Fetal Scalp Blood Sampling
Fetal scalp blood sampling allows for the determination of the fetal acid–base status during labor. The technique requires dilation of the cervix, rupture of the
membranes, and access to the fetal presenting part. A lighted plastic endoscopic cone is inserted into the vagina and through the cervical os so that it rests against the fetal presenting part. Care should be taken to ensure that it is not placed over a fontanelle. The area to be sampled is dried with a sponge and coated with a thin layer of silicone to facilitate the formation of a blood globule. The scalp is then punctured with a microscalpel, and blood is collected by capillary action in a heparinized capillary tube. After the sample is mixed, it is transported on ice to the laboratory for blood gas analysis. During labor, a normal scalp blood pH is 7.25 to 7.35. A fetal scalp pH of greater than or equal to 7.25 provides evidence of a nonacidotic fetus. A scalp pH of 7.20 to 7.25 is considered suspicious, and sampling should be repeated within 30 to 60 minutes. Historically, pH values less than 7.20 have been considered acidotic; however, minor deviations below normal correlate poorly with perinatal outcome. Because abnormal perinatal outcome has not been consistently observed with values greater than 7.0, there is debate regarding the specific pH value that should be considered acidotic. In addition, fetal scalp sampling reflects the status of the peripheral blood, where acidosis is inherent, owing to the
accumulation of CO 2. Since respiratory acidemia is generated in the blood and metabolic acidemia is generated in the tissues, a scalp sample may not reflect the state
of the fetus. In light of the technical difficulty of the procedure and the uncertainty regarding interpretation of results, many centers have reduced their reliance on fetal scalp blood sampling.
Percutaneous Umbilical Blood Sampling
Electronic FHR monitoring, ultrasound, and fetal scalp blood sampling can provide useful information regarding the acid–base status of the fetus. Occasionally, however, direct access to circulating fetal blood is necessary. A classic example is the fetus with severe anemia secondary to Rh-isoimmunization. In this condition, maternal antibodies directed against fetal red blood cell antigens cross the placenta and bind to fetal red blood cells, resulting in hemolysis. At term, suspected fetal anemia can be managed by delivery and treatment of the neonate. Earlier in pregnancy, however, it may be preferable to delay delivery by assessing the fetal
hematocrit and, if necessary, performing intrauterine blood transfusion. Percutaneous umbilical blood sampling (PUBS) is a procedure that affords direct access to fetal venous blood. Using sterile technique and direct ultrasound guidance, a fine needle is passed transabdominally into the umbilical vein. Medications or blood may be infused through the needle once fetal blood samples have been obtained. Other indications for PUBS include suspected antibody-mediated fetal thrombocytopenia and fetal cardiac arrhythmias requiring assessment of fetal drug levels or direct fetal administration of antiarrhythmic agents.
Fetal blood sampling can be used along with cardiotocography (CTG) to assess fetal acid–base status during labor. However it requires additional expertise, is time-consuming, has some significant risks, and has only intermittent information. For these reasons, it is not widely used.
Fetal Scalp Stimulation and Fetal Vibroacoustic Stimulation
FHR accelerations of 15 beats per minute for 15 seconds in response to fetal scalp stimulation have been shown to predict a scalp pH greater than or equal to 7.19.
Among fetuses without an acceleratory response to scalp stimulation, 39% were considered acidotic (pH < 7.19). A similar relationship has been reported between fetal scalp pH and the FHR response to vibroacoustic stimulation with an artificial larynx applied to the maternal abdomen over the fetal head for 1 to 3 seconds. Among 30 fetuses with FHR accelerations in response to this stimulus, all had scalp pH values greater than or equal to 7.25. Half of the fetuses that did not respond to acoustic stimulation had pH values less than 7.25. FHR accelerations in response to external stimuli are thought to have the same predictive value as spontaneous
accelerations. Fetal stimulation is used in antepartum testing to shorten the time of the NST and in the intrapartum period to confirm fetal well-being when spontaneous accelerations are absent. There is no evidence in humans of adverse long-term effects of vibroacoustic stimulation.
