Hydration Revisited – Myths Versus Facts

[dreamdive]

The majority of divers drink all this water before the dive because they believe that by being hydrated they avoid getting bent! I wrote the article to educate how many divers have this backwards.

 

[Duke Dive Medicine]

And does over hydrating increase risk for for Immersion Pulmonary Edema (IPE)? That is another important question.

We are finding that overhydration is definitely correlated with increased risk of IPE in susceptible individuals.

The normal humidity at the bottom of a healthy human lung should be pretty high -- nearly 100%. The amount of water vapor inside a clean, topped off AL80 should be nearly zero, if the water traps in the compressor room were working correctly.

When we dive, we suck the dry air in. Huge amounts of moisture leave the moist lung tissue into the inhaled dry gas. Our lungs act as humidifiers.

Then we blow the humidified gas out into the water column and inhale some more dry air.

For me, the wonder is not that a few people get IPE. The amazing thing to me is that everybody doesn't get it.

It looks like you're saying that breathing dry air is a risk factor for IPE. Can you expand on that?

Best regards,
DDM

 

[TSandM]

It's a good question and one that has been proposed as a possibility especially in people who inadequately vasodilate their pulmonary vasculature. . . .

In layman's terms: When you exercise, your muscles need more oxygen and glucose, so your cardiac output needs to increase. Blood going out of the left side of the heart to feed the muscles has to come through the pulmonary vascular bed to pick up oxygen, and gets pumped through there by the right side of the heart. Normally, the pressures in the pulmonary vessels are quite low, and the right side of the heart is designed to pump against very little resistance. It isn't as thick or as strong as the left side, which pumps against the entire arterial system of the body. When the output from the right heart goes up, if the pulmonary vessels stayed the same, the pressure head would increase and the right heart would have trouble coping with it. So, in most people, when they exercise, those vessels relax and increase their capacity, keeping the pressures more or less the same.

Some of the IPE research has suggested that people who are susceptible to the problem may not expand these vessels to the degree necessary. High pressure heads, in addition to straining the heart, can also cause fluid to leak out of the capillaries and into the air spaces, which is what IPE is.

 

[dreamdive]

In layman's terms: When you exercise, your muscles need more oxygen and glucose, so your cardiac output needs to increase. Blood going out of the left side of the heart to feed the muscles has to come through the pulmonary vascular bed to pick up oxygen, and gets pumped through there by the right side of the heart. Normally, the pressures in the pulmonary vessels are quite low, and the right side of the heart is designed to pump against very little resistance. It isn't as thick or as strong as the left side, which pumps against the entire arterial system of the body. When the output from the right heart goes up, if the pulmonary vessels stayed the same, the pressure head would increase and the right heart would have trouble coping with it. So, in most people, when they exercise, those vessels relax and increase their capacity, keeping the pressures more or less the same.

Some of the IPE research has suggested that people who are susceptible to the problem may not expand these vessels to the degree necessary. High pressure heads, in addition to straining the heart, can also cause fluid to leak out of the capillaries and into the air spaces, which is what IPE is.

Thank you for the nice clarification. I meant to answer it but got side tracked

 

[ev780]

My understanding is that the sense of thirst lags behind the body's need for water, so by the time you feel thirsty you are already insufficiently hydrated. I don't know whether that's exaggeration/myth. I try to drink water a little at a time throughout the hours preceding a dive so that by the time I get in the water I don't feel thirsty. However, I suppose I'm "guilty" of taking a swig of water just before the dive, in the belief it's going to provide some extra measure of insurance.

From an evolutionary standpoint this does not make sense to me. The reptilian brain will tell you "drink stupid, you need water" and we will. I think we may drink the wrong thing or maybe we have trained ourselves to ignore the signals but I have a hard time believing that there is a lag of any metabolic significance. The body has adequate capacity to withstand short term variation in fluid levels. I come down more on the myth side but again I only have reasoning no data.

 

[tech_diver]

Random thoughts...

The data seems mixed (to my civilian eyes.) Could it be that we are looking at competing systems? The state of hydration could play a different role during the 'ongassing' phase of the dive then it does during the 'offgassing' phase? Maybe dehydration is better during the ongassing?


I've always thought, "I can never be too hydrated." But now, IPE is making me rethink that.


