Martini’s law: Each 15-metre (50-foot) depth is equivalent to the intoxication of one martini.
Although there is marked individual variation in susceptibility to IGN, all divers breathing compressed air are significantly affected at a depth of 60 to 70 metres (Figure 15.1). The minimum pressure producing signs is difficult to define, but some divers are affected subjectively at less than 30 metres.
The higher functions, such as reasoning, judgement, recent memory, learning, concentration and attention are affected first. The diver may experience a feeling of well-being and stimulation similar to the overconfidence of mild alcoholic intoxication. Occasionally, the opposite reaction, terror, develops. This is more probable in the novice who is apprehensive in this new environment. Further elevation of the partial pressure of the inert gas results in impairment of manual dexterity and progressive deterioration in mental performance, automatisms, idea fixation, hallucinations and, finally, stupor and coma. Some divers complain of a restriction of peripheral visual field at depth (tunnel vision). They are less aware of potentially significant dangers outside their prescribed tasks (perceptual narrowing). More recently, abnormal emotional processing has been described, with a suggestion that the emotional responses to threat are muted with increasing IGN.
From a practical point of view, the diver may be able to focus attention on a particular task, but the memory of what was observed or performed while at depth may be lost when reporting at the surface. Alternatively, the diver may have to abort the dive because of failure to remember instructions. Repetition and drills can help overcome these problems through a ‘practice effect’. Conversely, anxiety, cold, fatigue, sedatives, alcohol and other central nervous system depressant drugs aggravate narcosis.
Nitrogen narcosis has often been likened to alcoholic intoxication, especially the euphoria, lightheadedness and motor incoordination. There is some evidence that correlates subjective feelings of alcohol consumption and IGN, especially the variation in intensity experienced among individuals. In one elegant experiment reported in 2008, Hobbs2 found evidence that heavier drinkers did not show a reduction in the effects of nitrogen narcosis at depth, but they did show tolerance for the combined effects of narcosis and alcohol. At much greater depths the parallel with general anaesthetic agents is probably closer.
Some of the reported observations at various depths breathing compressed air are shown in Table 15.1.
The narcosis is rapidly evident on reaching the given depth (partial pressure) and is not progressive with time. It is said to be more pronounced initially with rapid compression (descent). The effect is rapidly reversible on reduction of the ambient pressure (ascent).
Other factors have been observed to affect the degree of narcosis. Cold, reduced sensory input, and both oxygen and carbon dioxide disturbances are interrelated in impairing the diver’s underwater ability. In experimental conditions, with an attempt to control variables, alcohol and hard work have been shown to enhance narcosis. Moderate exercise and amphetamines may, in certain situations, reduce narcosis, but some studies have conversely suggested unpredictable or increased narcotic effects with amphetamines. Increased carbon dioxide and nitrogen tensions appear to be additive in reducing performance. Task learning and positive motivation can improve performance. Frequent or prolonged exposure produces some acclimatization, but this may reflect a reduction in psychological stress rather than representing true adaptation. For example, Hamilton and associates3 have experimental evidence to suggest that the reported experience of adaption is more subjective than behavioural.
Direct pathological injury to the diver as a result of the high pressure of inert gas is unlikely. The danger is rather a result of how the diver may react in the environment while under the narcotic influence of nitrogen. Impaired judgement can lead to an ‘out-of-air’ drowning sequence, with no other apparent cause of death found. The diver affected by IGN may also be at increased risk of insidious hypothermia (see Chapter 28) because of decreased perception of cold and decreased shivering thermogenesis4,5. Jacques Cousteau6 shared his experience of nitrogen narcosis (Case Report 15.2).
CASE REPORT 15.2: A PERSONAL DESCRIPTION OF NITROGEN NARCOSIS BY JACQUES COUSTEAU
We continue to be puzzled with the rapture of the depths, and felt that we were challenged to go deeper. Didi’s deep dive in 1943 of 210 feet had made us aware of the problem, and the Group had assembled detailed reports on its deep dives. But we had only a literary knowledge of the full effects of I’ivresse des grandes profondeurs, as it must strike lower down. In the summer of 1947 we set out to make a series of deeper penetrations.
…I was in good physical condition for the trial, trained fine by an active spring in the set, and responsive ears. I entered the water holding the scrap iron in my left hand. I went down with great rapidity, with my right arm crooked around the shotline. I was oppressively conscious of the diesel generator rumble of the idle Elie Monnier as I wedged my head into mounting pressure. It was high noon in July, but the light soon faded. I dropped through the twilight, alone with the white rope, which stretched before me in a monotonous perspective of blank white signposts.
At 200 feet I tasted the metallic flavour of compressed nitrogen, and was instantaneously and severely struck with rapture. I closed my hand on the rope and stopped. My mind was jammed with conceited thoughts and antic joy. I struggled to fix my brain on reality, to attempt to name the colour of the sea around me. A contest took place between navy blue, aquamarine and Prussian blue. The debate would not resolve. The sole fact I could grasp was that there was no roof and no floor in the blue room. The distant purr of the diesel invaded my mind – it swelled to a giant beat, the rhythm of the world’s heart.
I took the pencil and wrote on a board, ‘Nitrogen has a dirty taste’. I had little impression of holding the pencil, childhood nightmares overruled my mind. I was ill in bed, terrorised with the realisation that everything in the world was thick. My fingers were sausages. My tongue was a tennis ball. My lips swelled grotesquely on the mouth grip. The air was syrup. The water congealed around me as though I were smothered in aspic.
I hung stupidly on the rope. Standing aside was a smiling, jaunty man, my second self, perfectly self-contained, grinning sardonically at the wretched diver. As the seconds passed the jaunty man installed himself in my command and ordered that I unloose the rope and go on down.
I sank slowly through a period of intense visions.
Around the 264 foot board the water was suffused with an unearthly glow. I was passing from night to an imitation of dawn. What I saw as sunrise was light reflected from the floor, which had passed unimpeded through the dark transport strata above. I saw below me the weight at the end of the shotline, hanging twenty feet from the floor. I stopped at the penultimate board and looked down at the last board, five metres away, and marshalled all my resources to evaluate the situation without deluding myself. Then I went to the last board, 297 feet down.*
The floor was gloomy and barren, save for morbid shells and sea urchins. I was sufficiently in control to remember that in this pressure, ten times that of the surface, any untoward physical effort was extremely dangerous. I filled my lungs slowly and signed the board. I could not write what it felt like fifty fathoms down.
I was the deepest independent diver. In my bisected brain the satisfaction was balanced by satirical self-contempt.
I dropped the scrap iron and bounded like a coiled spring, clearing two boards in the first flight. There, at 264 feet, the rapture vanished suddenly, inexplicably and entirely. I was light and sharp, one man again, enjoying the lighter air expanding in my lungs. I rose through the twilight zone at high speed and saw the surface pattern in a blaze of platinum bubbles and dancing prisms. It was impossible not to think of flying to heaven (Cousteau JY. The Silent World. London: Reprint Society; 1954).
* 297 feet is 90.5 metres (don’t try repeating this experiment!).