Nitrox and Fatigue: the Evidence

[tursiops]

There is mixed evidence for and against a reduction in fatigue after a dive using Nitrox, compared to air.

DAN has summarized this in 2014; the article is often misinterpreted.

Their summary gives three critical references, and uses interviews with two notable DCS researchers, namely Richard Harris and Neal Pollock.

The oldest (2003) of the three references is: Harris RJD, Doolette DJ, Wilkinson DC, Williams DJ. Measurement of fatigue following 18 msw dry chamber dives breathing air on enriched air nitrox. Undersea Hyperb Med. 2003; 30(4): 285-91. (Yes, that is the same Harris.)

That publication involved 11 divers doing one “dive” in a dry chamber, using air or 36% Nitrox. The abstract is: Many divers report less fatigue following diving breathing oxygen rich N2-O2 mixtures compared with breathing air. In this double blinded, randomized controlled study 11 divers breathed either air or Enriched Air Nitrox 36% (oxygen 36%, nitrogen 64%) during an 18 msw (281 kPa(a)) dry chamber dive for a bottom time of 40 minutes. Two periods of exercise were performed during the dive. Divers were assessed before and after each dive using the Multidimensional Fatigue Inventory-20, a visual analogue scale, Digit Span Tests, Stroop Tests, and Divers Health Survey (DHS). Diving to 18m produced no measurable difference in fatigue, attention levels, ability to concentrate or DHS scores, following dives using either breathing gas.

Conclusion: no lessening of “fatigue” (as defined and measured) was found.​

The next publication (2008) is: Chapman SD, Plato PA. Measurement of fatigue following 18 msw open-water dives breathing air or EAN36. In: Brueggeman P, Pollock NW, eds. Diving for Science 2008. Proceedings of the American Academy of Underwater Sciences 27thSymposium, 2008; 1-11. (Yes, that is the same Pollock.)

That publication involved actual open-water dives, again with 11 divers, using air or 36% Nitrox. Unlike the earlier study, two repetitive dives were made on air and on Nitrox, with a least a day off in between each pair of dives. The abstract is: SCUBA divers often report feeling fatigued upon conclusion of diving activities. Post-dive fatigue is thought to be induced by increased energy demands of submersion in a hyperbaric environment and decompression stress. Anecdotal reports indicate a reduction in post-dive fatigue when using enriched-air nitrox (EAN). The purpose of this double-blind study was to compare subjective fatigue levels experienced by SCUBA divers after two repetitive air dives and two repetitive EAN36 dives on separate, nonconsecutive days. Eleven male participants completed pre- and post-dive fatigue assessment using the Multidimensional Fatigue Inventory and a Visual Analogue Scale, while general health was assessed using the Diver Health Survey. Divers did tend to be more fatigued after diving; however, breathing gas mixture exhibited no statistically significant effect. Participants did have significantly lower Diver Health Survey scores upon the conclusion of EAN36 test sessions, possibly indicative of reduced decompression stress.

Conclusion: no statistically significant effect of the breathing gas on “fatigue,” which was measured the same way as in the earlier study.​

The most recent reference is: Lafère P, Balestra C, Hemelryck W, Donda N, Sakr A, Taher A, Marroni S, Germonpré P. Evaluation of critical flicker fusion frequency and perceived fatigue in divers after air and enriched air nitrox diving. Diving Hyperb Med. 2010 September; 40(3): 114-8.

