[Investigations on the mechanism of the effects of ketamine (Ketanest) on circulation and respiration (author's transl)].

The effects of ketamine 5 mg/kg on blood pressure, heart-rate and the efferent action-potentials of the sympathetic and phrenic nerves were investigated in 17 relaxed and artificially ventilated cats. In part of the experiments (8 cats) ketamine was given as the sole narcotic agent, in the rest ketamine was applied a light basic as anaesthesia with N2O/O2 in relation of 2:1. The findings were, that ketamine given as monoanaesthetic agent increase blood pressure and heart-rate, as well as efferent sympathetic activity. The registration of the phrenic nerve activity showed a prolongation of the inspiratory periods, but no change of the general activity. In contrast, ketamine combined with nitrous-oxide resulted in a reduction of blood pressure and in a clear depression of the activity of both the sympathetic and phrenic nerves. These results may lead to the conclusion, that the blood pressure increasing effect of ketamine is, at least to a large extent, due to a suppression of inhibitory neurons in central-nervous sympathetic areas, whereas the increase of the heart-rate may be caused by a depression of parasympathetic centres. The characteristic changes of the inspiratory activity of the respiration centre may be due to a disinhibition of inspiratory neurons. Because these results were obtained after application of anaesthetically effective doses, these experiments lead to the conclusion, that after application of ketamine, effects were registered in the stage of tolerance, which with other anaesthetics only appear in the stage of excitation.