Dehydration and Diving


Divers are almost always dehydrated during/after.

The body is 40 to 75 percent water and we need to maintain a relatively constant amount of water in the body.  Dehydration is simply defined as a reduction in water content.  The question to ask is how much water loss can we tolerate? Divers frequently start a dive with some amount of dehydration and become more dehydrated during and after the dive.

Urine must be produced to eliminate the nitrogen waste from metabolizing the protein in our food.  Therefore, eating a high protein diet will increase the amount of urine that must be produced and predispose you to dehydration.

The easiest way to understand and determine your state of hydration is to observe the color of your urine.  Dark yellow/orange urine means that your kidneys are trying to conserve water and are concentrating the urine as much as possible.  This means you are dehydrated.  Very light yellow or clear urine means that your kidneys are eliminating excess water from the body and that you are well hydrated.  At the same time, if you’re producing only a small volume of urine you’re dehydrated; if you’re producing a lot of urine you’re well hydrated.  These generalizations are usually true but there are a few medical situations where they do not apply.

If you are chronically dehydrated and producing concentrated urine you increase your risk of forming kidney stones and your risk of developing urinary tract infections.  Therefore, everyone should attempt to stay well hydrated and produce large volumes of dilute urine.  Drinking two liters of water daily (in addition to whatever else you drink) will achieve this goal in most people in temperate climates.

If a person loses one percent of their body weight due to dehydration, their rectal temperature will be significantly higher during exercise compared to the same exercise without dehydration.  In one experiment, 1.9 percent dehydration was associated with a 22 percent reduction in endurance performance and a 10 percent reduction in maximum oxygen utilization.  In the same experiment, 4.3 percent dehydration resulted in a 48 percent reduction in walking endurance and a 22 percent reduction in maximum oxygen utilization.  To put this into perspective, an athlete exercising on a hot day can easily lose 5 percent of his or her body weight.  Marathon runners frequently lose six to 10 percent of their body weight during a competition and competitive wrestlers/weightlifters can intentionally lose nine to 13 percent of their body weight before a competition to make their weight class!  Needless to say, both groups of athletes attempt to rehydrate as much as possible before and during competition.

Mild dehydration of five percent is usually accompanied by increased heart rate, dry mouth, decreased urine output, loss of appetite, nausea, constipation, irritability, fatigue, drowsiness and sometimes, thirst (thirst is a poor indicator of dehydration and is not always present).  Simply working outside on a hot day in Canada can result in this much dehydration.

Moderate dehydration of five to 10 percent usually results in headache, dizziness, difficulty breathing and numbness. 

Severe dehydration of 10 to 20 percent also has visual/auditory hallucinations, delirium and may result in death.  Moderate and severe dehydration often require hospitalization and intravenous fluid replacement while mild dehydration can usually be treated with oral fluids.  One myth is that men have more sweat glands than women.  This is not true, however, men do tend to start sweating at a lower body temperature and men tend to sweat a higher percentage of their body weight.  When you remember that men tend to have a higher percentage of body water than women, the relative amount that they sweat is probably similar. I am advised, incidentally, that only men “sweat”, women merely get “get dewy”!

Water Inside, And Out

There are several reasons why you should avoid dehydration while diving.  One is that you may not be able to safely finish your dive.  When the body is dehydrated it has a greatly reduced ability to keep cool and to do work.  When your body temperature rises, it responds by redirecting blood to the skin to eliminate heat and it increases sweating.  If you are dehydrated you will have a reduced blood volume. If blood is redirected to the skin your blood pressure will fall.  If you then attempt to do strenuous work, more blood will be directed to the working muscles and your blood pressure can become so low that you pass out, injure yourself or even drown.  Swimming back to the boat or shore, getting out of the water (especially climbing a ladder onto the boat), carrying your gear or dealing with an emergency, all these will be very difficult if you are dehydrated and can cause you to lose consciousness.

Another reason divers should avoid dehydration is that it is a major risk factor for decompression sickness (DCS).  Dehydration reduces the circulating blood volume, the perfusion (blood flow) of tissues, and therefore the elimination of inert gas.  If the elimination of inert gas is reduced, the diver will surface with more inert gas in the body and thus have an increased risk of developing DCS.  Dehydration also increases the viscosity (thickness/stickiness) of blood, thereby reducing perfusion even more.  To make matters worse, the signs and symptoms of dehydration could easily be confused with the signs and symptoms DCS!

Unfortunately, there are many reasons why divers are almost always dehydrated after a dive (and often before/during the dive).  In the last column I discussed dehydration due to immersion in water, being cold, and consuming alcohol or caffeine before the dive.  In addition to the fact that alcohol and caffeine often cause more water to be lost as urine than was contained in the drink (there is some evidence that a hangover is due to brain dehydration), alcohol reduces the output of glucose by the liver, is not a ready source of energy, interferes with mental and motor control, enhances the effect of narcosis, etc.  Obviously, alcohol should not be consumed at any time near a dive.

Technical Considerations

A certain amount of water is lost through breathing.  The gas we inhale is warmed to body temperature and 100 percent humidified by the time it gets to the alveoli in our lungs.   The gas in dive tanks (especially in cold countries) will contain almost no moisture and therefore you can lose a significant amount of water this way.  On a long technical dive where you are breathing several tanks of gas this loss can become significant. 

Using a Re-breather, the gas you inhale is already 100 percent humidified.  If the inhaled gas is the same temperature as your body, no additional water is required.  However, if the inhaled gas is colder than your body, as in Trimix divers using Helium, additional moisture must be added to the gas to maintain 100 percent humidification as it is warmed up to body temperature.  As the gas moves around the breathing loop it loses heat to the water but gains heat as it passes through the CO2 scrubber.  The net effect is to cool down unless the water is very warm.

On a hot day divers can lose a lot of water via sweat and seriously overheat before the dive.  This is a major problem where the water might be very cold but the air temperature hot.  The diver needs to wear a thick wetsuit or a dry-suit with thick insulating undergarments.  I try to get into the water as soon as I am dressed so that I can float on the surface, relaxing and cooling off while waiting for my partner.  Help getting geared up for a dive is greatly appreciated under these circumstances!

During the dive a diver might become quite hot and lose a lot of water via sweating.  In a wetsuit or swimsuit the diver will be completely unaware of this loss. 

For boat dives, especially in rough water, vomiting can be another cause of dehydration and diarrhea from infections, especially if you are traveling, can be another cause.

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