you are not exerting much effort, and therefore not generating much internal heat

On a more practical note, I found that wearing a hood, even in the tropics, dramatically reduced my tendency toward hypothermia.

 

[SeaHorse81]

My air consumption definitely goes up with more effort, as does my ability to stay warm. Drift dives are a killer for me thermally, for example, because I'm just not moving. I've never gotten cold while working against a bit of current (in otherwise warm waters). Relative to most other divers on a given dive, my consumption is pretty low.

I wear a 5mm hood with the 7mm suit, and dedicate all surface interval time to restoring and conserving core heat as much as humanly possible. I don't see anybody else, including women of roughly my size and age, having to struggle with this, this much. Maybe I'm just an unfortunate outlier on the sprectrum of such things.

 

[buiszie]

When u dive more and getting more experienced ur air consumption will improve. In my experience it didnt went with i am getting colder.For me getting colder comes when i dive more than once in a day and a few weeks after each other or when i am little bit sick or not feeling well.

 

[Mr Carcharodon]

I tend to agree with Thal on this. You get cold because you have relatively more surface area to volume compared to larger people. Also people with relatively smaller respiratory systems tend to have lower SAC rates since the dead spaces that do not exchange gas are smaller too. Go get yourself a dry suit. Once you get the undergarments sorted out you will be warm and happy (even if your pocketbook is not).

By the way it is nonsense that you are cold because you are breathing too slowly. Breathing is driven by the need to eliminate carbon dioxide and not the need to get oxygen. Hemoglobin is close to saturated for healthy individuals and you can not be more than 100% saturated by breathing faster.

 

[spectrum]

SeaHorse81

If you are executing the dives and not ending up with a headache then you are just in the groove. Your body is as active as it needs to be. You are probably efficient, very at ease and are in at least reasonable shape. Rejoice in your low SAC.

Getting chilled I see as a separate though intertwined issue. Sure, if you were flapping around, swimming like a nutcase and making random movements there would be more muscle activity, energy burned and heat generated but the fact is that your diving is right where you want it at that barely aerobic level.

Once you do hit that moderate hypothermic point and begin to shiver you are dealing with you body responding to a threat. That's not something you want to emulate

You can go heavier on the wetsuit, assuming what you have is a good snug fit or go dry. Everyone's sensitivity to heat loss and excess thermal energy is unique. You just happen to be burning just enough to get by hence the low SAC and the predisposition to the chills.

Pete

 

[j2s]

You said you wear a hooded vest and I'm assuming you wear gloves and boots too ??....Try bringing a thermos of hot chocolate w/you [triple strength] no tea or anything w/caffiene/alchohol...And drink some before you enter the water [save some for the second tank].....It's a simple way to keep you warm(er)......