Fetal Pulse Oximetry (Fetal Oxygenation)
The aim of fetal pulse oximetry is to provide a continuous assessment of fetal oxygen saturation. The fetal pulse oximeter provides a noninvasive continuous measurement of fetal arterial oxyhemoglobin saturation once membranes have been ruptured. A sensor, similar to an IUPC is placed transvaginally between the uterine wall and fetal face and measures oxyhemoglobin saturation. The system includes an optoelectronic sensor and a microprocessor-based monitor. It has two low-voltage, light-emitting diodes as light sources and one photodetector. One of the light-emitting diodes emits red light (735 nm) and the other emits infrared light (890 nm). When light from each light-emitting diode passes through fetal tissues at the sensor application site, a fraction is absorbed. The photodetector measures the reflected (nonabsorbed) light using a process similar to measurement of transmitted light in conventional pulse oximetry. The amount of light absorbed is related to the amount of oxy- and deoxyhemoglobin. Their ratio determines fetal oxygen saturation during each arterial pulse. The potential is that this technology will provide a practical and easy method for better evaluation of fetal well-being during labor.
In 1994, fetal oximetry was marketed in countries outside of the U.S., including Europe and Canada. In January 2000, the Obstetrics and Gynecology Devices Panel of the Medical Devices Advisory Committee of the Food and Drug Administration (FDA) recommended approval of a fetal oximeter for use in obstetrics (OxiFirst: N-400 Fetal Oxygen Monitoring System, Nellcor).
A U.S. multicenter, randomized, controlled trial of fetal oximetry enrolled 1010 women with abnormal FHR patterns. The objective of the trial was to determine whether fetal pulse oximetry with fetal monitoring would reduce the rate of cesarean delivery performed for non-reassuring fetal status. The rate of cesarean delivery for fetal distress was significantly lower in the fetal oximetry group (4.5%) compared to 10.2% in the conventional fetal monitoring group. Unexpectedly, however, the cesarean rate for dystocia was significantly higher in the oximetry group (18.5%) versus 8.6% in the control group), thus the overall cesarean rates were not different (29%
oximetry plus FHM vs. 26% FHM).
Clearly, continuous measurement of fetal oxygen saturation could be an important addition to the interpretation of heart rate patterns. The device is attractive in that it offers a direct measure of fetal oxygenation during labor. However, like its predecessor, electronic FHR monitoring, oximetry has the potential for widespread
application without conclusive evidence that it is efficacious. The use of this technology is the focus of several ongoing clinical studies and trials.
Fetal Cardiotocography Plus Pulse Rate-Interval Analysis Normally there is a negative relationship between the pulse rate (PR) interval and the FHR: as the FHR slows, the PR interval lengthens, and vice versa. In acidemic infants, this relationship is reversed. A retrospective study of 265 women suggested that the addition of time-interval analysis of the fetal ECG would decrease the rate of unnecessary fetal blood sampling or assisted delivery for presumed fetal distress. However, in a prospective randomized trial of the use of fetal ECG time-interval variables in addition to cardiotocography in fetal surveillance during labor there was no difference in operative intervention or neonatal outcome. The use of fetal cardiotocography plus PR-interval analysis is presently considered investigational.
