Compression of animals causes excitation, which has recently posed a barrier to deeper diving. The broad question addressed here is how far the inert gas breathed modifies the excitatory effects of hydrostatic pressure. By using aquatic animals we first show that
helium postpones the onset of pressure-induced
paralysis by some 35 atm. Next we show that in mammals compressed with
helium, five anaesthetic
gases (
nitrogen,
argon,
nitrous oxide,
carbon tetrafluoride, sulphur hexafluoride) all elevated dose-dependently the median pressure of four distinct phases of the
high pressure neurological syndrome (h.p.n.s.) (complete
spasms, clonic convulsions, tonic convulsions and non-tonic death). All the
gases were equally efficacious relative to their anaesthetic potency. However, the sensitivity of each phase of the h.p.n.s. to anaesthetic
gases differed. Most notably, the median pressure for tonic convulsions was elevated about three times more by a given partial pressure of anaesthetic gas than were the median pressures for complete
spasms or non-tonic death. These observations can be fitted remarkably well by the hypothesis that a given phase of the h.p.n.s. is activated when some hydrophobic region is compressed beyond a certain critical amount by the application of pressure. Absorption of an inert gas in this region will cause it to expand, tending to elevate the median pressure for that phase of the h.p.n.s. Our data and analysis allow the following conclusions relevant to diving practice. All
gases protect against the h.p.n.s. but some phases of this complex syndrome are more effectively controlled than others. Although addition of a second inert gas to
helium allows substantial increases in the pressure at which h.p.n.s. occurs, the onset of anaesthesia (or
inert gas narcosis) will limit the ultimate gain. The composition of therapeutic gas mixtures becomes more narrowly defined as the pressure increases. The optimum mixture may be different for each phase of the h.p.n.s., and the order of presentation of the h.p.n.s. symptoms may be changed by the second inert gas. We may also predict that physiological sites may exist where
helium acts like an anaesthetic. If such sites resulted in physiological dysfunction, addition of a second gas would exacerbate the situation.