Decreased thiopental requirements in early pregnancy.
BACKGROUND
Anesthetic requirements for inhalational agents are decreased during pregnancy, but there are no data regarding requirements for intravenous agents. The quantal dose-response curves for thiopental were calculated for 70 nonpregnant women having gynecologic surgery and for 70 pregnant women of 7-13 weeks' gestation having elective abortions.


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 for hypnosis. Patients who did not open their eyes were given a 10-s, 50-Hz, 80-mA transcutaneous tetanic electrical stimulus to the ulnar nerve as a test for anesthesia. Purposeful movement indicated that there was no anesthesia. Log dose-response curves for hypnosis and anesthesia were calculated after logit transformation.


RESULTS
In the nonpregnant women, the median effective doses (ED50s) (95% confidence interval) 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 ED50s were 2.6 (2.3-2.8) mg/kg and 4 (3.7-4.4) mg/kg. In the non-pregnant women, the ED95s (95% CI) for hypnosis and anesthesia were 4.4 (3.9-5.4) mg/kg and 6.4 (5.7-7.9) mg/kg, whereas in the pregnant women the corresponding ED95s 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 hypnosis 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 that for anesthesia was 18% less in pregnant women of 7-13 weeks' gestation compared with that in nonpregnant women.
Gin
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Department of Anaesthesia and Intensive Care, Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, New Territories, Hong Kong.
Mainland
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Chan
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Short
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D000328:Adult; D018686:Anesthetics, Intravenous / Q000008:administration & dosage; D001835:Body Weight; D004305:Dose-Response Relationship, Drug; D005260:Female; D006801:Humans; D011247:Pregnancy; D011261:Pregnancy Trimester, First; D013874:Thiopental / Q000008:administration & dosage*
D018686:Anesthetics, Intravenous; D013874:Thiopental