Rapture
The Phenomenon With Many Names
I've heard countless divers say something along the line of "If you go deep enough, you start to feel drunk because of all the nitrogen." Are they wrong? Not entirely, but they aren't correct either.
As with anything related to dive theory, it serves us well to pursue a greater understanding of what happens to us underwater. The "drunk" feeling being described here can actually present in several different ways, and has gone by many different names.
Back in the old days, when "Sea Hunt" introduced the world to scuba diving, the coined term was most often "Rapture of the Deep." This described a sense of euphoria, overconfidence, and a dangerous loss of judgment.
Later, as we learned more about the science behind this challenge, the term "Nitrogen Narcosis" was popularized. It was the diving community's understanding that Nitrogen buildup in the brain led to these challenges, and that decreasing the partial pressure of nitrogen (PPN2) was the only way to address it. Unfortunately, man (if not most) divers today still abide by this term, largely a consequence of lacking continued education.
Gas Narcosis - the Modern Interpretation
With the assistance of modern technologies and an enormous amount of data being made available, the old interpretations of narcosis are being left behind.
As it would turn out, nitrogen is not the sole culprit, nor even the primary cause of what is now referred to as gas narcosis. In fact, every major gas we inhale while diving has a narcotic effect at depth, and nitrogen is among the least potent.
When determining narcotic impact, we need to consider both the Lipid Solubility Coefficient and the Effective Dose of each relevant gas. We'll get to the math shortly, but its worth noting that even oxygen is more narcotic at depth than the inert gas nitrogen.
As for the symptoms of gas narcosis, I've found that most everyone tends to oversimplify them. This doesn't sound like a big issue, but lack of understanding can lead to failure to recognize the signs while diving 120' beneath the surface.
In addition to cognitive slowing, divers may also experience euphoria or paranoia, (typically an exaggerated form of their mental state prior to affectedness.) Other signs include but are not limited to hyperfixation, short-term memory loss, time dilation, impaired judgement, visual/auditory distortion, numbness/tingling, and loss of fine motor skills.
The Math (Simplified)
The narcotic potency of any given gas is dictated by its lipid solubility coefficient and effective dosage. By combining these factors, we gain a great understanding of just how powerful an effect each gas has on our mind underwater.
The lipid solubility coefficient is related to the Meyer-Overton Rule, which states that narcosis is tied to how well a gas dissolves in lipids. The greater the coefficient, the more efficiently it dissolves.
Helium - 0.015
Nitrogen - 0.067
Oxygen - 0.14
Carbon Dioxide - 1.34
As you can see, nitrogen is actually among the least soluble gases encountered in diving. Oxygen is over twice as soluble, and carbon dioxide is twenty times more. Mind you, we still aren't done here.
The effective dose for the 50th percentile (ED50) for each gas is measured as atmospheres required for anesthetic response. The greater the value, the less potent its effects.
Helium - >190
Nitrogen - 35
Oxygen - 24
Carbon Dioxide - 0.25
Considering both factors, it becomes apparent that oxygen is only slightly more narcotic than nitrogen, whereas carbon dioxide is roughly 130x as potent.
Carbon Dioxide - The Quiet Antagonist
Having looked at the numbers, we can now tell that carbon dioxide is by far the most potent narcotic agent we might encounter while diving. Thankfully, there is no significant quantity of this in a properly filled gas tank, but its worth noting that hypercapnia )an excess of CO2) is still a very real problem in both recreational and technical diving.
CO2 is a natural byproduct of human respiration, and is typically exhaled in a fairly even exchange as we consume oxygen. That being said, many divers at various levels of training have encountered the effects of carbon dioxide retention and/or excessive production.
To put it simply, working harder underwater means expending more energy, which in turn results in more CO2 being produced by the body. This gas is stored in the blood in three different ways, mostly as bicarbonate ions.
Because of its high lipid solubility, CO2 crosses the blood-brain barrier with ease and causes a narcotic effect due to its strong potency (130x that of nitrogen.) In additon, CO2 is a vasodilator, meaning it increases blood flow to the brain and therefore exposes the brain to more narcotic gases.
Hypercapnia is also known to cause "air starvation" and shortness of breath, leading to shallow hyperventillation. This form of breathing is undersireable not only because of its effect on heart rate, but also because it limits gas exchange by failing to clear "dead space" in the trachea and mouth of CO2. This results in further increased levels of the gas, leading to a very unfortunate positive feedback loop.
In Summary
Gas narcosis is a real threat underwater, especially at depth. You may hear it referred to as nitrogen narcosis, but this is a dangerous misnomer that removes the blame from CO2 and leads newer divers to think that enriched air nitrox is less narcotic.
Management of gas narcosis is often simplified to something like "go up fifteen feet if you start feeling funny." While this is sound advice, its worth noting that prevention is just as important, if not more so.
Deep, slow, full breathing is non-negotiable. Taking our time underwater and avoiding an excess of exertion will help with this, and avoid overproduction of carbon dioxide to begin with.
Dive with a buddy, ensuring that you both check on each other frequently to monitor for signs of narcosis. Perform self checks, including your ability to estimate the passage of time using a dive watch or computer.
Never dive when you aren't feeling well, especially if the struggle is largely psychological. I've personally had to rescue a panicking diver underwater who was narced out of their mind, and later determined that CO2 and psychological distress were the most likely culprits.
Most importantly, ensure that you're getting the proper training required for your intended dive profile. If you're looking for options to further your dive education, or even just have some questions left over after reading this article, don't hesitate to reach out.
As with all content on this site, this article was typed out key-by-key by Cuda, who now needs a nap.