Why could divers feel less tired after a nitrox dive?

By Matias Nochetto, MD for DAN Europe

For decades, compressed air has been the standard and the most used respiratory mix in recreational diving. Gaseous mixtures with a lower nitrogen content and a higher oxygen concentration (enriched air ou nitrox) have grown in popularity among divers who wish to increase their time at the bottom or reduce the decompression stress of dives of typical duration. Now that 25 years have passed since the introduction of nitrox in recreational diving, divers and scientists have gained valuable experience in its use. Some divers, however, have made an observation that has yet to find any scientific basis. Indeed, they say they feel less tired after a nitrox dive. Is there any evidence to support that nitrogen reduces fatigue? Can we find a scientific explanation for this phenomenon?

Physical fatigue is a physiological consequence of extraordinary physical activity. It is characterized by a subjective impression of a temporary reduction in the ability to perform ordinary physical activities. It is often associated with a state of drowsiness and lethargy, as well as a decline in cognitive performance. It is not uncommon to experience fatigue after dives without special difficulty and in the absence of signs of decompression sickness (DCS), although it can be a symptom of the latter.

The fact that some divers can apparently feel less tired after a nitrox dive is the subject of much controversy. Indeed, the objective studies carried out on this subject did not find any reliable difference between air and nitrox dives with regard to fatigue or cognitive performance.

For exposure at the same depth for the same duration, it is generally accepted that the higher the partial pressure of the oxygen inhaled (PO₂) is high and the partial pressure of the inert gas (PN₂) is low, the less decompression stress will be felt. It is then tempting to deduce that lower decompression stress reduces the feeling of fatigue. Nevertheless, a PO₂ higher is also associated with oxidative stress, and the OP₂ nitrox becomes greater than that of air from a certain depth.

Various studies conducted on divers suggest that the oxidative stress generated by the OP₂nitrox could cause a slight malfunction endothelial, which in turn may lead to some vague symptoms that may be perceived as fatigue. This theory is supported by several physicians specializing in hyperbaric treatments, who observed that increased oxidative stress during hyperbaric oxygen therapy was associated with increased fatigue after treatment.

We consulted two experts in this field to try to reconcile the impressions of divers who support the beneficial effects of nitrox on post-dive fatigue and the controversial findings of some studies that have looked into this phenomenon.

What can be the causes of fatigue experienced after diving? 

Richard Harris : There are several factors that can cause fatigue after diving. Some are linked to diving itself, such as heat stress, decompression stress, energy expenditure, prolonged exposure to high oxygen concentration, anxiety and seasickness. But there are also many other potential causes unrelated to diving, such as lack of sleep, alcohol consumption, jet lag, etc.

Neal Pollock: We do not always imagine the physiological impact that diving can have. As an immediate reaction to the hydrostatic pressure, a significant amount of the blood normally present in the capacitive vessels (veins) of the legs is sent to the central volume (the chest). According to a well-known study carried out on the subject, an average of 700 ml of blood is delivered to the heart during the resting phase of the cardiac cycle. The heart stretches as the blood volume increases and responds immediately with stronger contractions and then, over a short period of time, by suppressing certain hormones to increase fluid removal from the kidneys. This is a normal response of the body to a physiological perception of the presence of excess fluids. On a practical level, this is the reason why divers need to urinate even after short periods of immersion.

Upon exiting the water, the diver may experience an immediate drop in blood volume returning to the brain. I say “may” because wearing a tight wetsuit can cause blood to move to the chest without the diver being submerged. After the dive (or after removing the wetsuit), the diver may experience a drop in core blood volume and blood pressure. This is not only due to the decrease in hydrostatic pressure. Indeed, during diving or wearing a wetsuit, there is an active reduction in fluid volume within the body. This set of factors could largely explain the fatigue experienced after a dive. Note, however, that fatigue is mainly due to immersion, regardless of depth and decompression stress.

Is there any reliable evidence to support the hypothesis that nitrox reduces fatigue after diving more than air? 

Richard Harris : No. Three articles in peer-reviewed publications (the first three listed in the box of references) provide data on nitrox and fatigue, but I am not convinced that they are of the size or scope necessary to answer the question satisfactorily.

Neal Pollock: The data supporting these assumptions are not convincing. This is hardly surprising given the substantial increase in OP₂ with depth alone. However, we can not discuss the fact that a person feels more or less tired. Research has shown that the placebo effect has a physiological impact, so let the divers enjoy this sensation. Most important for these is to make sure you do not exceed the PO limits₂ to avoid oxygen poisoning.

Since the increased oxygen content of nitrox seems to cause greater oxidative stress, how would you explain these seemingly opposite effects of nitrox? 

Richard Harris : Pierre Lafère makes an interesting comment in his article: he says that oxidative stress can inhibit neuronal activity (like alcohol does, for example), and that this can affect inhibitory neurons and therefore (temporarily) increase levels. activation. On a personal note, my fellow divers and I have felt great after cave dives lasting 8 to 17 hours in water at 6 ° C and at depths exceeding 200m.
How to explain this phenomenon taking into account the thermal stress, decompression, physical, oxidative and psychological endured during these dives? Maybe we are just happy to be alive! However, I have also had occasion to feel exhausted and have a “weight” on my chest after much shallower, shorter dives, with less exposure to oxygen and in warmer waters. Too many variables come into play to be able to detect a difference on the basis of a simple change of gas.

Neal Pollock: Oxidative stress can certainly have adverse effects, but only to a lesser extent in short exposures typical of most recreational dives. Further studies are needed to assess the physiological impact.

Would you consider fatigue to be a sign of subclinical DCS? 

Richard Harris : I think fatigue can be a symptom of MDD, but it must be very marked to impress me. Severe fatigue, like that of the flu, indicates a more serious problem than feeling "a little more tired than usual" after a dive. Rather than using the term Subclinical MDD, which I am not a partisan, I would attribute fatigue to a private label only if it is part of a set of symptoms.

Neal Pollock: Normal fatigue patterns following a dive are not indicative of an MDD. On the other hand, exceptional fatigue, well above the usual levels, may be a sign, or more likely a symptom, of MDD. It is important to ask the diver to describe the nature and degree of his fatigue to determine whether it is habitual fatigue or not.

Although many divers report feeling less tired when diving with nitrox rather than air, scientific research to date does not reveal any evidence to support such a hypothesis. As mentioned before, the placebo effect should not be underestimated. Its study still needs to be deepened.

Regardless of the answers that future research may bring to this subject, caution should be exercised when using oxygen enriched mixtures. It is not just a question of lengthening the times at the bottom, but above all of minimizing decompression stress. If nitrox allows divers to feel less tired at the end of the day, whether this impression is based on scientific data or not, let's use this breathing mixture without forgetting our safety!

     Presentation of the experts   
Richard Harris, BMBS, FANZCA, DipDHM, FFEWM, is an Australian anesthetist practicing in the field of diving medicine and medical evacuation.

Neal W. Pollock, Ph.D. is Director of Research at DAN and Research Assistant at the Center for Hyperbaric Medicine and Environmental Physiology at the Duke University Medical Center in Durham, North Carolina, United States.