This column is adapted from a chapter in my book, Scuba Physiological – Think you know all about Scuba Medicine? Think Again! The chapters in this book were originally written by scientists in the field of decompression research as part of a three-year project called PHYPODE (Physiology of Decompression). My (self-appointed) task was to rewrite their sometimes-complex research in a form accessible to all divers.
One interesting aspect they addressed was the concept of preconditioning as it may apply to scuba diving safety. In other sports, preconditioning strategies such as warming up, passive heat maintenance and prior exercise are used to ensure that athletes perform as well as possible on game day.
Tiny gas bubbles in the bloodstream are thought to be the main cause of decompression sickness (DCS), so the PHYPODE researchers looked at six preconditioning strategies that divers might be able to deploy before a dive to reduce the quantity of tiny bubbles produced during the dive, thus reducing both decompression stress and the risk of DCS.
1. Pre-dive endurance exercise
An aerobically fit diver has a lower risk of developing DCS than an unfit diver and aerobically trained runners produce fewer bubbles on a dive than people who are mostly sedentary. Why this should be the case is not yet clear. In the past, it was thought that a bout of aerobic activity immediately before diving had exactly the opposite effect. Pre-dive exercise was seen as a factor that increased the risk of DCS, because it was thought that muscle contractions and tissue movement might produce gas nuclei leading to increased bubble formation.
This theory has now been seriously challenged. In studies conducted in a hyperbaric chamber, divers produced fewer bubbles when they had performed a bout of aerobic exercise 24 hours before a dive. Another study tested the effect of cycling for 45 minutes, two hours before a dive in the ocean. The results confirmed the data obtained in the chamber and found that both moderate and strenuous exercise pre-dive reduced bubble production. A further study showed that running on a treadmill for 45 minutes, one hour before a dive, also significantly reduced the bubble count. Nobody knows yet what the optimal timescale is for doing pre-dive exercise, or even if there is one. Nor is it clear why pre-dive exercise should have this effect: but it seems clear that divers are best advised to keep aerobically fit.
2. Pre-dive hydration
Drinking water before a dive is an easy way to reduce the risk of DCS. When you are well hydrated during a dive, you minimise the negative effects associated with post-dive dehydration. The best way to stay well hydrated is to drink before you get thirsty, a little at a time, say a cup of water every 15-20 minutes. Drinking a large amount of water too fast will increase diuresis, the phenomenon that makes you want to pee, and will not hydrate your tissues. A recent experiment involving military divers showed that drinking a saline-glucose beverage before diving significantly decreased the quantity of bubbles in a diver’s circulation after the dive.
A further study found that loss of body fluids during a dive correlated with bubble count, as measured approximately one hour after surfacing: the greater the fluid loss, the higher the bubble count. This suggests that it is also very important to rehydrate AFTER a dive, especially if you are doing more than one dive a day.
3. Pre-dive oxygen breathing
Oxygen breathing has been extensively investigated as a way of reducing DCS risk before altitude decompression and space walks. Oxygen breathing is also routinely employed during decompression from deep air dives to accelerate the washout of nitrogen from the tissues, thus both shortening decompression time and lowering the risk of DCS.
In 2009, a study examined what effect breathing oxygen at ambient pressure for 30 minutes BEFORE a dive would have on post-dive bubble formation. The divers participating in the experiment stopped breathing oxygen 15 minutes before the dive. They did two dives, 100 minutes apart, to 30m (100ft) for 30 minutes with a 6-minute stop at 3m (10ft) on each dive. The divers were randomly assigned to one of four profiles: “air-air” (the control profile), “O2-O2”, “O2-air” and “air-O2” where “O2” was a dive with oxygen pre-breathing and “air” was a dive without oxygen pre-breathing.
The study found that oxygen pre-breathing resulted in a significant reduction in decompression-induced bubble formation, regardless of the profile. The beneficial effect of pre-dive oxygen was observed after the first dive and was maintained after the second dive even when oxygen pre-breathing did not precede the second dive. The “O2-O2” profile resulted in the greatest reduction in bubble scores measured after the second dive. The results also indicated that the beneficial effects were cumulative and long lasting.
This might be due to the physical effects of breathing gas without nitrogen (denitrogenation) although there are no data that suggest this. Instead, it might be because of the antioxidant and anti-inflammatory properties of hyperbaric oxygen itself. More research is needed.
4. Pre-dive heat exposure
Researchers also conducted studies to determine what effect pre-dive heat exposure in a sauna would have on bubble formation after a dive. Sixteen divers underwent a 30-minute infrared dry sauna session, followed one hour later by a dry chamber dive to 30m (100ft) for 25 minutes. Test results showed that the sauna exposure significantly decreased circulating bubbles after the dive.
Another experiment was carried out, involving a group of divers who were known normally to produce particularly large numbers of bubbles after diving. In this study, the divers first performed three control dives in a 34m (113ft) deep swimming pool with no preconditioning. Then, further dives were carried out with a preconditioning procedure that involved a 30-minute infrared sauna session two hours before the dive. The preconditioning led to a significant reduction in post-dive bubbles.
So, it seems heat stress may give some degree of protection against bubble-induced injury from decompression.
5. Pre-dive vibration
In the old days, combat divers and commercial divers would drive their boat fast out to the dive site but return to shore slowly after the dive in the belief that this strategy would reduce the risk of DCS. The PHYPODE researchers decided to see if science could back up the theory and found that 30 minutes of whole-body vibration, (such as you would receive in a speedboat driven fast) before a dive could indeed reduce the quantity of bubbles produced after the dive.
6. Biochemical preconditioning (with dark chocolate)
The vascular endothelium is an organ you probably do not know you have. It is a single layer of cells that completely covers the inner surface of all the blood vessels in your body. Several studies on both animal and human subjects have shown that hyperbaric exposure results in dysfunction of the vascular endothelium. This may be the result of oxidative stress resulting from hyperoxia during diving and recent experiments have shown that taking antioxidants prior to diving can reduce the negative effects that diving has on endothelial function. Preconditioning by taking an antioxidant such vitamin C might reduce endothelial inflammation at depth and thus limit gas bubble formation.
Recently, some Belgian scientists studied the effects of dark chocolate on bubble production and endothelial impairment associated with diving and found that consumption of dark chocolate had a positive effect on the endothelium, although it had no significant effect on the quantity of bubbles developed during a dive.
Other recent studies discovered that eating 30 grams of dark chocolate two hours before a breath-hold free dive can prevent endothelial dysfunction, which is normally observed after free diving as well as scuba diving. The flavonoids in dark chocolate seem to be the key ingredients. They generate nitric oxide secretion and decrease platelet adhesion two hours after ingestion and this makes it less easy for bubbles to form and achieve stability.
The timing is critical. The decrease in platelet adhesion peaks two hours after chocolate consumption. Bubble production while scuba diving is not directly related to the preservation of endothelial function. Neither is it directly related to nitric oxide. However, preservation of good endothelial function after diving may reduce the adverse effects of the bubbles that have formed during the dive.
Further research is required into all of the effects described here, but a few things are clear:
1. Divers should stay in good physical shape and maintain cardiovascular fitness.
2. Pre-dive procedures can help reduce decompression stress. Some help maintain endothelial function. Others are better at reducing bubble production.
3. Pre-dive oral hydration, exposure to heat, whole body vibration and oxygen breathing may represent relatively easy ways of reducing DCS risk. ■
For a more detailed summary of the PHYPODE findings on pre-conditioning, read Simon’s book Scuba Physiological – Think you Know All About Scuba Medicine? Think Again! available as an e-book via Amazon stores worldwide. For more information, go to: Simonpridmore.com.