Effects of variations in time pattern of nitrogen addition on development of HPNS in mice.

The effect of change in injection pattern in nitrogen on threshold pressures of three symptoms associated with compression in helium/nitrogen atmospheres was explored. Excitement threshold pressures decrease with increasing concentrations of N2 and are not affected by changing from continuous to equivalent bolus N2 injection. Coarse tremor onset is delayed in direct proportion to the amount of N2 present with the same relative potency in compression at 60 atm/h as at 1000 atm/h. Bolus injection of N2 is less than half as effective as continuous injection in this respect. Threshold pressures of convulsion from high pressure neurological syndrome (HPNS) increase with increasing amounts of N2, the relative anticonvulsant potency of the gas being independent of compression rate when this is introduced by continuous injection. A bolus effect similar to, though smaller than with coarse tremors, is encountered at a compression rate of 60 atm/h but is absent at 1000 atm/h. Late injection of the bolus likewise abolishes this bolus effect. Possible mechanisms to give rise to these effects are discussed, and the bearing of the data on time sequence of HPNS development is explored.