Nitrox and Narcosis

[Steve_C]

You hear folks say that using Nitrox reduces Narcosis. I think it is a bit more nuanced than that. My understanding is the following. Suppose that I do two dives to the identical depth with identical ascent and descent profiles. One is on air and one on Nitrox, say 30%. For each dive I stay down until within a minute or two of NDL and then start to ascend. It would seem to me that

1. The Nitrox dive would last longer but since NDL is based on compartment loading I would be equally narced at the end of bottom part of both dives since I have equal Nitrogen loading. So Nitrox does not reduce the maximum ammount of narcosis reached although it does reduce it early on in the bottom time.

2. Since I start the ascent with the same Nitrogen loading I will offgas Nitrogen faster with the Nitrox and thus might be a bit clear headed at the end of the Nitrox dive. This being diver dependent.

Anything wrong with this?

 

[DivemasterDennis]

I'm not so sure that narcosis is necessarily related to total loading or length of time at depth, though it can be. It can come on during initial descent. I think that since there is individual variation in susceptibility to being "narc'ed," the same would be true as to the impact, if any, of nitrox on the process- different for different people. I would focus on depth as it relates to narcosis, and suggest that "on paper" a nitrox diver is likely to initially feel the effects of nitrogen narcosis at a greater depth that if that same diver was diving air. But then I could be wrong. It is an interesting question. Someone smarter than I am should weigh in soon.
DivemasterDennis

 

[Nirvana]

As has been stated in recent threads, most if not all training agencies consider both nitrogen and oxygen narcotic. It means the central nervous system is affected, in terms of narcosis, by PPN2 and by PPO2. I admit I do not know the diffusion rates of O2 and N2, but I would be surprised if that of O2 was significantly slower than that of N2. This means that the sum PPO2 + PPN2 the CNS cells are exposed to shouldn't be much different when using air or nitrox at any point during the dive, for the same depth and duration of dive.

I haven't ever read anything relating time at depth and narcosis. It may be because the CNS is very well perfused, causing the partial pressure of gases in the CNS to reach levels similar to the blood rapidly enough for the time of exposure until the commencement of narcotic effects to be insignificant. Or maybe I should read more.

 

[boulderjohn]

Narcosis is not well understood, but it is apparently not like O2 toxicity or decompression, which are both a function of both partial pressure and time. Narcosis can occur in different ways to different people at any time in a dive. People can be highly narced immediately upon reaching depth, and they can recover to some extent with time. A friend of mine, a technical diving instructor, was still at the early stages of a dive, still only at 85 feet, when he had an overwhelming sensation of doom. He thought it might be a narcosis symptom, so he ascended about 15 feet. The feeling went away. After a minute or two he descended. The feeling never returned.

 

[TSandM]

Narcosis and decompression status are not the same. Narcosis is a function of the nitrogen tension in the blood perfusing the brain; this equilibrates very rapidly as you descend. Decompression is a function of the total nitrogen which has been absorbed by the tissues FROM the blood. It takes time to do this.

You can hit your no-decompression limits at a depth where narcosis is unlikely to be noticeable, and you can be narced out of your gourd two minutes after hitting a depth where you have significantly more than that in no-deco time.

 

[redacted]

Perhaps a good test would be for a diver (sample of divers) to descend on air to a depth where narcosis is apparent and then switch to an EAN mix which would have the same effect on PPN as ascending some small distance. For example, descending to something in the range of 100 to 130 feet on air where narcosis can become apparent and then switching to 32% has the same effect on PPN as ascending about 20 feet. If the switch clears up the narcosis in a frequency similar to a 20 ft ascent, then that would be evidence that narcosis is reduced by nitrox. This method has the advantage of minimizing the effects of many other recognized variables such as subject susceptibility to narcosis on an incident by incident basis.

 

[nimoh]

Perhaps a good test would be for a diver (sample of divers) to descend on air to a depth where narcosis is apparent and then switch to an EAN mix which would have the same effect on PPN as ascending some small distance. For example, descending to something in the range of 100 to 130 feet on air where narcosis can become apparent and then switching to 32% has the same effect on PPN as ascending about 20 feet. If the switch clears up the narcosis in a frequency similar to a 20 ft ascent, then that would be evidence that narcosis is reduced by nitrox. This method has the advantage of minimizing the effects of many other recognized variables such as subject susceptibility to narcosis on an incident by incident basis.

The first problem is that it is very difficult to measure the intensity of inert gas narcosis, the second is that the onset of narcosis can be different in the same diver on different dives, so you couldn't conclusively attribute a change in narcosis intensity solely to changing the gas blend.

Although with perhaps a very large test group, a trend could be established.

 

[boulderjohn]

Perhaps a good test would be for a diver (sample of divers) to descend on air to a depth where narcosis is apparent and then switch to an EAN mix which would have the same effect on PPN as ascending some small distance. For example, descending to something in the range of 100 to 130 feet on air where narcosis can become apparent and then switching to 32% has the same effect on PPN as ascending about 20 feet. If the switch clears up the narcosis in a frequency similar to a 20 ft ascent, then that would be evidence that narcosis is reduced by nitrox. This method has the advantage of minimizing the effects of many other recognized variables such as subject susceptibility to narcosis on an incident by incident basis.

In an attempt along those lines, my trimix instructor did a dive with me in which he was using air at 160 feet and I was using trimix (21/35). We switched regulators for a few minutes. He talked about how different they felt to him. I could not tell a bit of difference.

If I were a test subject describing how I felt during narcosis, I would not be much help. I have never felt as if I were impaired in any way. On the other hand, I have been stupid enough at such depths on enough occasions to know that if I do an activity that truly reveals the degree of my impairment, then it becomes evident, no matter how "just fine" I may feel. Also, it is different on different dives.

As far as testing the degree to which high PPO2s are intoxicating, remember that we have a limitation on how we can test it. Nitrogen's narcotic potential can be measured by putting divers at very high PPN2. If you do that with O2, you risk or even guarantee a toxicity event.

 

[redacted]

Those are some of the variable that the right test protocol should be able to deal with. Clearly, divers that do not show any narcosis effects in the 100 to 130 ft range would not make good subjects. I'm sure the test would also have to be baselined with subjects that stay on air but actually use depth change to counter the narcosis.

Although, I guess the real answer is it just does not matter. We don't dive EAN to reduce the effects of narcosis. Even if it does have that effect, it is only a 10 or 20 ft of depth difference - not that interesting. If you really need to overcome the effects of narcosis. then that is what helium is for.

 

[wve]

Narcosis and decompression status are not the same.

I agree completely. As I read the OP, it seems to conflate narcosis with nitrogen loading and the risk of DCS.

Another way to think of it is this: narcosis happens when you are loading nitrogen, and DCS happens when you are off-loading nitrogen.

No-decompression limits (NDLs) are our way of trying to estimate how much nitrogen has already been loaded in order to prevent problems when it starts off-loading.