What Dive Computers Don\'t Know

Text by Dennis Guichard

1. TEMPERATURE

Dive computers might measure water temperature to display on your screen readout. Still, almost none of them actually incorporate that data into their decompression algorithm calculations.

Fluctuations in temperature may affect diving safety by increasing the risk of developing decompression sickness (DCS). The reason for this is two-fold – if the body is cold, it can retain more inert gas. Then, when the diver warms up after a dive, the recovery to normal body temperature may bring bubbles out of solution. Hot showers or baths and exercise may have the same effect: This is why exercise is not recommended within 24 hours of diving. If the diver is adamant about keeping to their exercise routines, then 4 hours before and after diving should be considered the bare minimum due to the influence of exercise and increased body heat on blood perfusion and gas solubility.

After the dive, exposure to sudden and excessive heat, such as in a hot shower, produces increased superficial blood flow and lower solubility of gas, which could induce decompression sickness.

As mentioned previously, cold tissues in-gas more and have a higher affinity to hold excess nitrogen. Active movement may help eliminate nitrogen during decompression before bubbles are likely to form. Suppose a diver becomes mildly hypothermic during the ascent phase of a dive - in that case, the ability to eliminate excess nitrogen is decreased, and DCS can become more likely - in some studies, the perfusion rate was halved, thus doubling the required duration of decompression.

2. BODY FAT CONTENT

Some say that fat is only important as an indicator of physical condition and efficient gas exchange and is of no consequence to sport divers being a slow tissue, unlikely to affect those who stay within accepted NDT dive time limits as it in-gasses relatively little on regular sports dives. We know that fat has a much higher affinity for nitrogen than other tissues, but how this impacts the risk of DCS is still uncertain. ‘Chubbier’ subjects may be at slightly greater risk of DCS, as nitrogen is 5-6x more soluble in fatty tissues than aqueous ones. Also, chubbier individuals may have elevated blood lipid levels that may increase susceptibility to DCS.

3. OUR ADVANCING AGE

The ageing process causes a decrease in the efficiency of all biological systems. Generally, the older the person, especially over 40 years, the greater the susceptibility to DCS. A 28-year-old has twice the likelihood of an 18-year-old in aviation decompression sickness.
The number and area of ‘Active Hydrophobic Spots’ (AHS) in our blood vessels, a known seed source for producing the microbubbles, might lead to neurological DCS with increasing age. AHS are formed by (DPPC) surfactants which leak over time from our lungs and settle on the walls of the blood vessels in our vascular system.

4. DIVE EXERTION

Exercise performed at depth is likely to increase the blood supply to the muscular tissues and the rate of inert gas absorption at that site. It can increase decompression requirements by a factor of three. During or after decompression, strenuous exercise increases the speed of bubble development and the number of bubbles, perhaps due to increased cavitation from tribonucleation of tissues or turbulence, similar to shaking a bottle of fizzy juice. Mild exercise during decompression is of value in increasing the rate of gas elimination, perhaps by increasing tissue perfusion, if supersaturation and bubble growth have not occurred.

5. HYDRATION STATUS

Influenced by the environment, exercise, respiration, immersion, and alcohol consumption, dehydration remains one of the primary causes of decompression sickness worldwide. Dehydration is a loss of body fluids that inadvertently leads to a ‘thickening’ of the blood, reducing the efficiency of circulation and gas elimination and inducing bubble coalescence. One way to monitor your hydration level is to observe the colour of your urine - it should be virtually colourless. If it’s dark or cloudy, you’re dehydrated! It has been recommended that divers drink 500ml of water before and after a dive to maintain adequate levels lost through diving.

6. ALCOHOL

Alcohol is a diuretic and induces body fluid loss. It is also a vascular dilator, causing peripheral blood vessels to expand, increasing blood flow, nitrogen absorption, and heat loss. It also reduces blood serum surface tension, which can cause bubbles to form more easily. Alcohol also appears to reduce surface tension in bubbles that might remain too small to cause DCS. Neurological DCS seems to be the primary ‘target’ of dehydration and alcohol abuse. It is one of the most severe and common types of DCS we see in the chamber. A few social drinks can be fun the night before, but no one wants to dive hungover.

7. ADAPTATION  

Repetitive recent exposure to scuba diving has been shown to reduce the likelihood of DCS. It is believed that frequent diving removes the naturally occurring gas nuclei that are precursors to bubble formation. Regeneration times for these classes of micronuclei are estimated to be near a week, so a lay-off of this period puts you back at square one.
Detached ‘Active Hydrophobic Spot’ microbubbles strip away the phospholipid AHS cell structure in our blood vessels which may also contribute to acclimation in repetitive diving. A rest day is recommended after 3-days of continuous diving.

8. DRUG USE
 
The many variables in diving complicate drug use. Quite apart from the unknown effect of many therapeutic drugs in the hyperbaric environment, it is often the condition for which these drugs are taken that renders diving unwise. Drugs can affect DCS risk, particularly if they impact circulation or blood chemistry. Sedatives can aggravate nitrogen narcosis, whilst stimulants, excessive caffeine, decongestants, and many weight loss products, can cause fatal arrhythmias of the heart, provoke heart attacks, and may precipitate oxygen seizures. So, generally speaking, all drugs affecting the central nervous system, the autonomic nervous system, and the heart are contra-indicated in diving.