Predicting Fetal Acidemia Using Umbilical Venous Cord Gas Parameters To the Editor: We read with interest the article by Cantu et al1 regarding the prediction of fetal umbilical artery base deficit through the umbilical vein base deficit value. The authors indicate that venous blood gas parameters are powerful predictors of arterial blood gas pH and that base deficit and can be used to predict the likelihood of fetal acidemia when cord arterial blood gas is not available. Although the research was wellperformed and provides interesting results, there are a number of concerns. The authors provide receiver operator characteristic (ROC) curves for more than 11,000 patients with paired arterial and venous blood gas values. These ROC curves confirm that, in the vast majority of cases, umbilical venous base deficit predicts umbilical artery base deficit, with the arterial values being slightly more acidotic. However, only 1.1% of cases had fetal pH acidemia and only 2.1% had umbilical artery base deficit 12 mmol/L or greater. Thus, to utilize the predictive value of the 98% nonacidemic cases in an ROC curve does not provide an assessment of the predictive value in the 1–2% of acidemic cases. The authors acknowledge that acute cord compression or bradycardia can cause a large difference between the arterial and venous pH (and base deficit), although they do not provide an analysis of their own subset of cases in which a large difference occurred. The authors note that important medical–legal applications can be derived from the ability to predict the arterial base deficit based on the venous values. However, it is specifically among the cases with elevated base deficit values, and oftentimes the discrepancy between the umbilical artery and umbilical vein base deficit, due to sudden cord occlusion or bradycardia that may result in liability allegations. It would be informative to report the predictive value of umbilical vein base deficit in the 2% of cases with a base deficit greater than 12 mmol/L. Because these cases of acute cord compression typically are accompanied by elevated umbilical artery carbon dioxide
partial pressure values, it is also important to assess both base deficit (blood) and base deficit (extracellular fluid), because the latter value adjusts for markedly elevated carbon dioxide partial pressure levels to provide a more precise index of the degree of metabolic acidosis.2 Of note, assuming appropriate resuscitation, early newborn arterial base deficit values often provide an appropriate index of the umbilical artery base deficit when this value is unavailable.3 Financial Disclosure: The authors did not report any potential conflicts of interest.
Michael G. Ross, MD, MPH David Geffen School of Medicine at UCLA and, Fielding School of Public Health at UCLA, Los Angeles, California; Department of Obstetrics and Gynecology, Harbor-UCLA Medical Center, Torrance, California Kevin Amaya, DO Department of Obstetrics and Gynecology, Harbor-UCLA Medical Center, Torrance, California
REFERENCES 1. Cantu J, Szychowski JM, Li X, Biggio J, Edwards RK, Andrews W, et al. Predicting fetal acidemia using umbilical venous cord gas parameters. Obstet Gynecol 2014;124:926–32. 2. Morgan TJ. Partitioning standard base excess: a new approach. J Clin Monit Comput 2011;25:349–52. 3. Ross MG, Gala R. Use of umbilical artery base excess: algorithm for the timing of hypoxic injury. Am J Obstet Gynecol 2002;187:1–9. Editor’s Note: Cantu et al declined to respond.
Outpatient Cervical Ripening by Nitric Oxide Donors for Prolonged Pregnancy: A Randomized Controlled Trial To the Editor: I read with interest the article by Schmitz et al.1 Nulliparous pregnant women with Bishop scores of less than 6 who needed induction of labor were randomized to receive 40 mg vaginal isosorbide mononitrate or a placebo at 41 0/7, 41 2/7, and 41 4/7 weeks of
gestation. At 41 5/7 weeks of gestation, in women who had not yet given birth, labor was induced with oxytocin or prostaglandins according to Bishop score and local protocols. The primary outcome measure (cesarean delivery rate) was not significantly different between the two groups.1 Pharmacologic agents used for induction of labor that are likely to have any effect on the cesarean delivery rate are those associated with the initiation and maintenance of human parturition in term pregnancies (at least 37 weeks of gestation). Both oxytocin and prostaglandins are associated with initiation and maintenance of human parturition,2 but the role of isosorbide mononitrate—a nitric oxide donor—has not yet been proven.3 This begs the question as to whether the primary outcome measure of this study would ever be realized. The Cochrane review of randomized controlled trials of oxytocin for cervical ripening and induction of labor demonstrates that fewer nulliparous women achieved significant change in cervical ripening and vaginal delivery within 24 hours when the use of oxytocin alone was compared with the addition of prostaglandins.4 I am of the opinion that the first choice of induction of labor agent in the study protocol, irrespective of Bishop score, should have been prostaglandins as opposed to oxytocin alone. This approach is likely to minimize the risk of cesarean delivery owing to prolonged labor or failed induction of labor. Financial Disclosure: The author did not report any potential conflicts of interest.
