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Acute increase of umbilical artery vascular flow resistance in compromised fetuses provoked by uterine contractions

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Acute increase of umbilical artery vascular flow resistance in compromised fetuses provoked by uterine contractions
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  Acute increase of umbilical artery vascular flowresistance in compromised fetuses provoked byuterine contractions Hui Li a,b , Saemundur Gudmundsson a  , Per Olofsson a, * a   Department of Obstetrics and Gynecology, Malmo¨  University Hospital, Lund University,S-205 02 Malmo¨  , Sweden  b  No. 2 Clinical Hospital, China Medical University, Shenyang, PR China Accepted 3 July 2003 Abstract  Background  : An impaired placental circulation is involved in the mechanism of late fetal heart rate (FHR) decelerations.  Objectives : To explore umbilical artery (UA) Doppler velocimetry changesin response to uterine contractions during the oxytocin challenge test (OCT) and assess the potentially clinical value of ‘OCT Doppler velocimetry’.  Methods : 111 women with pregnancycomplications were subjected to an OCT with simultaneous recordings of FHR and UA pulsatilityindex (PI). Positive OCT cases (late FHR decelarations) were compared with negative OCT cases(normal FHR patterns). Only negative OCT cases were allowed a trial of vaginal delivery.  Results :The UA PI was higher in OCT positive (  N  =21) than in negative cases (  N  =90) during uterinecontractions and relaxations (  P  <0.05), but not during basal measurements. A positive OCT, but not negative, was associated with an increase of PI during contractions and relaxations compared with basal measurements (  P  <0.05). Fetal growth restriction (  N  =58), operative delivery for fetal distress(ODFD) in labor (  N  =10), and birth asphyxia (  N  =8) were not associated with PI changessignificantly different from cases without these complications.  Conclusions : During uterinecontractions and relaxations, but not during basal measurements, a significantly higher UA PIevolved in OCT positive cases compared with OCT negative cases. This indicates a pathophysiological mechanism disclosed only during uterine activity. Although these changeswould not have been revealed by traditional Doppler velocimetry, the data suggest a limited predictive value of ‘OCT Doppler velocimetry’ on the short-term neonatal outcome. D  2003 Elsevier Ireland Ltd. All rights reserved.  Keywords:  Artery; Contraction; Doppler; Oxytocin; Pregnancy; Umbilical; Uterine0378-3782/$ - see front matter   D  2003 Elsevier Ireland Ltd. All rights reserved.doi:10.1016/S0378-3782(03)00084-7* Corresponding author. Tel.: +46-40-331000; fax: +46-40-962600.  E-mail address:  per.olofsson@obst.mas.lu.se (P. Olofsson).www.elsevier.com/locate/earlhumdevEarly Human Development 74 (2003) 47–56  1. Introduction The fetal heart rate (FHR) pattern in labor is interpreted relative to the occurrence of uterine contractions. Typically, late FHR decelerations are provoked by the transient interruption of oxygen delivery to the fetus by uterine contractions, and are regarded a signof impending fetal hypoxia. An impaired placental circulation is also involved in thismechanism. During uterine contractions with late FHR decelerations, the umbilical artery(UA) vascular flow resistance increases significantly more than with a normal FHR pattern[1]. Fetuses with a marked increase of the UAvascular flow resistance during contractions run an increased risk of distress in labor  [2]. In uncompromised fetuses, both t he umbilical arterial and venous circulations are unaffected by uterine contractions [2–6].We have previously studied UA blood flow changes by means of Doppler velocimetry performed during the oxytocin challenge test (OCT) [7]. Although OCT is a laborious method and has at large been outmoded by more easy methods for antenatal fetalsurveillance, we have continued to use the OCT in selected cases of complicated pregnancies, e.g., in cases of intrauterine growth restriction (IUGR) with abnormal UADoppler flow velocity waveforms (FVW). In such cases, there is often uncertainty about when to interrupt the pregnancy and decide the best mode of delivery. We found the OCTto be abnormal (positive) twice as often in cases of IUGR with abnormal UA FVWs as incases with normal FVWs, but in OCT negative cases that allowed a trial of labor, thevaginal delivery rate was the same irrespective of normal or abnormal UA FVWs [8]. Asignificantly higher flow resistance evolves in both the uteroplacental and fetoplacentalcirculation in response to uterine contractions in OCT positive cases compared with OCTnegative cases [7]. These observations suggest an association between transitory utero-  placental and fetoplacental circulatory changes and late FHR decelerations. The aim of the present study was therefore to explore UA blood flow changes in response to uterinecontractions during the OCT, and to assess the potentially clinical value of ‘OCT Doppler velocimetry’ in a series of high-risk pregnancies. 