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[Effect of Doppler ultrasound on perinatal mortality of high risk pregnancies: required patient sample?]
1Universitäts-Frauenklinik Tübingen.
The clinical value of Doppler ultrasound in obstetrics is related to the question whether pregnancy course and outcome will be improved when this method is introduced into obstetrical management. Recently a couple of randomized controlled management studies have been published and the majority of these trials showed significant beneficial effects. Even perinatal mortality may possibly be reduced, but none of these trials was large enough to achieve a significant reduction. Giles and Bisits [12] performed a metaanalysis by combining evidence from 6 of these trials in high risk pregnancies and could demonstrate a 50% reduction in perinatal mortality at a significance of 5%. But they did not report whether the sample size was large enough to detect this difference with the usually required power of at least 80%. Therefore the question remains whether obstetrical management should be changed in view of these results. Additionally, a clinically useful estimate is needed to determine the chance to reproduce the assumed beneficial effect. The present paper describes the statistical method for calculating the appropriate sample size needed in each group of a trial to detect a predefined treatment difference with specified significance and power. This method is applied in detail to the results of the meta-analysis of Giles and Bisits [12] and a minimal sample size of 1458 high risk pregnancies in each group is calculated. As the Doppler and the control groups actually contained 2102 and 2133 high risk patients respectively, these sample sizes proved to be large enough to detect a 50% reduction in perinatal mortality with a power of at least 80%. Furthermore, 59 high risk pregnancies would need to be managed by Doppler in order to prevent 1 case of perinatal death. In other words, it should be possible to prevent 4 perinatal death cases per 1000 total births when the high risk proportion is 25%.
The clinical value of Doppler ultrasound in obstetrics is related to the question whether pregnancy course and outcome will be improved when this method is introduced into obstetrical management. Recently a couple of randomized controlled management studies have been published and the majority of these trials showed significant beneficial effects. Even perinatal mortality may possibly be reduced, but none of these trials was large enough to achieve a significant reduction. Giles and Bisits [12] performed a metaanalysis by combining evidence from 6 of these trials in high risk pregnancies and could demonstrate a 50% reduction in perinatal mortality at a significance of 5%. But they did not report whether the sample size was large enough to detect this difference with the usually required power of at least 80%. Therefore the question remains whether obstetrical management should be changed in view of these results. Additionally, a clinically useful estimate is needed to determine the chance to reproduce the assumed beneficial effect. The present paper describes the statistical method for calculating the appropriate sample size needed in each group of a trial to detect a predefined treatment difference with specified significance and power. This method is applied in detail to the results of the meta-analysis of Giles and Bisits [12] and a minimal sample size of 1458 high risk pregnancies in each group is calculated. As the Doppler and the control groups actually contained 2102 and 2133 high risk patients respectively, these sample sizes proved to be large enough to detect a 50% reduction in perinatal mortality with a power of at least 80%. Furthermore, 59 high risk pregnancies would need to be managed by Doppler in order to prevent 1 case of perinatal death. In other words, it should be possible to prevent 4 perinatal death cases per 1000 total births when the high risk proportion is 25%.