The effect of methotrimeprazine on arterial blood gases in human volunteers.

Since methotrimeprazine proved to be both an effective tranquilizer and analgesic, its effect in a tranquilizing dose of 0.15 mg/kg on the arterial blood gases was determined in human volunteers. Because of the known potentiating effect of some phenothiazines on the narcotic-analgesic induced respiratory depression and analgesia, the effect of methotrimeprazine on the meperidine-induced respiratory depression was also studied. Before, and at five minute intervals after the administration of the test drugs, PaO2, PaCO2 and pH were determined by a Radiometer Copenhagen Blood Gas Analyzer (Radiometer Copenhagen, 72 Endruvej, Denmark) through a Riley-needle. Continuous ECG lead II tracings were taken during the experiment. No significant decrease in PaO2 or increase in PaCO2 (P less than 0.01) was observed in 6 healthy volunteers (mean age = 25 yrs) after 0.15 mg/kg i.v. methotrimeprazine. In 19 volunteers (mean age = 32 yrs), the intravenous infusion of 1.5 mg/kg meperidine caused significant decrease in PaO2 and increase in PaCO2 five minutes after its administration. The combined administration of both drugs to 6 volunteers (mean age = 23 yr) caused initially the same decrease in PaO2 as after meperidine alone with subsequent increase in PaO2 over normal levels, however, the PaCO2 significantly increased both as compared to baseline values and as compared with meperidine alone. The pH reductions after the combination of both drugs were greater than after meperidine alone, which in combination with the PaCO2 values confirms the potentiation of meperidine-induced respiratory depression by methotrimeprazine. The results indicate the methotrimeprazine alone causes no significant respiratory depression, but it potentiates the respiratory depression caused by meperidine.