The article talks about the pressure of the surrounding environment affecting fluids in the body. Could the osmotic effects come into play? A human in fresh water is absorbing water. A human in sea water in losing water. Right?


The hydration thing is something I've believed for years, like the reverse profile thing. Even as contrary evidence starts to become available, it will take me a little time to get past my superstitions.

 

[dreamdive]

Random thoughts...

The data seems mixed (to my civilian eyes.) Could it be that we are looking at competing systems? The state of hydration could play a different role during the 'ongassing' phase of the dive then it does during the 'offgassing' phase? Maybe dehydration is better during the ongassing?


I've always thought, "I can never be too hydrated." But now, IPE is making me rethink that.


The article talks about the pressure of the surrounding environment affecting fluids in the body. Could the osmotic effects come into play? A human in fresh water is absorbing water. A human in sea water in losing water. Right?


The hydration thing is something I've believed for years, like the reverse profile thing. Even as contrary evidence starts to become available, it will take me a little time to get past my superstitions.

First of, thank you for your post.
Yes, it has been postulated that mild to moderate dehydration aids in less ongassing. The proposed mechanism cited was a decrease in cardiac output. Advocators of hydration cite that if blood is too viscous (due to dehydration), there is inadequate or less-than-optimal perfusion for offgassing.

Regarding your question of osmotic effect: A 2001 published study by Hope, Aanderud, and Aakvaag looked at the difference of water loss due to sweating in salt water and fresh water. They concluded that one sweats more in salt water than fresh water and thus fluid losses are greater in salt water.

Here is the article citation:
Hope A, Aanderud L, Aakvaag A - Dehydration and body fluid-regulating hormones during sweating in warm (38 C) fresh- and seawater immersion
J Appl Physiol 2001; 91: 1529-1534

I believe this article might still be available free of charge. Look for it using "Scholar.google.com".

Regards,

Claudia Roussos MD

 

[drrich2]

People in the medical field often view the body in terms of compartments; an intravascular compartment (your blood), an extravascular compartment (outside the blood vessels) & an intracellular compartment. And the adipose compartment (fat). I'm coming up with my own names; not inclined to go look it up for formality's sake.

Water is a substantial component of the 1st 3 I named. Therefore, all 3 (blood, tissue fluid, inside the cells) will have hydration as a factor.

I'm not sure how quickly hydration equilibrates across compartments, or in which it is most important to minimize DCS risk, if that is indeed a factor.

Any ideas on the current thinking in this regard?

Oh, and since getting submersed is said to shunt around 700 ml of blood to the heart, which then tries to trigger diuresis (peeing off perceived extra fluid), I wonder what drinking a liter of water right before jumping in does in the context that the body is trying to cut water volume, not absorb more?

Richard.

 

[boulderjohn]

I just want to comment that I think this is a valuable topic considering the degree to which it is discussed in diving circles without anyone having good access to solid information, and I am glad this thread was started.

 

[DocVikingo]

Hi tech_diver,

Skin consists of 3 main layers, internal to external they are hypodermis-->dermis--> epidermis. Blood vessels only extend to the top of the dermal layer. Epidermal cells die as they migrate outwards because they lose access to blood circulation and in doing so function as a "waterproofing" layer with a low permeability. It is a rather effective barrier against most nano-sized particles. There are other barriers to osmosis via skin as well, which I won't get into here. But, in short, in submerged humans the osmotic movement of H2O through the skin is trivial.

Apparently sweating u/w may be another matter, but one that won't be adequately answered for the typical rec diver by the above cited article "Dehydration and body fluid-regulating hormones during sweating in warm (38 C) fresh- and seawater immersion" as that study involved commercial divers wearing open hot water suits that continuously circulate heated seawater over the skin surface.

As regards, "I can never be too hydrated," that is largely correct. One can over-hydrate and develop water intoxication from the extreme washout of blood sodium, but this typically occurs only under unusual circumstances, such as in marathon runners who overdo the hydration thing. However, IPE is not the result of over-hydration and very likely is little affected by such. IPE appears to be a clearly pathological condition caused by an as yet not fully understood mechanism(s) that results in capillaries leaking fluid into interstitial spaces of the lung.

Regards,

DocVikingo