Oligohydramnios: Antepartum fetal urine production and intrapartum fetal distress Lynn J. Groome, PhD, MD, John Owen, MD, Cherry L. Neely, RT, RDMS, and John C. Hauth, MD Birmingham, Alabama Animal and human studies suggest that fetal oliguria is a normal physiologic response to hypoxemia. To assess the clinical significance of this observation, we studied (before their admission) 51 fetuses of women whose pregnancies were complicated by oligohydramnios at ;?:38 weeks' gestation. We found that the mean hourly fetal urine production decreased significantly in relation to the severity of subsequent intrapartum fetal compromise. The mean antepartum rate was 95 ml/hr in the 21 fetuses with a normal intrapartum heart rate pattern; this fell to 59 ml/hr in the 18 fetuses who had an abnormal intrapartum heart rate pattern but who responded to intrauterine resuscitation. The rate was 33 ml/hr in the 12 fetuses who were delivered by cesarean section as a result of fetal distress. These findings suggest that oligohydramnios associated with fetal oliguria may be used to identify those fetuses who have less intrinsic or uteroplacental reserve than do those of women with oligohydramnios who have a higher rate of fetal urine production. (AM J OSSTET GYNECOL 1991 ;165:1077-80.)
Key words: Oligohydramnios, fetal urination, fetal distress The relationship between decreased amniotic fluid volume and adverse perinatal outcome 15 has led to the assessment of amniotic fluid volume as an integral part of antenatal fetal surveillance programs. 5.' Although a semiquantitative estimate of amniotic fluid volume is simple and rapid, there is currently no universally accepted definition of oligohydramnios. In 1981 Manning et al. l diagnosed oligohydramnios when the largest pocket of amniotic fluid was 51 cm in its greatest dimension. Chamberlain et al. 3 later suggested that this criterion should be changed to 52 cm. Subsequently, a number of other semiquantitative methods s. 9 were proposed in an effort to improve the predictive value of a reduction in amniotic fluid volume. Not only is there disagreement about what constitutes a clinically significant reduction in the amount of amniotic fluid, but the multiple pathways by which amniotic fluid is produced and the ways in which amniotic fluid volume is maintained lo . 12 make it difficult to determine the causes of oligohydramnios in every case. If no abnormalities exist in the genitourinary tract, oligohydramnios is often the result of a hypoxemiainduced reduction in renal blood flow that impairs fetal urine production. 13 Observations of the newborn infant
From the Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, University of Alabama at Birmingham. Presented at the Eleventh Annual Meeting of the Society of Perinatal Obstetricians, San Francisco, California, January 28 -February 2, 1991. Reprint requests: Lynn]. Groome, PhD, MD, the University of Alabama at Birmingham, Department of Obstetrics and Gynecology, UAB Station, Birmingham, AL 35294. 6/6/30893
support the premise that perinatal asphyxia, in which a decreased rate of urine production may be present, adversely affects renal function. 14 , 15 Additional support for this concept was recently provided by Nicolaides et al.,16 who reported a significant correlation between low hourly fetal urine production rates and fetal hypoxemia. In addition, animal studies 17 , 18 suggest that fetal oliguria may be a normal physiologic response to hypoxemia, However, because of the many possible pathways for amniotic fluid production and removal, other factors that are unrelated to hypoxemia may be responsible for the observed decrease in amniotic fluid volume. Of the possible fluid exchange routes, only the rate of fetal urine production can be assessed in utero by noninvasive means. 19 Because a hypoxemia-mediated decrease in fetal urine output may be responsible for a significant number of cases of oligohydramnios, it is reasonable to postulate that the condition is a disorder of varying severity and that those cases that arise as a result of fetal hypoxemia may be identified by a decrease in the rate of fetal urine production. This study was designed to assess perinatal outcome in terms of fetal urine production in patients with oligohydramnios at 2':38 weeks' gestation. If a decrease in fetal urine production before the onset of labor is a result of unrecognized chronic hypoxemia in patients with oligohydramnios, then the fetuses of these patients may have an increased risk of experiencing fetal distress during labor.
Material and methods Approximately 200 obstetric ultrasonographic examinations are performed each week by four experi-
Groome et al.
October 1991 Am J Ouslet G ynecol
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Fig. 1. Fetal bladder volume (FEV) in milliliters as a function of time (in minutes) for single fetus. Circl~s denote clinical measurements and solid line is least-squares best fit; slope of this line is rate of fetal urine production (HFUPR) (in milliliters per minute).
Fig. 2. Hourly fetal urine production rate (HFUPR) for three different outcomes: no fetal distress (open circles), fetal distress responsive to intrauterine resuscitation (half solid circles), and cesarean section because of fetal distress (solid circles).
enced technicians in the Ultrasound Laboratory at the University of Alabama at Birmingham. AGE RT-3000 is used for all obstetric scans. In this study, which was approved by the institutional review board, diagnosis of oligohydramnios was based on the visual qualitative estimate of amniotic fluid volume, which was assessed by an experienced ultrasonography technician. When patients with oligohydramnios were identified , the semi-quantitative method originally described byPheIan et al." was performed and the amniotic fluid index measured was recorded in the computerized data base. This information was not made available for decisions regarding management of patients. Patients with qualitative oligohydramnios were recruited for this study on ly if the amniotic fluid index was also ~8 cm. Exclusion criteria included estimated gestational age