2. Material and methods The study was performed at the Malmo¨ University Hospital, Sweden. All OCTs were performed on clinical indications at a gestational age of 36 completed weeks or more. A prerequisite for inclusion was the managing obstetricians’ uncertainty about the optimaltime and/or optimal mode of delivery. Hence, an increased UA vascular flow resistancewas often an additional indication to perform an OCT. The study comprised altogether 111women with pregnancy complications as shown in Table 1. The experiments with simultaneous Doppler velocimetry and FHR recordings during the OCT were approved by the Lund University Research Ethics Committee, and all participating women gavetheir informed consent. The results of the basal blood flow measurements were disclosedto the obstetrician, but the results of Doppler velocimetry during the OCT were concealed.The blood flow measurements were performed with an Acuson 128 or Acuson Sequoia512 real-time scanner (Acuson, Mountain View, CA, USA), using 3.5 and 5 MHztransducers or an automatic step-less 2.5–6 MHz probe, respectively, with pulsed and  H. Li et al. / Early Human Development 74 (2003) 47–56  48  color flow Doppler options. The high-pass filter was set at 100 Hz. The UA blood flowvelocity Doppler shifts were recorded in a free-floating part of the cord during fetalquiescence. The mean pulsatility index (PI) was calculated from three consecutive FVWsaccording to Gosling et al. [9]. The UA PI was classified normal when it is within mean F 2 standard deviations (S.D.) or high when it is above mean+2 S.D. according toreference values [10].A basal UA blood flow measurement was performed immediately before the OCT wasstarted. Cases with absent or reversed end-diastolic (ARED) flow were not included in thestudy, since this is an indication for prompt abdominal delivery at our department  [11– 14]. During the OCT, UA FVWs were recorded in-between and during uterine contrac-tions. The aim was to obtain Doppler shift signals during whole uterine contractionsrepeatedly during three to four consecutive contractions and relaxations, and to extract FVWs obtained during the peak of each contraction for analysis. The mean PI wascalculated from representative FVWs obtained during two to four contractions. Since FHR correlates negatively and linearly with the UA PI [15,16], and FHR decelerations were by definition present in OCT positive cases, the PI values were adjusted for heart rateaccording to the following equation: Corrected PI=observed PI+constant   (observedFHR   mean FHR). The PI values were corrected to an FHR close to the mean FHR obtained.The OCT was started with an intravenous oxytocin infusion at a rate of 6 ml/h (5 unitsof oxytocin in 500 ml 5.5% glucose, or saline in diabetics). The infusion rate was doubledevery 10 min until three uterine contractions per 10-min window were obtained and thenmaintained during the whole procedure. The maximum infusion rate was set at 96 ml/h.The infusion was stopped after the procedure or if a positive OCT occurred. The OCT wasclassified positive or negative according to Freeman’s criteria [17]. Although positive OCTcases may be optioned for a trial of vaginal delivery at other delivery units, a positiveOCT is an indication for a prompt cesarean section at our department. In negative OCT Table 1Distribution of complications and outcome among 111 pregnancies subjected to an oxytocin challenge test (OCT)Type of complication High basalUA PIPositiveOCTBirthasphyxiaODFD CS for other reasons thanfetal distressVaginaldeliveryIUGR, total (  N  =89) 37 18 3 10 13 49Preeclampsia a  (  N  =17) 6 1 – 1 7 8Diabetes  b (  N  =1) – – – – – 1Preeclampsia (  N  =2) 1 – 2 – 1 1Diabetes (  N  =2) 2 – – – – 2Oligohydramnios (  N  =8) 4 3 1 – – 5Poor obstetric history (  N  =3) 2 – – – – 3Prolonged pregnancy (  N  =2) 1 – 2 – – 2Decreased fetal movements (  N  =5) 2 – – – – 5Values are number of cases.UA PI=umbilical artery pulsatility index; ODFD=operative delivery for fetal distress; IUGR=intrauterinegrowth restriction; CS=cesarean section. a  IUGR cases with an additional diagnosis of preeclampsia.  b IUGR case with an additional diagnosis of diabetes mellitus.  