ST Waveform Analysis Since different fetuses may be able to tolerate varied oxygenation levels based on their situation prior to the hypoxic event, reliance on the level of oxygenation may not be the most applicable measure. In the search for other parameters, an evaluation of the changes in specific end organs, such as the fetal heart, have been pursued. One development is the ST waveform analysis to aid in the interpretation of ominous FHR patterns. ST analysis of the ECG during exercise is a method for assessing myocardial function in the adult. As a corollary, ST waveform analysis of the fetus during labor is analogous to a fetal “stress test” and should provide information on the ability of the fetal heart to respond. An ST segment rise indicates a fetus responding to hypoxia. A negative ST indicates a fetus that is unable to respond or has not had time to react. The fetal ECG is readily obtainable during labor from the same scalp electrode used to obtain the FHR. The evidence from experimental work indicates that ST waveform elevation reflects compensated myocardial stress and a switch to anaerobic myocardial metabolism. A study of 4966 women with CTG versus CTG plus ST analysis revealed that intrapartum monitoring with cardiotocography combined with automatic ST waveform analysis increased the ability of the obstetrician to identify fetal hypoxia. Currently, the use of ST segment elevation in clinical interpretation of fetal heart tracings is considered investigational and studies are ongoing to assess its effectiveness.
SUMMARY POINTS
Fetal evaluation and interpretation of the findings has made some significant advances. However, there are many limitations with the available technology.
Electronic FHR monitoring is a very sensitive tool for the detection of fetal compromise; truly compromised fetuses rarely fail to exhibit abnormal FHR patterns.
The converse, however, is not true. Abnormal FHR patterns frequently are observed in the absence of fetal compromise. The limited positive predictive value is the principal shortcoming of FHR monitoring.
Accuracy may be improved by combining FHR analysis with assessment of biophysical variables such as amniotic fluid volume, fetal movement, breathing, tone, and blood flow characteristics.
Other variables under investigation include continuous intrapartum fetal pulse oximetry, fetal ECG interpretation, and ST waveform analysis.
To date, the most effective combination of variables has not been defined, and no one approach to fetal surveillance has demonstrated clear superiority over the others. Yet, despite the limitations, antepartum testing in “high-risk” pregnancies has been reported to yield a fetal death rate nearly seven times lower than that in untested, “low-risk” pregnancies. If this observation is substantiated, future investigation will be needed to address the role of antepartum fetal
surveillance in uncomplicated, low-risk pregnancies.
SUGGESTED READINGS
American College of Obstetricians and Gynecologists. Neonatal encephalopathy and cerebral palsy: defining the pathogenesis and pathophysiology, a report. Washington: American College of Obstetricians and Gynecologists, 2003.
Asphyxia and Cerebral Palsy
Nelson KB, Ellenberg JH. Antecedents of cerebral palsy: multivariate analysis of risk. N Engl J Med 1986;315:81.
Fetal Heart Rate Monitoring
American College of Obstetricians and Gynecologists. ACOG Technical Bulletin No. 207, July 1995.
Hon EH, Quilligan EJ. The classification of fetal heart rate. Conn Med 1967;31:779.
Kubli FW, Hon EH, Khazin AF, et al. Observations on heart rate and pH in the human fetus during labor. Am J Obstet Gynecol 1969;104:1190.
Paul RH, Suidan AK, Yeh S, et al. The evaluation and significance of intrapartum baseline FHR variability. Am J Obstet Gynecol 1975;123:206.
Rosen KG, Luzietti R. Intrapartum fetal monitoring: its basis and current developments. Prenatal Neonatal Med 2000;5:155–168.
Fetal Scalp Stimulation/Vibroacoustic Stimulation
Smith CV, Nguyen HN, Phelan JP, et al. Intrapartum assessment of fetal well-being: a comparison of fetal acoustic stimulation with acid-base determinations. Am J Obstet Gynecol 1986;155:726.
Prolonged Decelerations in Fetal Heart Rate Baseline
Clapp JF, Peress NS, Wesley M, et al. Brain damage after intermittent partial cord occlusion in the chronically instrumented fetal lamb. Am J Obstet Gynecol 1988;159:504.
Contraction Stress Test/Oxytocin Challenge Test Nonstress Test
Boehm FH, Salyer S, Shah DM, et al. Improved outcome of twice weekly nonstress testing. Obstet Gynecol 1986;67:566.
