Wednesday 10 July 2013

Narcotics of the Deep... a Rapturous Dive

We all learn about Nitrogen Narcosis from our very first Diving Courses, be that with PADI, BSAC SSI, Naui, CMAS or GUE. It is an important consideration for any dives that we make but how well is it really understood in the Diving world?

What is Nitrogen Narcosis?

A Greek word (ναρκωσις) narcosis is derived from "narke", which means " temporary decline in, or loss of senses and movement".

Essentially, it is an alteration of consciousness as a result of the gases in our breathing mixes acting under pressure and a mixture of Dalton's and Henry's laws.

For those of you that don't remember them, this is the science bit... concentrate!!! (sorry L'oreal)

Dalton's Law

Dalton's Law states that the total pressure exerted by the mixture of non-reactive gases is equal to the sum of the partial pressures of individual gases.

daltons and henrys laws in scuba divingWe use it mostly in diving to work out our Partial Pressure of Oxygen at a given depth or our END (equivalent Narcotic Depth) for trimix

Henry's Law

 Henry's Law States that at a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.

This goes back to decompression theory and the fact that, in the most basic terms, as we go deeper and the ambient pressure increases, more of the inert gas that we breathe dissolves in the body to balance the pressures of gases in the body with the surrounding pressures outside.

How does that work then?

Very interesting, you say! I remember those laws now but what does that have to do with Nitrogen Narcosis?

Well, let's look at what we believe to be the history and mechanics of Narcosis first.

Narcosis was first described back in 1834 by Victor T Junod, who was a French Researcher. His description referred to symptoms of intoxication being present and he surmised this was due to the increased ambient pressure increasing the blood flow in the body, which stimulated the nerve centres.

There were a few more theories over the years and much of our knowledge of Narcosis today is based on the study of anesthetics. The best theory we have for Nitrogen Narcosis at the moment is the Meyer-Overton Rule.

Around 1900, Meyer and Overton (independently) observed the potency of general anesthetic gases by testing their solubility in a simple organic solvent. They used Olive Oil as the organic Solvent, which represents the lipid bilayer found in our bodies, which make up the cell membranes of almost all living organisms and viruses.

Inert gas dissolves into the nerve cells lipid bilayer (Henry's Law) and causes narcosis by altering the nerve transmission in many diffuse areas of the brain resulting in symptoms described as; Euphoria, Over Confidence, Anxiety, Loss of Judgement and/or Paranoia.

We Diving instructors often teach our students about Narcosis by describing the "Martini Effect". This is a comparison between the way we act as we go deeper underwater and the way we act when we consume alcohol. Essentially, for every 10m we descend, it is likened to drinking one martini.

Even Jacques Yves Cousteau famously called it the Raptures of the Deep!!

Nitrogen Narcosis is a depth related problem. As we go deeper, the ambient pressure increases and so does the partial pressure of our inert gas (Dalton's Law). Beyond 30m, where the partial pressure of Nitrogen (PPN2) is above 3.16ata, we usually can begin to feel these symptoms

Is it just Nitrogen that causes Narcosis?

As Divers, we don't just have to worry about Nitrogen causing Narcosis at depth. It is widely accepted that all breathing gases have a narcotic effect but to varying degrees. Below is a list comparing the solubility of some gases in the Organic Solvent. Remember the Bigger the number next to the gas the more soluble it is and the more potency it has for Narcosis

Helium  0.045 
 Neon 0.300 
 Hydrogen 0.600 
 Nitrogen 1.000
 Oxygen 1.700
 Argon  2.300
 Krypton  7.100
 Carbon Dioxide  20.00
 Xenon  25.60

As you can see, our two primary gases in the breathing mix; Nitrogen and Oxygen are similar in Narcotic Potency with Oxygen being slightly more Narcotic than Nitrogen.

So why don't we worry so much about Oxygen Narcosis?

It is believed that the Narcotic effect of Oxygen is actually lessened becasue our bodies metabolise it but, to be safe, we are taught to assume Nitrogen and Oxygen have the same Narcotic Potency.

The suprising Gas I think is Carbon Dioxide. With such high solubility in lipids, we can see why it is such a dangerous gas and how Hypercapnea can kill a Diver.

Carbon Dioxide causes increased blood flow to the brain and is therefore, believed to increase and augment the narcotic effect of the other Gases too. This is why it is dangerous to complete hard, strenuous activity while diving and Skip Breathing or Shallow Breathing is BAD!!!!

As you can see from the table, the least Narcotic breathing gas is Helium, which is why deeper divers tend to opt for a trimix containing lower percentages of Nitrogen and Oxygen and filling the gap with Helium.

Some agencies even advocate trimix within recreational limits. For example GUE (Global Underwater Explorers) suggest that below 30m, breathing mixes should contain helium for less narcosis and better Work of Breathing.

Prevention and Management

The full complexity of Narcosis is not yet fully understood. However, the mechanism and primary agents responsible seem sufficient at the moment for us to make good decisions with regards to the gases we should use at certain depths while diving.

Recognising symptoms can be difficult due to the impairment they cause. The most dangerous aspect of Narcosis is the loss of good judgement and decision making abilities. If not recognised, these can lead to drowning. Good job we have our buddy there to help us out then!!!

To prevent Narcosis, avoid deep diving, strenuous activity, being cold, stress and Carbon Dioxide retention. If you are diving deep, use the correct gas mixes for the depth and keep your Equivalent Narcotic Depth above 30m.

Should you feel the symptoms of Narcosis, simply ascend and you should feel the symptoms diminish, although I have read reports recently that they may not vanish altogether until hours after you have surfaced.

We just don't know!!

Narcosis seems to be another Diving Problem that we are still only just learning about. It is dependent on the individual Diver's susceptibility to Narcosis and their health on the day of the Dive and many other variables that we just do not know enough about.

Be prudent, be conservative with your dives...

and if Nemo looks like he needs your reg more than you do, you are probably narked!!!

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