Nitrox - Trimix

[Vtach]

Another thing to realize that might be missed with trimix is the reduced density of gas.  Somewhere around 6 ATA (165 ffw), the density of air is the limiting factor in your respiratory systems ability to exchange gas in and out.  Air becomes so dense at these depths that your anatomy is physically unable to exchange enough gas beyond the "relaxed" breathing state.  Combine this with a regulator that is reaching its physical limit to providing a dense gas on demand and you have a prescription for CO2 retention.  Add exertion to the equation and you've just filled that prescription. 

CO2 is 25 times more lipid soluble than nitrogen.  There is a strong correlation between lipid solubility and the narcotic potency of gas.  So you begin to increase your arterial PCO2 due to the inability to efficiently exchange oxygen and CO2.  Your normal PCO2 is 35-45 mmHg, at 70-75 mmHg PCO2 you have reduced awareness.  At 100-120 mmHg you become unresponsive.

So words to the wise, deep air can bite you in the ass hard.  You might get away with it when the dive is easy, but combine nitrogen narcosis, anxiety and a bit of physical work making an already bad situation much, much worse.  Can you say...Big O' Dang O' Ni-Night... :

Helium on the other hand, while being expensive, has a much lower density and breathing resistance than air along with a low narcotic potential.  This helps to reduce the physical limitations of regulators, upper and lower airways while being used at high ATA's.  Not to say that you can kill yourself on Trimix but it at least gives you the physical ability to handle a situation where it becomes necessary to breathe enough to fully inspire the necessary PO2 and eliminate the PCO2. The reasonable equivalent narcotic depths don't hurt either.  It's nice to remember the dive.

For me, depths over 100 feet, I prefer Trimix. 
Anything less and Nitrox or Highmix is fine ;D