THE ANALGESIA PRODUCED BY NITROUS OXIDE, ETHYLENE AND CYCLOPROPANE IN THE NORMAL HUMAN SUBJECT

¹ From the Departments of Pharmacology and Anesthesia, University of Wisconsin School of Medicine, Madison

A technique previously described for the comparative study of the analgesia produced by the opiates in the human subject has been modified, by the substitution of an improved algesimeter for the von Frey hairs, and adapted to a similar study of subanesthetic concentrations of nitrous oxide, ethylene and cyclopropane.

Determination of the concentration of the different gases, which by continuous inhalation will produce an equal elevation of the pain threshold to an arbitrarily established level, makes it possible to estimate the relative analgesic potencies of the different gases. Since the level of depression used is within the realm of consciousness, it has been possible to obtain values for weak agents like nitrous oxide without the accompaniment of anoxia.

Cyclopropane in the range of concentrations studied has been determined to have approximately eight times the potency of nitrous oxide and six times the potency of ethylene.

No significant difference in the degree of analgesia was observed whether the gases were mixed with air or oxygen.

The optimum concentrations of the three gases for continuous inhalation which were found to produce the maximum degree of analgesia, and yet retain the coöperation of the subject, were nitrous oxide 35 to 40 per cent; ethylene 25 to 35 per cent; cyclopropane 4 to 6 per cent. Poor coöperation and occasionally unconsciousness may be obtained if these levels are much exceeded.

The concentration of these gases required to produce unconsciousness shows such great individual variation that this criterion cannot be used satisfactorily for comparative purposes.