Nitrox and Narcosis

[dfx]

I understand your point but it does not seem to be that simple. The gases are not completely dissolved in liquid (say in blood), they do exist in a form of bubbles

I thought bubbles form only upon ascending. As long as you only descend or stay level, there should be no gasses out of solution. At least that's my understanding.

---------- Post added August 30th, 2013 at 12:03 PM ----------

If the saturation level of the CNS is not the trigger of narcosis, then what is? If it was the rate of transfer of gases from the blood to the nervous cells, then narcosis should alleviate with time, as the transfer rate gets smaller as the saturation increases. If it is neither of the above described mechanisms, what is it then?

It could be that the pressure simply alters how certain chemical reactions happen in the CNS and that the mere presence of N2 at elevated pressures does this. Disclaimer: totally unqualified opinion that isn't based on any hard facts.

 

[Nirvana]

I understand your point but it does not seem to be that simple. The gases are not completely dissolved in liquid (say in blood), they do exist in a form of bubbles and I would guess mean they will have partial pressure changed in the bubble almost instantaneous. How that will affect narcosis though I have no idea. And even if we do not take into account those bubbles and only consider ongassing blood is a fast tissue and saturates fairly quickly and I doubt it matters whether one breathes air of nitrox.

From my own experience the bigger enemy is not N2 but CO2. I clearly feel a difference in the level of narc between scootering and finning at 100ft. And you do not need to transfer it from lungs to the tissues as we know.

Just to be clear, I do not pretend to know the way N2, O2, CO2 or whatever other gas causes narcosis. All I know is that they do (setting aside the debate on whether O2 is narcotic), and that the intensity of narcosis is related to the partial pressure of the inspired narcotic gas.

That said, I do not believe the presence of bubbles have any impact on narcosis. For a bubble to form in a liquid, some gas has to be dissolved into the liquid and later, through some mean, go out of solution into a bubble. The presence of bubbles most definitely doesn't make gas transfer any faster. That is, if a person's blood contains bubbles (and as you know, divers try to make them as few and small as possible to prevent DCS), the transfer of gas from the blood to some other tissue will not occur any faster than if the blood did not have bubbles. The risk, in fact, is that the bubbles may block a vessel and cause the transport of gases by the blood to cease.

On the matter of CO2, I think you are right that it is the most narcotic of the gases usually found in a diver's lungs (N2, O2, CO2, He).

I thought bubbles form only upon ascending. As long as you only descend or stay level, there should be no gasses out of solution. At least that's my understanding.

---------- Post added August 30th, 2013 at 12:03 PM ----------


It could be that the pressure simply alters how certain chemical reactions happen in the CNS and that the mere presence of N2 at elevated pressures does this. Disclaimer: totally unqualified opinion that isn't based on any hard facts.

It has been known for some time that there are gas bubbles in the blood even before a diver begin to ascend. In fact, even before he enters the water. Most recent models on the appearance of DCS (VPM, RGBM, AMB) consider that there are microbubbles before the diver begins his ascent and to limit the size of the bubbles and the total number of bubbles. They consider that the risk of DCS is acceptable if this combination of number of bubbles/size of biggest bubbles stays bellow a certain limit.

 

[bada3003]

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?

Overall, IMO, a reasonable rule of thumb but for the import of depth: usually ~100 fsw is used as common onset of nitrogen narcosis (nn), but increased bottom time at shallow depths (say 30 fsw) is associated with decompression sickness (dcs) but not nn. Since the mechanism of nn is not well-understood, even this is speculative but empirical evidence seems to support the crucial role of high pressure, not the total level of nitrogen in tissue/bloodstream. In the limit, saturation models have been used as the de facto model which is not associated with nn. For example, workers at the turn of the 20th century (bridge/tunnel construction, etc.) stayed for extended periods of time (well beyond NDL) and suffered dcs, but nn (or onset of euphoria) was not considered a key factor. Same goes for underwater dwelling that was popular once upon a time.

As to causality of nn, it's mostly speculation, but one educated guess is that relative reduction of O2 (such as CO2 euphoria) tends to slow down neurotransmission (increased inhibitory activity, decreased excitatory activity) which tends to feel like a buzz, and eventually, can lead to unconsciousness. But what role pressure plays in the above is unclear, hence unsatisfactory.