That study looked at 219 divers using air or 32% Nitrox, and measured “fatigue” in a different way than did the two earlier studies, and also looked at critical flicker fusion frequency (CFFF) as a diagnostic of cortical arousal. The abstract is: Introduction: Many divers report less fatigue following dives breathing enriched air nitrox (EANx) compared with breathing air. A reduction of post-dive fatigue with EANx would suggest a pathological origin, possibly the presence of asymptomatic nitrogen bubbles in the body after a dive. Method: We studied fatigue in 219 healthy divers performing either an air (n = 121) or EANx32 (oxygen 32%, nitrogen 68%; n = 98) dive to 21.2 +/- 4 metres' sea water for 43.3 +/- 8.6 minutes in tropical open-water conditions. Divers were assessed pre-dive and 30-60 minutes after surfacing using a visual analog scale (VAS) of fatigue and critical flicker fusion frequency (CFFF). Results: The two groups were comparable in sex ratio, age and diving experience. The change in perceived fatigue level after a single dive was significantly lower when EANx was breathed compared to air dives (VAS; P < 0.001). Compared to pre-dive, CFFF decreased by 6% in the air group (P < 0.01) but increased by 4% in the EANx group (P < 0.05). The post-dive difference between the two groups was highly significant (P < 0.001). Conclusions: Three hypotheses should be considered to explain the difference in post-dive fatigue and alertness between the air and EANx groups: a nitrogen effect, an oxygen effect and a bubble effect. These involve complex phenomena in the functional modifications of the nervous system in hyperbaric environments according to the type of gas used for the dive, and more research will be required to elucidate them.

Conclusion: Significant reduction is “fatigue” between the air and Nitrox divers, and highly significant increase in CFFF. Three hypotheses were offered, and further study is needed.​

In addition to these three studies, which had quite varying methodologies and results, the DAN article interviewed the two experts Harris and Pollock. They argued that dry studies in chambers do not adequately replicate the effects on the human body; one must be immersed. (So throw out the 2003 study by Harris et al.) Neither of the experts felt the three studies offered any compelling proof one way or the other. They do agree that there may be a placebo effect, but do not offer that as evidence in any way. “More research is needed.”

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My conclusions: the studies do NOT argue against a lessening of fatigue after a Nitrox dive, nor do they support it. In terms of research, the question is open. To quote the DAN article otherwise is incorrect.

Meanwhile, if it feels good, go for it.

 

[Divin'Papaw]

Thanks for the summary!

I dive nitrox almost exclusively but only because for the majority of my dives (50-100') there is a meaningful increase in NDL. Since I live and dive in South Florida where Nitrox is plentiful and cheap, there is no reason not to. It is just easier to buy a fill card and keep my tanks full of 36%. But I've never noticed any sort of reduced fatigue associated with using Nitrox back when I would mix my diving between Air and Nitrox, mostly 32% in those days.

 

[Fastmarc]

My one experience with nitrox was at the end of several days of diving in Cayman several years ago. We arranged that the last dive be a shallow night dive on Nitrox before we flew out I think the following night (or two after). After several days of diving I was truly tired and contemplated sitting this dive out. It did involve a surface swim, but despite that when I got out it immediately hit me that I physically felt better post dive. I mean really noticeable. The wear of the previous dives had vanished. Puzzled, I came on here I saw that there was a debate on this.
I never got to dive nitrox again, so have no other experience for comparison. One thing for sure is that I have never had that experience with air, which is why I guess it jumped out at me.

I find it strange that it's so difficult to prove/disprove.

 

[BRT]

My guess is that at 60 feet for 40 minutes it will make little difference. 60 feet for 55 minutes might show more difference. I'm pretty sure the effect is from less deco stress.

 

[BlueTrin]

My one experience with nitrox was at the end of several days of diving in Cayman several years ago. We arranged that the last dive be a shallow night dive on Nitrox before we flew out I think the following night (or two after). After several days of diving I was truly tired and contemplated sitting this dive out. It did involve a surface swim, but despite that when I got out it immediately hit me that I physically felt better post dive. I mean really noticeable. The wear of the previous dives had vanished. Puzzled, I came on here I saw that there was a debate on this.
I never got to dive nitrox again, so have no other experience for comparison. One thing for sure is that I have never had that experience with air, which is why I guess it jumped out at me.

I find it strange that it's so difficult to prove/disprove.

I think it is organising the experiment that is hard: you’d need a very large sample of people diving a mix they do not know then being asked how they feel on a scale.