Olaleye Sanu, MRCOG Department of Obstetrics and Gynaecology, West Hertfordshire Hospitals NHS Trust, Watford. United Kingdom
REFERENCES 1. Schmitz T, Fuchs F, Closset E, Rozenberg P, Winer N, Perrotin F, et al. Outpatient cervical ripening by nitric oxide donors for prolonged pregnancy: a randomised controlled trial. Obstet Gynecol 2014;124:1089–97. 2. Fuchs AR, Goeschen K, Husslein P, Rasmussen AB, Fuchs F. Oxytocin and initiation of human parturition.111. Plasma concentration of oxytocin and 13, 14-dihydro-15-keto-protaglandin F2
VOL. 125, NO. 3, MARCH 2015
Copyright ª by The American College of Obstetricians and Gynecologists. Published by Wolters Kluwer Health, Inc. Unauthorized reproduction of this article is prohibited.
Letters
741
partial pressure values, it is also important to assess both base deficit (blood) and base deficit (extracellular fluid), because the latter value adjusts for markedly elevated carbon dioxide partial pressure levels to provide a more precise index of the degree of metabolic acidosis.2 Of note, assuming appropriate resuscitation, early newborn arterial base deficit values often provide an appropriate index of the umbilical artery base deficit when this value is unavailable.3 Financial Disclosure: The authors did not report any potential conflicts of interest.
Michael G. Ross, MD, MPH David Geffen School of Medicine at UCLA and, Fielding School of Public Health at UCLA, Los Angeles, California; Department of Obstetrics and Gynecology, Harbor-UCLA Medical Center, Torrance, California Kevin Amaya, DO Department of Obstetrics and Gynecology, Harbor-UCLA Medical Center, Torrance, California
REFERENCES 1. Cantu J, Szychowski JM, Li X, Biggio J, Edwards RK, Andrews W, et al. Predicting fetal acidemia using umbilical venous cord gas parameters. Obstet Gynecol 2014;124:926–32. 2. Morgan TJ. Partitioning standard base excess: a new approach. J Clin Monit Comput 2011;25:349–52. 3. Ross MG, Gala R. Use of umbilical artery base excess: algorithm for the timing of hypoxic injury. Am J Obstet Gynecol 2002;187:1–9. Editor’s Note: Cantu et al declined to respond.
Outpatient Cervical Ripening by Nitric Oxide Donors for Prolonged Pregnancy: A Randomized Controlled Trial To the Editor: I read with interest the article by Schmitz et al.1 Nulliparous pregnant women with Bishop scores of less than 6 who needed induction of labor were randomized to receive 40 mg vaginal isosorbide mononitrate or a placebo at 41 0/7, 41 2/7, and 41 4/7 weeks of
gestation. At 41 5/7 weeks of gestation, in women who had not yet given birth, labor was induced with oxytocin or prostaglandins according to Bishop score and local protocols. The primary outcome measure (cesarean delivery rate) was not significantly different between the two groups.1 Pharmacologic agents used for induction of labor that are likely to have any effect on the cesarean delivery rate are those associated with the initiation and maintenance of human parturition in term pregnancies (at least 37 weeks of gestation). Both oxytocin and prostaglandins are associated with initiation and maintenance of human parturition,2 but the role of isosorbide mononitrate—a nitric oxide donor—has not yet been proven.3 This begs the question as to whether the primary outcome measure of this study would ever be realized. The Cochrane review of randomized controlled trials of oxytocin for cervical ripening and induction of labor demonstrates that fewer nulliparous women achieved significant change in cervical ripening and vaginal delivery within 24 hours when the use of oxytocin alone was compared with the addition of prostaglandins.4 I am of the opinion that the first choice of induction of labor agent in the study protocol, irrespective of Bishop score, should have been prostaglandins as opposed to oxytocin alone. This approach is likely to minimize the risk of cesarean delivery owing to prolonged labor or failed induction of labor. Financial Disclosure: The author did not report any potential conflicts of interest.
Olaleye Sanu, MRCOG Department of Obstetrics and Gynaecology, West Hertfordshire Hospitals NHS Trust, Watford. United Kingdom
REFERENCES 1. Schmitz T, Fuchs F, Closset E, Rozenberg P, Winer N, Perrotin F, et al. Outpatient cervical ripening by nitric oxide donors for prolonged pregnancy: a randomised controlled trial. Obstet Gynecol 2014;124:1089–97. 2. Fuchs AR, Goeschen K, Husslein P, Rasmussen AB, Fuchs F. Oxytocin and initiation of human parturition.111. Plasma concentration of oxytocin and 13, 14-dihydro-15-keto-protaglandin F2
VOL. 125, NO. 3, MARCH 2015
Copyright ª by The American College of Obstetricians and Gynecologists. Published by Wolters Kluwer Health, Inc. Unauthorized reproduction of this article is prohibited.
Letters
741
