DECREASED THIOPENTAL REQUIREMENTS IN EARLY-PREGNANCY

Background: Anesthetic requirements for inhalational agents are decrea sed during pregnancy, but there are no data regarding requirements for intravenous agents. The quantal dose-response curves for thiopental w ere calculated for 70 nonpregnant women having gynecologic surgery and for 70 pregnant women of 7-13 weeks' gestation having elective aborti ons. Methods: Groups of 10 patients were given 2, 2.4, 2.8, 3.3, 3.8, 4.5, or 5.3 mg/kg thiopental as a bolus dose during a period of 10 s. Two minutes later, patients were asked to open their eyes as a test fo r hypnosis. Patients who did not open their eyes were given a 10-s, 50 -Hz, 80-mA transcutaneous tetanic electrical stimulus to the ulnar ner ve as a test for anesthesia. Purposeful movement indicated that there was no anesthesia. Log dose-response curves for hypnosis and anesthesi a were calculated after legit transformation. Results: In the nonpregn ant women, the median effective doses (ED(50)s) (95% confidence interv al) for hypnosis and anesthesia were 3.1 (2.8-3.4) mg/kg and 4.9 (4.5- 5.4) mg/kg, whereas in the pregnant women the corresponding ED(50)s we re 2.6 (2.3-2.8) mg/kg and 4 (3.7-4.4) mg/kg. In the nonpregnant women , the ED(50)s (95% CI) for hypnosis and anesthesia were 4.4 (3.3-5.4) mg/kg and 6.4 (5.7-7.9) mg/kg, whereas in the pregnant women the corre sponding ED(50)s were 3.7 (3.3-4.5) mg/kg and 5.2 (4.7-6.3) mg/kg. The pregnant to nonpregnant relative median potency (95% CI) ratio for hy pnosis was 0.83 (0.67-0.96) and for anesthesia it was 0.82 (0.62-0.94) . Conclusions: The dose of thiopental for hypnosis was 17% less and th at for anesthesia was 18% less in pregnant women of 7-13 weeks' gestat ion compared with that in nonpregnant women.