The respiratory depressant effects of barbiturates and narcotic analgesics in the unanaesthetized rabbit.

The effects of ten narcotic analgesics and three barbiturates have been investigated on respiratory rate, respiratory minute volume and blood Pco3 in the unanaesthetized rabbit. Both groups of drugs produced respiratory depression, and whilst the pattern of depression was similar for drugs of the same group, there were three main differences in the pattern of depression between the narcotic analgesics and the barbiturates. (1) In the case of the narcotic analgesics the relationship between the log dose and depression of minute volume was fairly steep and was linear over a wide range, whilst in the case of the barbiturates there was a distinct plateau in this relationship. (2) The log doseresponse for depression of respiratory rate was linear for both groups, but that produced by the narcotic analgesics was steeper than that produced by the barbiturates. Rabbits tolerated severe depression of respiratory rate when it was produced by a narcotic analgesic, whilst fatalities occurred after barbiturates with doses which produced far less depression. (3) Narcotic analgesics produced a dose-dependent elevation of blood PcOj which paralleled the changes in respiratory minute volume. Barbiturates produced little Pcoa elevation, and fatalities occurred with Pco2 elevations which were relatively insignificant The implications of these findings are discussed.