What Dive Computers Don't Know - Part 2

BY DENNIS GUICHARD

1 DONATING BLOOD

A typical blood donation is about half a litre (500mL), and normal adults have 56 litres of blood in their bodies As such, donors give up about 10% of their total blood volume per donation It takes about 24 hours for the body to replace the donated blood plasma and about 46 weeks to replace the donated red cells However, blood iron levels may not reach pre-donation until eight weeks after donation Although blood volume will return to normal quickly, a lower red blood cell count may contribute to fatigue and dehydration Such conditions increase one's risk of a diving accident or possible DCS As defined by the UHMS, medical standards for divers should not dive for at least 48 to 72 hours after blood donation. DAN routinely recommends 72 hours.

2 PREVIOUS DCS

Guidelines for a return to diving after DCS tend to depend mainly on the type of bend a diver experienced and the actions that might've led to that It is believed that if you get DCS in a particular location of your body that you may be susceptible to repeat hits in the same location This however probably depends on whether you change your dive habits and/or just keep doing the same things the same way that got you bent in the first place DAN guidance for a layoff from diving might be required to include anything between two to six weeks with mild DCS Anything up to 6months layoff might be prescribed if more severe DCS symptoms were experienced Divers may experience lingering symptoms from tissue damage or inflammation. A dive doctor's medical clearance is always recommended.

3. CARBON DIOXIDE

Elevated levels of carbon dioxide can occur in our bodies due to the effects of breathing increased gas density at depth, over-exercising, skip-breathing, or excessive breathing resistance from equipment. This can cause dilation of the blood vessels, which may increase perfusion during a dive, increasing nitrogen loading. Carbon dioxide is carried to the lungs and dissolved in our blood plasma. It has a gas tension which also contributes to tissue supersaturation in decompression Carbon dioxide has a very high narcotic potential, 20 times more than nitrogen, also adding to the narcotic effects of nitrogen at depth It is essential to breathe efficiently on scuba and avoid the deep water when breathing compressed air

4 DEEP DIVING

Recommendations are that we should not dive deeper than 30m on air This is because the exertion of breathing enhanced gas density (beyond 5g/L) at that depth triggers a risk of carbon dioxide build-up in the body Therefore, 40m is considered the absolute maximum safe limit for diving on air because of gas density (6g/L at this depth) and the severe risk of carbon dioxide accumulation due to the effort of breathing that increased gas density Similarly, if increased exertion is experienced, the maximum depth should be reduced Carbon dioxide has an anaesthetic effect under pressure and may even lead to deep water blackouts. The very reason why deep technical divers add helium to their breathing mixture is to reduce the gas density being breathed and also to minimise DCS risk.

5 GENDER

It is frequently suggested that females might have a higher incidence of DCS than men, mainly due to a larger body mass index (which is an amusing misconception looking about on any beach anywhere) There is no conclusive evidence to show that women divers have any higher susceptibility to DCS than males In fact, I am pretty confident that female DCS susceptibility is less than men's due to their typically lower tidal volumes (ie nitrogen exposure) and usually more conservative diving habits Fluid retention during menstruation and being on the contraceptive pill have not been proven to increase the susceptibility to DCS The DDRC in Plymouth, England, has done extensive research on 'women in diving', and you can read much more on the subject by visiting their website.

6 SMOKING

Cigarette smoke contains some 50 times the carbon monoxide (CO) content considered to be a safe air purity standard Blood flow to the brain increases by up to 400% when exposed to CO The ramifications of this on CNS oxygen toxicity and DCS are concerning Such an increase in blood flow would impact nitrogen perfusion, absorption, and cerebral DCS risk on an otherwise ordinarily safe dive Cigarette smoke inhalation affects pulmonary bubble mechanics that may hinder the effective elimination of inert gas and predispose to DCS As silent bubbles accumulate in the lung filters quite extensively in the initial 45-60 minutes after a dive, it would probably be prudent to avoid that desperate cigarette for a good hour or so after surfacing from any dive.

7 DIVE FITNESS

Swimming is invaluable for enhancing your cardiovascular fitness by getting into a swimming pool and swimming lengths with your mask and fins on Fitness is essential for optimal cardiovascular functionality, minimising the risk of DCS-stress, and reducing post-dive fatigue Also, for minimising the risk of clinical DCS and for managing in-water stress You can build up a basic level of diving fitness by just diving often It is valuable to exercise your diving-specific muscles with your own scuba equipment Still, nothing beats working on your swimming and cardiovascular fitness to increase your enjoyment of being underwater on scuba To minimise your risk of DCS and enhance your ability to manage stress in the water, invest some swim time at your local gym to be the best diver you can be.

8 SARS-COV-19

COVID-19 is here to stay It's become a part of us all It invades every cell of our bodies and takes residence there asymptomatically It has been shown to be harbouring in the bacterial microbiomes of people's large colons for well over a year or more Many people worldwide suffer from lingering fatigue, brain fog, and other symptoms from long-covid Long-covid is shown likely to be caused by amyloid micro clots forming on the endothelial surface of our blood vessels The virus may also cause damage to the lung alveoli by causing cells to fuse We don't know yet what impact SARSCOVID-19 might have on scuba diving in the longer term. There may be an impact on how inert gas perfuses into and out of tissues, and DCS risk might be elevated.

9 PATENT FORAMEN OVALE

A 'PFO' or a hole in the heart that does not close completely after birth may become a problem for divers later in life Up to 25% of the population is believed to have a PFO However, only 5% are likely to have one considered detrimental enough to scuba diving Under some conditions, i e , when you equalise your ears or cough underwater, the blood pressure in the heart's right atrium can increase slightly over the left Nitrogen-enriched blood can shunt from the right to left side of the heart atria allowing microbubbles to pass directly back into the arterial circulation The majority of divers we treat in the chamber are indeed rather suffering with a PFOinduced bend of this nature. These types of bends can often be severe, causing neurological insult often with lower limb paralysis.

10 BIO INDIVIDUALISM

Dive computer algorithms are based on benchmarking of average risk and do not consider us individually We are each quite remarkable and unique, of course We all have different heights, weights, and BMI scores We have different levels of cardiovascular efficiency Our blood health varies with different levels of the various biomarkers Our cardiovascular health varies depending on lifestyle and genetic predispositions It is known that some people naturally generate high venous gas bubble scores on the same dive that others don't Even individually, we can generate microbubbles on a dive today that our bodies might not do tomorrow, due to biological variables from day to day. We are each very unique and individual.