Inert Gas Narcosis: Measurement of Central Nervous System Effects

Although suitable and reliable indices of IGN are not yet available, the search continues. Such tests would be useful in predicting individual susceptibility (diver selection), comparing the relative narcotic potencies of different respiratory diluents for oxygen, delineating the role of factors other than inert gas in producing depth intoxication and monitoring the degree of impairment during practical tasks.

Attempts to quantify the effects of IGN can be roughly divided into two methods. The first is a psychological behavioural approach measuring performance on tasks such as mental arithmetic, memory, reaction time and manual dexterity. The second relies on observing a change in some neurophysiological parameter. Some representative studies are discussed to illustrate points.

Behavioural approach

The aspects of behaviour usually studied may be divided into three categories: cognitive ability, reaction time and dexterity. The cognitive functions are the most affected and dexterity the least. One early study measured the performances of 46 men on simple arithmetic tests; reaction time and letter cancellation were measured at pressures from 3.7 to 10 ATA. This study demonstrated quantitatively the previously observed qualitative progressive deterioration with increasing pressure of compressed air. It also showed that individuals of high intelligence were less affected. The impairment noted on arrival at the target pressure was exacerbated by rapid compression.

Another study using simple arithmetic tests of manual skill showed that narcosis was maximal within 2 minutes of reaching depth, and continued exposure did not result in further deterioration, but rather there was a suggestion of acclimatization. Muscular skill was much less affected than intellectual performance. Other studies involving reaction time, conceptual reasoning, memory and psychometric tests all showed progressive deterioration with increasing pressure. Narcosis has also been measured by tests of intelligence and practical neuromuscular performance.

Some work on open water divers suggested a greater impairment of performance on manual tasks at depth when anxiety was present. Plasma cortisol and urinary adrenaline/noradrenaline (epinephrine/norepinephrine) excretion ratios were used to confirm the presence of anxiety noted subjectively. Divers were tested at 3 and 30 metres at a shore base and in the open sea. Intellectual functions, as assessed by memory test, sentence comprehension and simple arithmetic, showed evidence of narcosis in both 30-metre dives, but the decrement was greater in the ocean dives, possibly because of the greater psychological stress in the open sea. Unsurprisingly perhaps, these effects have not been reproduced in the laboratory.

Many experimental protocols have been criticized because the effects of motivation, experience and learning, for example, are difficult to control. Caisson workers participating in a card-sorting test showed some impairment at 2 to 3 ATA, especially those who had relatively little exposure to pressure. However, with repeated testing, i.e. practice, this difference disappeared, and no loss of performance was noted even deeper than 3 ATA. These experiments were repeated using 80 naval subjects at 2 and 4 ATA while breathing air and helium-oxygen mixtures. The only significant impairment was found at 4 ATA breathing air.

The effects of IGN on behaviour, as measured by the psychologist, were well reviewed by Fowler, Ackles and Porlier7, and there has been relatively little work in this area since that time. More attention has been paid to the molecular mechanisms involved. Psychological studies suggest that the behavioural effects of all inert gases that produce narcosis are identical. Human performance under narcosis is explained using the ‘slowed processing’ model. Slowing is said to result from decreased activation or arousal in the central nervous system, manifested by an increase in reaction time, perhaps with a fall in accuracy. Increases in arousal, such as by exercise or amphetamines, may explain improved performance. Manual dexterity is less affected than cognitive functioning because dexterity requires fewer mental operations and there is less room for cumulative slowing of mental operations (processes). Although memory loss and impaired hearing are features of narcosis, these effects are more difficult to explain using the slowed processing model. A similar alteration in the processing of emotional experience has more recently been proposed by Löftdal and colleagues.

Studies of the subjective symptoms of narcosis have indicated that the diver can identify these symptoms and that they could relate the effect to the ‘dose’. Euphoria, as described by terms such as ‘carefree’ and ‘cheerful’, is only one of these symptoms and may not always be present. Other descriptive symptoms such as ‘fuzzy’, ‘hazy’ (state of consciousness) and ‘less efficient’ (work capability) and ‘less cautious or self-controlled’ (inhibitory state) may be more reliable indicators of effect on performance.

Behavioural studies have cast doubt on some traditional concepts of narcosis. True adaptation to narcosis has not been found in many performance tests. Where adaptation has been found, it is difficult to distinguish learned responses or an adaptation to the subjective symptoms from physiological tolerance. Carbon dioxide probably has additive and not synergistic effects in combination with nitrogen and probably acts by a different mechanism. Behavioural studies have not been able, so far, to demonstrate clearly the potentiating effects on IGN of anxiety, cold, fatigue, anti–motion sickness drugs and other sedatives (except alcohol).

Neurophysiological changes

Attempts have been made to confirm the subjective experiences and obtain objective evidence of performance decrement, with some neurophysiological parameter. The investigations included electroencephalographic records of subjects exposed to compressed air in chambers. Contrary to the expected findings of depression, features suggesting cortical neuronal hyperexcitability were noted at first. These included an increase in the voltage of the basal rhythm and the frequent appearance of low-voltage ‘spikes’ elicited by stimuli that do not have this effect at 1 ATA. Experiments in which the partial pressures of oxygen and nitrogen were controlled showed that in compressed air these changes are caused by the high oxygen partial pressure. If nitrogen-oxygen mixtures containing 0.2 ATA oxygen are breathed, these changes are absent. The depressant effects of nitrogen are then revealed. These consist of a decrease in the voltages of the basal rhythm and the appearance of low-voltage theta waves.

Blocking of electroencephalographic alpha rhythm by mental activity can be observed in half of the population. The observation that there is an abolition of this blocking on exposure to pressure introduced the concept of ‘nitrogen threshold’. It was found that the time to abolition of blocking was inversely proportional to the square of the absolute pressure (T is proportional to 1/P2) for an individual, although there was marked variation among subjects. In some persons abolition of blocking was noted at depths as shallow as 2.5 ATA, where no subjective narcosis was evident.

Flicker fusion frequency was investigated in an attempt to obtain a measurement that could be applied to the whole population. Subjects were asked to indicate when the flickering of a neon light, at a steadily increasing rate, appeared continuous. This is termed a ‘critical frequency’ of flicker. After a certain time at pressure, the critical frequency dropped. The same relationship, T is proportional to 1/P2, resulted. Critical flicker fusion tests have been adapted for the in-water environment and continue to be used in the context of IGN assessment.

A more direct measure of central nervous system functioning may be obtained by observing the effect of inert gas exposure on cortical evoked potentials. Evoked potentials are the electroencephalographic response to sensory stimuli. A depression of auditory evoked responses on exposure to hyperbaric air has been shown to correlate with the decrement in mental arithmetic performance under the same conditions. The conclusion was that auditory evoked response depression was an experimental measure of nitrogen narcosis. However, other work was unable to support this hypothesis and concluded that there is a complex relationship among hyperbaric oxygen, nitrogen narcosis and evoked responses.

Auditory evoked responses as a measure of narcosis are problematic because of sound alteration with pressure and the ambient noise during hyperbaric exposure. Visual evoked responses (VERs) have been used in an attempt to produce more reliable information. VERs were studied in US Navy divers, and reliable and significant differences were reported while the divers were breathing compressed air versus helium-oxygen mixtures at pressure. A further study using VERs during a shallow 2-week saturation exposure with excursion dives suggested that some adaptation to narcosis occurred, but it was not complete. Reduction of frequency and amplitude of alpha activity when compared with pre-exposure and post-exposure surface levels were also noted. Nevertheless, the value of current methods of measurement of IGN, by the use of neurophysiological changes, is questionable.