Diving a random mix for thousands of people may not be an easy experiment to organise

 

[wnissen]

I think it is organising the experiment that is hard: you’d need a very large sample of people diving a mix they do not know then being asked if they feel on a scale.

Diving a random mix for thousands of people may not be an easy experiment to organise

Exactly, 11 people is not an experiment, that's an anecdote. If the magnitude of the effect was very large, you'd be able to see it, but you'd need a much bigger pool to locate even a 20% effect with certainty.

 

[MaxBottomtime]

Fatigue is so subjective that it would be nearly impossible to measure. I'm in pretty good shape. I ride my bike 20-44 miles each morning. Sometimes I feel great, other days I feel tired just watching TV.

I dive with a steel backplate and steel 130 cu ft tanks. After making a couple of dives I wash the boat, take my tanks as well as Merry's 100 cu ft tanks to the dive shop, fill them, then take them back to the boat. I then go home and work on my photos from that day for hours. I'm pretty drained after I get home, even though I'm breathing nitrox.

I breathed air using steel doubles and made long, deep dives for years. I usually breathe nitrox now to get a shorter surface interval or greater ndl. I don't feel any different than I did diving heavier gear on air.

Even though the certifying agencies' textbooks mention the placebo effect, it seems that many people hold onto beliefs from something they read on the internet.

 

[rhwestfall]

"feeling"..... yeah, that is scientific....

Anyhow, my thought is that with the same dive time on different mixes, the dive on the richer mix may cause less decompression stress thus you could feel better. If diving both mixes to the NDL limits (thus one a longer dive than the other) would put both bodies into the same "level" of decompression stress, thus the feeling would be the same.....

 

[Jay]

The most recent reference is: Lafère P, Balestra C, Hemelryck W, Donda N, Sakr A, Taher A, Marroni S, Germonpré P. Evaluation of critical flicker fusion frequency and perceived fatigue in divers after air and enriched air nitrox diving. Diving Hyperb Med. 2010 September; 40(3): 114-8.

The change in perceived fatigue level after a single dive was significantly lower when EANx was breathed compared to air dives (VAS; P < 0.001). Compared to pre-dive, CFFF decreased by 6% in the air group (P < 0.01) but increased by 4% in the EANx group (P < 0.05). The post-dive difference between the two groups was highly significant (P < 0.001). Conclusions: Three hypotheses should be considered to explain the difference in post-dive fatigue and alertness between the air and EANx groups: a nitrogen effect, an oxygen effect and a bubble effect. These involve complex phenomena in the functional modifications of the nervous system in hyperbaric environments according to the type of gas used for the dive, and more research will be required to elucidate them.

Conclusion: Significant reduction is “fatigue” between the air and Nitrox divers, and highly significant increase in CFFF. Three hypotheses were offered, and further study is needed.​

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My conclusions: the studies do NOT argue against a lessening of fatigue after a Nitrox dive, nor do they support it. In terms of research, the question is open. To quote the DAN article otherwise is incorrect.

Meanwhile, if it feels good, go for it.

In the third study, which seems more rigorous and is statistically significant, why do you feel this study doesn't support the proposition that nitrox reduces fatigue? (or at least the feeling of fatigue).

Are VAS and CFFF flakey measuring tools?

 

[mi000ke]

In the third study, which seems more rigorous and is statistically significant, why do you feel this study doesn't support the proposition that nitrox reduces fatigue? (or at least the feeling of fatigue).

Are VAS and CFFF flakey measuring tools?

As a recovering academic who had to both do research and review it for peer reviewed journals, one of my pet peeves was recognizing the difference between statistical significance and "practical" significance. Statistically, it is possible to get a very high level of statistical significance for any relationship if your sample size is large enough. But it may be folly to say that a relationship that causes, say, a 0.1% change in something, while statistically significant, is of any practical significance. My question here would be, while statistically significant, were the results meaningful from a practical perspective. What really is the practical impact of a 6% change, for example? If not much, then I would certainly agree that the three studies are inconclusive for all practical intents and purposes (and two for statistical reasons).