Nitrogen narcosis also goes by its rather dramatic stage name: the rapture of the deep.
Because we are medical professionals and not thespians, indulge us in continuing to use the former name as we discuss it as a clinical entity. It also gets called "inert gas narcosis" which is a rather pointless term enjoyed by pedants the world over, but, as a pedant myself, I would point out that other inert gases (like helium) don't produce a similar narcosis, and when diving, one is exposed to one gas in particular that produces it: nitrogen. Thus, we shall continue to call it what it is: nitrogen narcosis.
Though N2 is usually a physiologically inert gas, at high partial pressures it can affect the CNS. The reason for this is thought to be the relatively high ratio of fat-to-water solubility (which is also a property of anaesthetic agents), and though N2 is poorly soluble gas at atmospheric pressure, the high partial pressure present during descent forces it into solution (à la Henry's law). To explain how this generates any sort of psychotropic effect would necessitate an explanation of how any volatile anaesthetic works - and since we don't really know, that will be difficult (anaesthetists at this point will be stuttering something about the Meyer-Overton hypothesis or Mullin's critical volume hypothesis - but let me just say that the fact that there are multiple hypotheses means by definition that we don't really know yet, do we).
In terms of toxidrome, nitrogen narcosis most closely resembles the effects of alcohol, which has led to yet another interestingly named "law" known as the Martini Law - for every 10m descended after 20m, the dissolved nitrogen should have an effect equivalent to one martini. Experimental evidence would suggest there is some disinhibitory effect on impulsivity at 20m, but otherwise no change in reaction time or simple cognitive tasks [1]. For recreational divers descending to 30m the effect should therefore be mild in all but those with the very weakest of tolerances, and be limited to mild euphoria. The mildly impaired reasoning and dexterity may still contribute to error, but is unlikely to cause issues in isolation. For technical divers however, this is a different story. The main risks here are impaired concentration and cognition, causing errors of judgement or calculation, disinhibition and overconfidence, resulting in risk-taking behaviours, and eventually hallucinations and unconsciousness, one or both of which will eventually lead to drowning. At these depths a High Pressure Neurological Syndrome (HPNS) has also been proposed to occur as a result of the direct compressive effects of depth, possibly altering lipid bilayers and changing neuronal conduction velocity [2]. Environmental factors like cold temperatures further exacerbate this, itself impairing neurocognition [3], as will alcohol [4]. Most alarmingly, while divers are usually aware they are suffering a degree of narcosis, their self-assessment of function is often not great [5].
Prevention largely revolves around the use of He-O2 mixtures (Heliox), as helium is only half as soluble as nitrogen and therefore less gets dissolved in tissues. When a series of masochists researched neurocognitive function at a whopping 360m, the 18 previously healthy participants did demonstrated marked cognitive impairment but otherwise didn't come to harm. Enriched-air nitrox mixtures (EANx) may or may not reduce the risk - the jury is still somewhat out [5,6], though there are of course other benefits to its use. Calculations used to calculate Equivalent Nitrogen Depth (END) should be used but will not be replicated here for the time being.
Don't forget that after ascending, there may still be a residual effect. Balestra et al demonstrated that changes persisted for up to 30 minutes post-dive in divers descending to 33m, as measured through something known as critical flicker fusion frequency (CFFF) [7]. Understanding how this works exceeds the limitations of my own cognitive impairment, as I would've simply had participants do algebra or something as a means of neurocognitive testing - I guess this is more difficult to standardise. Karakaya et al demonstrated a similarly delayed post-dive recovery in volunteers pressurised to 500kPa (40m), using the even more incomprehensible auditory event-related potentials (AERPs) [8]. This is significant for a number of reasons: recently ascended divers may be impaired when attending to tasks like rendering aid to others or driving. This may assume additional importance in combat divers or underwater demolitions.
The residual neurocognitive effects of diving may persist longer than just the immediate post-dive period. High-frequency high-depth divers have been found to have a reduction in both cerebral blood flow and cognitive performance when compared to those who dived at lower frequencies and to lesser depth [9]. When visuospatial skills were compared between experienced divers and healthy controls, the experienced diver group also demonstrated poorer function [10]. This may be the first evidence of a cognitive decline occurring as a result of long-term diving, or may simply reflect an absolutely wild selection bias relating to those who choose to dive in the first place. I have two comments to make on this subject. One being that if we require advanced neuropsychological testing in order to detect an impairment that is otherwise not obvious, then the clinical relevance of this information may be limited, however interesting it may be. Secondly, if there genuinely is a causal relationship between diving and neurocognitive impairment, this may well relate to other pathologies like chronic exposure to decompressive illness, rather than nitrogen narcosis specifically.
In summary:
Recreational divers operating within accepted depths are unlikely to be significantly affected by nitrogen narcosis, though it may contribute to death due to other issues (and for the love of all that is holy, avoid alcohol). Technical divers are much more prone, and should be trained to recognise narcosis and employ risk mitigation strategies like Heliox. Some degree of narcosis could still be present in the early post-dive period.
Don't underestimate the effects of pressure that diving puts on the human body. The barometric pressure you would be exposed to at the top of Mt Everest is two-thirds that of sea-level. Conversely, to increase pressure by additional two-thirds that of sea-level you only need to dive just 6.7m below its surface. That 6.7m of water is equivalent to 8,849m of air. Underestimate it at your peril.
Credit:
DR1
References:
[1] Steinberg F, Doppelmayr M. Executive functions of divers are selectively impaired at 20-meter water depth. Front Psychol. 2017;8:1000.
[2] Talpalar AE. High pressure neurological syndrome. Rev Neurol. 2007;45(10):631-6.
[3] Martin K, McLeod E, Périard J, et al. The impact of environmental stress on cognitive performance: a systematic review. Hum Factors. 2019;61(8):1205-46.
[4] Fowler B, Adams J. Dissociation of the effects of alcohol and amphetamine on inert gas narcosis using reaction time and P300 latency. Aviat Space Environ Med. 1993;64(6):493-9.
[5] Hobbs MB. Impairment from gas narcosis when breathing air and enriched air nitrox underwater. Aviat Space Environ Med. 2014;85(11):1121-4.
[6] Germonpré P, Balestra C, Hemelryck W, et al. Objective vs. subjective evaluation of cognitive performance during 0.4-mPa dives breathing air or nitrox. Aerosp Med Hum Perform. 2017;88(5):469-75.
[7] Balestra C, Lafère, P Germonpré. Persistence of critical flicker fusion frequency impairment after a 33 mfw SCUBA dive: evidence of prolonged nitrogen narcosis. Eur J Appl Physiol. 2012; 112(12):4063-8.
[8] Karakaya H, Aksu S, Egi SM, et al. Effects of hyperbaric nitrogen narcosis on cognitive performance in recreational air SCUBA divers: an auditory event-related brain potentials study. Ann Work Expo Health. 2021;65(5):505-15.
[9] Slosman DO, Ribaupierre SD, Chicherio C, et al. Negative neurofunctional effects of frequency, depth, and environment in recreational scuba diving: the Geneva "memory dive" study. Br J Sports Med. 2004; 38(2):108-14.
[10] Ergen M, Uslu A, Caglar O, et al. Evaluation of cognitive performance in professional divers by means of event-related potentials and neuropsychology. Clin Neurophysiol. 2017;128(4):579-88.