H. Li et al. / Early Human Development 74 (2003) 47–56   49  cases, labor was induced or spontaneous labor was awaited. The median time from OCT todelivery in the total series was 1 day (range, 0–21).The UA PI values during basal measurements and during the OCTwere compared withregard to the OCT result (positive or negative OCT), operative delivery for impending fetaldistress (ODFD; abdominal, ventouse, or forceps delivery due to abnormal FHR pattern)in cases of negative OCT allowed a trial of labor, and birth asphyxia (defined as a 5-minApgar score <7 and/or an umbilical cord arterial blood pH <7.10 and/or venous blood pH <7.15).Suspected IUGR was defined as a fetal weight estimated to be below the mean valueminus 2 S.D. by ultrasound fetometry: appropriate-for-gestational age (AGA) as a birthweight within mean F 2 S.D., small-for-gestational age (SGA) as below mean minus2 S.D., and large-for-gestational age (LGA) as above mean plus 2 S.D. The weight deviation (WD) at birth was expressed as a percentage of deviation from the mean birthweight. All weight variables and definitions were gestational age-adjusted accordingto reference values [18]. 2.1. Statistical analyses The Chi-square test and Fisher’s Exact Test were used for comparison of categoricalvariables, the Mann–Whitney  U   test for comparison of cross-sectional non-pairedvariables, and the Wilcoxon matched-pairs signed-rank test for variables recordedlongitudinally. Simple linear regression analysis was used to show relationships betweenvariables. A two-tailed  P   value of <0.05 was considered significant. Statistics were performed with StatView R  (SAS Institute, NC) and MedCalc R  (MedCalc Software,Belgium) computer software. 3. Results The OCT became positive in 21 cases and negative in 90 cases. Gestational age wasequal in the two groups (269.9 F 7.5 days vs. 272.2 F 10.3 days;  P  =0.5). The basal UAPI was not significantly different between OCT positive and negative cases (Fig. 1,1.05 F 0.28 vs. 1.02 F 0.26;  P  =0.8).The perinatal outcome relative to pregnancy complications is displayed in Table 1. Inthe negative OCT group, in which women were allowed a trial of labor, nine women weredelivered by a cesarean section and one by ventouse because of ominous FHR patterns inlabor; none of these 10 neonates had signs of birth asphyxia. Overall, there were eight cases of birth asphyxia (prevalence 8/111=7.2%), of whom two had a positive OCT (onelethal malformation) and subsequent abdominal delivery (prevalence 2/21=9.5%), and sixhad a negative OCT with normal vaginal delivery (prevalence 6/90=6.7%) (  P  =0.3).Umbilical cord venous blood pH was similar in the two OCT groups (19 OCT positivecases, pH=7.32 F 0.04, vs. 78 OCT negative cases, pH=7.31 F 0.08;  P  =1.0).Simple linear regression analysis showed negative and significant correlations betweenFHR and UA PI during basal measurements as well as during uterine contractions andrelaxations. The PI values were accordingly adjusted to an FHR of 135 bpm (equations not   H. Li et al. / Early Human Development 74 (2003) 47–56  50  shown). The PI values with and without adjustment for FHR differences were compared,showing no difference in the OCT negative group (  P  >0.2), the OCT positive group(  P  >0.5), or when taking all cases together (  P  >0.2).The UA PI was significantly higher in OCT positive cases compared with OCTnegative cases during uterine contractions and relaxations, but not during basal measure-ments (Fig. 1). The significances of difference were the same whether adjusted or not for FHR differences.In cases of a positive OCT, the UA PI was significantly higher during OCTcontractionsand relaxations compared with basal measurements, and the PI during contractions washigher than during relaxations as well (Fig. 1). In negative OCT cases, no such differences were found. The significances of difference were the same irrespective of adjustment for FHR. No cases of ARED flow were observed during the OCT.Regarding the 10 cases of ODFD in labor in the OCT negative group, there was nodifference in UA PI during OCT contractions and relaxations compared with basalmeasurements (  P  >0.5). Furthermore, there were no differences compared with caseswithout ODFD regarding basal PI values or values obtained during the OCT (  P  >0.4).   Fig. 1. Umbilical artery pulsatility index before and during the oxytocin challenge test (OCT) relative to outcomeof the test. Mean and standard deviation values after adjustment for fetal heart rate changes are indicated. Double- pointing arrows denote a significant difference of   P  <0.05 between groups calculated by the Mann–Whitney  U  test and between measurements by the Wilcoxon matched-pairs signed-ranks test.  H. Li et al. / Early Human Development 74 (2003) 47–56   51
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