It has been widely believed that local anesthetics injected into the epidural space during uterine contractions may produce a high level of anesthesia. The purpose of this study was to test the validity of this clinical impression.Thirty-four full-term, healthy primiparous patients scheduled for epidural labor analgesia were studied. The patients were divided into two equal groups, A and B. In group A the inducing dose of 8 ml of 2 per cent chloro-procaine was injected at a rate of 1 ml/second during a uterine contraction. After the block had worn off (75 ± 3 minutes), a second dose was injected during the interval between uterine contractions. The protocol in group B was similar to that in group A except that the first injection was made during the interval between two uterine contractions and the second during a uterine contraction. The interval between the injections was 77 ± 7 minutes. The progression of labor and the descent of the presenting part during the two injections were similar to those in group A. The total number of segments blocked and changes in maternal blood pressure and heart rate obtained after the first injection were compared with those obtained after the second in each group by the f-test at levels of significance of P 0.05.In group A, the first injection (during contraction) blocked 16.7 ± 0.44 (± 1 SE) segments and decreased mean blood pressure by 4 ± 2 torr and HR by 2 ± 2.5 beats per minute. The second dose (between contractions) blocked 16.6 ± 0.46 segments and decreased blood pressure by 5 ± 2 torr and heart rate by 3 ± 2 beats per minute. In group B (reversed injection sequence), the first injection blocked 16.2 ± 0.44 segments and reduced blood pressure and heart rate by 4 ± 1 torr and 1 ± 2 beats per minute, respectively. There was no statistically significant difference in any of the measurements between the two injections in either group.Results of this study indicate that uterine contractions do not affect the spread of local anesthetic solution in the epidural space.
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