Changes in rat respiratory behavior under varying PCO2 levels and argon content in hyperbaric Ar-O2-CO2 atmospheres

An anesthetized rat preparation has been used for breathing studies in hyperbaric Ar-O2-CO2 atmospheres in which total saturation pressure was varied in the range 1-19.1 ATA and PCO2 was set at levels of 0.0, 0.05 or 0.10 ATA. The variables monitored were respiration rate and an isotonic measure of diaphragm-twitch amplitude during contracture/relaxation (twitch index). The results indicate that: (1) hyperbaric Ar-O2 atmospheres are able to maintain the animal in stable condition for several hours; (2) elevation in PCO2 level produces enhancements in respiration rate and twitch index that are markedly dependent on the PAr level; (3) successive increases in PAr are able to lower progressively the effects on both breathing parameters produced by either 0.05 or 0.10 ATA of CO2, with full abolition of both effects of inhaled CO2 in the PAr range 14-19 ATA; and (4) the intrinsic ability of hyperbaric argon to abolish respiratory enhancement by CO2 is greater than that of hyperbaric helium. These results have been discussed in terms of possible CNS loci at which hyperbaric argon (or helium) is able to antagonize centrally induced effects from dissolved molecular species derived from inspired CO2, and in relation to the broader task of control of excitatory/inhibitory centers of the CNS during diving by manipulation of the partial pressures of diluent components of breathing-gas mixtures.