It is unlikely that HPNS will be able to be entirely prevented. Nevertheless, several approaches are possible either to delay onset or to modify its clinical effects.
Reduction in the rate of compression reduces the incidence and severity of HPNS. This can be achieved by a slower overall rate or by inserting stops to allow for acclimatization as greater depths are reached. Very slow rates of compression do improve or even prevent symptoms of HPNS in some subjects. Indeed, nausea can be virtually eliminated. Nevertheless, at depths exceeding 300 metres, even with 6 days of compression, some signs of HPNS are still present. With increasing depth, symptoms become more serious and severely limit the ability to perform useful work13. Performance decrements induced by compression improve during a stop at constant pressure, but total recovery has not been recorded at pressures greater than 300 msw14. Very slow compression rates are economically disadvantageous in occupational (saturation diving) settings, and they simply cannot be considered in technical ‘bounce’ diving applications. Thus, at a practical level, slowing compression rates is frequently not particularly useful.
Modification of the breathing gas mixture has been used to delay or modify HPNS. Interestingly, the narcotic effect of nitrogen has been used to counter some of the symptoms. Small amounts of nitrogen (5 to 10 per cent) introduced into the helium-oxygen mixture have been shown to markedly reduce tremor but not EEG abnormalities.
Hydrogen has also been studied in breathing gas mixtures14,15. Hydrogen is less dense than helium and thus would be even better for respiratory mechanics. Being more lipid soluble than helium, hydrogen has a greater narcotic potency, and this can be used to reduce some of the symptoms of HPNS. It appears that the narcotic potency of hydrogen is too great for it to be used alone with oxygen in very deep applications, but hydrogen-helium-oxygen mixtures with about 50 per cent hydrogen have allowed working dives to 500 metres to be achieved without significant symptoms of HPNS and with minimal performance decrement15. Electrophysiological changes and sleep disruptions were still present.
The advantages of added nitrogen include decreased cost, increased thermal comfort, a reduced distortion of speech and a reduction in the HPNS. In adding nitrogen to a deep diving mixture to reduce HPNS, the user must be careful not to increase the narcotic potency of the gas to a higher level than desired at the intended depth of use (see Chapter 62). The advantages of helium and hydrogen include a reduction in the narcotic effect and a reduction in the work of breathing.
Drugs such as alcohol, anaesthetics and anticonvulsants have been suggested to control HPNS. Ketamine is effective in preventing HPNS in rats. Barbiturates have an anticonvulsive effect over a wide range of pressures. Valproate is effective in baboon experiments in reducing HPNS at pressures greater than 40 ATA16. Other anticonvulsants have only a limited effect. Common anticonvulsant drugs such as phenytoin and carbamazepine had no effect on prevention of tremor, myoclonus and seizures in rats. This finding suggests that HPNS-induced seizures are of an unusual type, and that conventional anticonvulsant treatment would be of limited value for HPNS in humans17. Currently, the use of drugs to modify HPNS has no place in human exposure.
Brauer raised two possible problems in trying to control HPNS. First, the efforts to ameliorate HPNS may be effective only on the early manifestations. In so doing, a situation may be created where the first sign may be more serious HPNS, which, in animals, has been fatal. The second problem is that, in baboons, delaying the development of HPNS can induce a new set of symptoms that may involve brain damage5. This is all speculative, and it is clear that more work is required in this area.
HPNS is a major limiting factor in deep diving14. The extremely long duration of deep dives that involve very slow rates of compression to mitigate HPNS followed by slow decompression to avoid decompression sickness have curbed commercial interest in such diving. Thus, research into HPNS has progressed little since the 1990s. Extrapolation from lower primates to humans suggests that human divers are approaching depths at which seizures may be anticipated.