Compressed air has been the standard and most widely used breathing mix in recreational diving for decades. Breathing gas mixtures with a lower nitrogen content and higher oxygen content (enriched air nitrox) have gained popularity among divers looking to increase their bottom times or reduce the decompression stress of typicalduration dives. Today, after some 25 years of nitrox use in recreational diving, divers and scientists have gained very valuable operational experience. But some divers have made an observation for which science does not provide any evidence. Namely, they often report feeling less tired after diving on nitrox.
IS THERE ANY SOLID EVIDENCE TO SUPPORT THE OBSERVATION THAT NITROX REDUCES FATIGUE? WHAT IS A POSSIBLE SCIENTIFIC EXPLANATION FOR THIS PHENOMENON?
Tiredness, or physical fatigue, is a physiological consequence of extraordinary physical activity. It is characterised by a subjective feeling of a transient reduced capacity to perform ordinary physical activities; it is often associated with somnolence and lethargy and sometimes with suboptimal cognitive performance. Fatigue is not uncommon following inconsequential dives in the absence of apparent decompression sickness (DCS), but it’s also frequently reported as a symptom of DCS.
Controversy surrounds divers’ subjective reports of reduced fatigue following nitrox dives. Contrary to these perceptions, objective studies have not found a reliable difference in fatigue or cognitive performance following dives on air versus nitrox.
Given an equal depth/time exposure, it is generally accepted that the higher the partial pressure of the inspired oxygen (PO2) and the lower the partial pressure of the inert gas (PN2), the less decompression stress will be experienced. It is then tempting to assume that lower decompression stress accompanies reduced perceptions of tiredness. However, a higher PO2 elicits more oxidative stress, and the PO2 of nitrox will be higher than that of air at a given depth.
Some studies of divers suggest that oxidative stress generated by nitrox’s higher PO2 might cause mild endothelial dysfunction, which may itself lead to some vague symptoms that may be perceived as tiredness. Observations from hyperbaricmedicine clinicians, who report that greater oxidative stress during hyperbaric oxygen therapy is associated with greater fatigue following treatment, support these findings.
We consulted two subject-matter experts to help us reconcile divers’ impressions of nitrox’s beneficial effects on post-dive fatigue with some controversial findings in studies of the phenomenon.
WHAT ARE SOME POSSIBLE CAUSES OF POST-DIVE TIREDNESS?
Richard Harris: Post-dive tiredness can be the result of multiple factors. Some of them could be dive related, such as thermal stress, decompression stress, energy expenditure, high and prolonged oxygen exposure, anxiety and seasickness. But there are many more potentially unrelated causes such as lack of sleep on holidays, alcohol, jet lag, etc.
Neal Pollock: Many people do not fully appreciate the physiological impact of being immersed in water. An immediate response to the hydrostatic pressure is that a substantial amount of blood normally remaining in the capacitance vessels (veins) of the legs is pushed to the central volume (in the chest). A well-known study of this effect found that an average of 700 ml of blood is pushed to the heart during the resting phase of the cardiac cycle. The heart is stretched by the increased blood volume and responds immediately by contracting harder and then, over a short period, by suppressing certain hormones to promote increased fluid elimination through the kidneys. This is a healthy response to the physiological perception of the presence of excess fluid volume. Practically, this is why people have to urinate even after fairly short periods of immersion.
After leaving the water, the diver may experience an immediate drop in the volume of blood returning to the heart. I specifically said “may” because a constricting wetsuit may shift some of the blood to the chest even without immersion. Post-dive (or post-wetsuit removal), the diver will experience a decline in the central blood volume and blood pressure. The effect exceeds the simple loss of hydrostatic pressure because the body has been actively reducing the fluid volume during the diving (or wetsuitwearing) period. Effectively, this set of events likely explains a substantial portion of the normal post-dive tiredness. Most important, fatigue is due to immersion, independent of depth and decompression stress.
ARE THERE ANY RELIABLE DATA TO SUPPORT CLAIMS ABOUT NITROX CAUSING LESS POSTDIVE TIREDNESS THAN AIR?
Harris: No. Three articles in the peer-reviewed literature (see the first three in the references box at the end of this article) contribute evidence to the nitrox and fatigue question, but I am not convinced that the size and power of any of them have addressed the problem comprehensively.
Pollock: The data supporting these claims are not compelling. This is not surprising since PO2 increases substantially in response to depth alone. However, there is no reason to argue over whether a person feels less fatigued. Research has shown that the placebo effect has physiological impact, so let the diver enjoy the sense. The important thing is for divers to stay within the PO2 limits to avoid oxygen toxicity.