Fascinating subject.
I have always noted that fatigue was a primary indicator for my personal diving that I was pre-DCS and approaching Type I. I figured that it was reasonably believed, and I'm shocked to consider that not everyone is sold on the idea of fatigue as a DCS indicator.
Note that the "London Diving Chamber" lists "General symptoms of profound fatigue or heaviness, weakness, sweating, or malaise" as a primary indicator of DCS at
How to diagnose DCI, decompression sickness / DCS / decompression illness / DCI / diving and the bends, London Recompression & Hyperbaric facilities - The London Diving Chamber . Wikipedia explains that a general symptom is "Unexplained extreme fatigue or behaviour changes" but cites that "extreme fatigue" is present in less that 2% of subjects studied. This is at
Decompression sickness - Wikipedia, the free encyclopedia . EMedicine cites a symptom of DCS as "◦General symptoms of profound fatigue or heaviness, weakness..." at
Decompression Sickness: eMedicine Emergency Medicine . There are dozens of other references to a connection to DCS and fatigue on Google.
Now, having said that, obviously, just because it's on the Internet doesn't make it so - and I am hard-pressed to find a conclusive study on the subject.
Having said that, however, and because during my studies at Duke University's Hyperbaric Facility (I was a test subject, not a doctor), I got the impression that it was generally accepted that fatigue could be used as an indirect indicator to pre-DCS. It's worked for me over the years - feeling tired and heavy and basically horrible (I liken it to the first day of catching the flu, prior to becoming symptomatic) basically tells me that I'm too close to actual, diagnosed DCS. In other words, that I need to "cut back" on my saturation levels.
The ways to do that, obviously, are by reducing my nitrogen exposure... In other words, by breathing elevated oxygen mixes and/or flattening my ascent rates.
I guess what I'm saying is that for me, my experience has been that yes, nitrox can have a pretty dramatic effect on my general state of feeling at the end of a long dive day... But not really until I'm near my limits. If I did a dive to 60 feet for 5 minutes, then whether I was on air or EAN36 I wouldn't be able to tell the difference. If I did a dive to 60 feet for 60 minutes, however, I can tell a huge difference. It's a matter of saturation levels.
Of course, this is a totally unscientific observation - but a real observation nonetheless, and no, it's not a placebo effect.
I'm shocked to find that there are people who do not generally accept the theory that reduced nitrogen exposure reduces fatigue while diving.
I always assumed that the fatigue was caused by my body fighting "invading" microbubbles... Which would cause fatigue for the same reason that an invading virus causes fatigue in the body.
On a side note - someone mentioned a PFO... The real problem with a PFO isn't that the arterial and ventrical blood mixes... It's that bubbles in the bloodstream can "leak" through the PFO and therefore never make it to the lungs for dispersal. Those with extended PFOs are dramatically more susceptible to DCS.
At Duke, testing negative for a PFO was foremost on the requirements for entry.