Dehydration and Diving

Dehydration Defined

Dehydration means your body does not have as much water and fluids as it should. Dehydration can be caused by losing too much fluid, not drinking enough water or fluids, or both. Vomiting and diarrhea are common causes. Pressurized airplane cabins, scuba air, alcoholic and other diuretic drinks, diuretic medications and certain medical conditions such as diabetes can cause dehydration.

Children divers are more susceptible to dehydration than adults because of their smaller body weights and higher turnover of water and electrolytes. The elderly and those with illnesses are also at higher risk.

Dehydration is classified as mild, moderate, or severe based on how much of the body’s fluid is lost or not replenished. When severe, dehydration is a life-threatening emergency. It is a known hazard to divers by increasing the risk for decompression illness.

Some of the smptoms of dehydration include:

* Dry or sticky mouth
* Low or no urine output; concentrated urine appears dark yellow
* Not producing tears
* Sunken eyes
* Lethargic or comatose (with severe dehydration)

Signs and tests indicating dehydration include:

* Low blood pressure
* Blood pressure that drops when you go from lying down to standing
* Rapid heart rate
* Poor skin turgor — the skin may lack its normal elasticity and sag back into position slowly when pinched up into a fold by the doctor; normally, skin springs right back into position
* Delayed capillary refill
* Shock

Tests include:

* Blood chemistries (to check electrolytes, especially sodium, potassium, and bicarbonate levels)
* Urine specific gravity (a high specific gravity indicates significant dehydration)
* BUN (blood urea nitrogen — may be elevated with dehydration)
* Creatinine (may be elevated with dehydration)
* Complete Blood Count (CBC) to look for signs of concentrated blood

Other tests may be done to determine the specific cause of the dehydration (for example, a blood sugar to check for diabetes).


Diuretics add to the dehydration that occurs with exercise from sweating, insensible and respiratory water loss; the diver also has to take into consideration some additional concerns for fluid loss and replacement.

1. Scuba tanks have extremely dry air inside. As this air is taken into the lungs and saturated–nearly twice the normal amount of water is lost from the body.

2. Negative pressure breathing causes divers to lose about 350 cc/hour from their circulating blood volume, a phenomenon called immersion diuresis and seen also in snorkelers and swimmers.

3. Cold inhibits anti diuretic hormone, causes peripheral vasoconstriction, driving fluid back into the core and stimulating diuresis resulting in losses of plasma volume.

4. The hypercarbia (high blood carbon dioxide) associated with diving decreases anti diuretic hormone, promoting fluid loss from the plasma volume.

5. Diuretics such as alcohol, coffee, tea also contribute to fluid losses, contributing to predive dehydration.

6. Khosla in 1979 found that there is a 4% hemodilution in early immersion from ECF (extracellular fluid) shift into the bloodstream, resulting in an increase in central blood volume. This alters stimulation of pressure and volume receptors, leading to further inhibition of ADH (anti-diuretic hormone) and fluid loss.

7. In addition to the above, the state of chronic hypovolemia (low fluid vloume) and hypokalemia (low serum potassium) caused by oral diuretics is dangerous to divers by increasing their susceptibility to decompression illness and cardiac arrhythmias, respectively.

A good start in reading about this problem would be in “Diving Medicine”, Bove and Davis, Philadelphia, WB Saunders, 1990, ed 2.

Also try Edmonds C., Lowry C, Pennefather J.:Diving and Subaquatic Medicine.
Oxford, Butterworth-Heinemann, 1992,ed 3., and Bennett and Elliott (Eds): The Physiology and Medicine of Diving. London, WB Saunders, 1993, ed 4.

The importance of entering a dive well-hydrated cannot be over-stated. Prehydration of divers should include regular ingestion of fluids several hours before, 15-20 minutes before and between dives, particularly if multiple dives are to be made each day. The urine should be “clear and copious”, the urine test for divers proposed by Dr. Jeff Davis.


Ernest S. Campbell, M.D., FACS

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