Influence of nitrous oxide, nitrogen, neon, and helium on the beating frequency of the mouse sinus node at high pressure.

The beating frequency (BF) reducing effect of 150 atm of hydrostatic pressure on mammalian cardiac pacemaker tissue (hyperbaric bradycardia) was counteracted by dissolved gas only if the gas was added after hydrostatic compression. The effect on BF seemed to be related to the narcotic potency of the gas and the effect was reversible. The gases tested were N2O, N2, Ne, and He, in decreasing order of potency. If N2O was added at a moderately raised ambient pressure prior to hydrostatic compression to 150 atm, there was no difference in the degree of hyperbaric bradycardia, compared to compression without gas. During decompression, however, experiments performed with gas showed a significantly higher gain in BF compared to experiments without gas. Autonomic blockade seemed to eliminate the difference between decompression with and without N2O. The results demonstrate that N2O, N2, and Ne, and to a small extent He, may counteract the retarding effect that increased hydrostatic pressure has on cardiac pacemaker activity. These effects on the cardiac pacemaker are similar both to the effects of increased hydrostatic pressure and of gases at elevated pressures on the central nervous system, but some important differences remain to be explained.