Freeman RK, Anderson G, Dorchester W. A prospective multi-institutional study of antepartum fetal heart rate monitoring. II. Contraction stress test versus nonstress test for primary surveillance. Am J Obstet Gynecol 1982;143:778.
Manning FA, Lange IR, Morrison I, Harman CR. Determination of fetal health: methods for antepartum and intrapartum fetal assessment. Curr Probl Obstet Gynecol 1983;7:3.
Yanagihara T, Ueta M, Hanaoka U, et al. Late second trimester nonstress test characteristics in preterm delivery before 32 weeks of gestation. Gynecol Obstet Invest 2001;1:32–35.
Modified Biophysical Profile
Clark SL, Sabey P, Jolley K. Nonstress testing with acoustic stimulation and amniotic fluid assessment: 5973 tests without unexpected fetal death. Am J Obstet Gynecol 1989;160:694.
Lagrew DC, Pircon RA, Nageotte M, et al How frequently should the amniotic fluid index be repeated? Am J Obstet Gynecol 1992;167:1129.
Miller DA, Rabello YA, Paul RH. The modified biophysical profile: ante-partum testing in the 1990's. Am J Obstet Gynecol 1996;174:812.
Nageotte JP, Towers CV, Asrat T, et al. Perinatal outcome with the MBPP. Am J Obstet Gynecol 1994;170:1672.
Fetal Scalp Blood Sampling
Goodwin TM, Milner-Masterson L, Paul RH. Elimination of fetal scalp blood sampling on a large clinical service. Obstet Gynecol 1994;83:971.
Doppler Ultrasound
Alfirevic Z, Neilson JP. Doppler ultrasonography in high-risk pregnancies: systematic review with meta-analysis. Am J Obstet Gynecol 1995;172:1379.
Fetal Pulse Oximetry
Garite TJ, Dildy GA, McNamara H, et al. A multi-center controlled trial of fetal pulse oximetry in the intrapartum management of nonreassuring fetal heart rate patterns. Am J Obstet Gynecol 2000;183: 1049–1058.
Fetal Cardiotocography Plus Pulse Rate-Interval Analysis
Strachan BK, van Wijngaarden WJ, Sahota D, et al. Cardiotocography only versus cardiotocography plus PR-interval analysis in intrapartum surveillance: a randomized, multicenter trial. Lancet 2000 355:456–459.
Strachan B, Sahota D, van Wijngaarden WJ, et al. The fetal electrocardiogram: relationship with academia at delivery. Am J Obstet Gynecol 2000;182:603–606.
ST Waveform Analysis
Amer-Wahlin I, Hellsten C, Noren H, et al. Cardiotocography only versus cardiotocography plus ST analysis of fetal electrocardiogram for intrapartum fetal monitoring: a Swedish randomized controlled trial. Lancet 2001 358: 534–538.
Chapter 10 Preterm Labor and Delivery
Summary of Screening Tests Used to Identify Women at Risk for Preterm Labor PRETERM LABOR DEFINITION
Over the past two decades we have seen a marked increase in survival of very low birth weight infants. This increase in survival has been attributed to increased use of corticosteroids, regionalization of perinatal care, improved methods of mechanical ventilation, availability of exogenous surfactant, and improved nutritional therapy.
However, the reduction in mortality has not been accompanied by a reduction in neonatal morbidity or long-term handicaps. It is estimated that 50% of all major neurologic handicaps in children result from premature births.
Despite widespread awareness of the problem and use of therapies believed to be beneficial to prevent preterm births, the rate of preterm delivery is increasing in the United States. The majority of spontaneous premature births occur to women who develop preterm labor or preterm premature rupture of the membranes (PPROM).
Cervical incompetence may also result in preterm delivery. Historically, researchers and epidemiologists have approached these conditions as being distinct processes that were mutually exclusive of one another. Recent evidence would suggest that many women have overlapping conditions, which predispose them to deliver preterm.
This concept is depicted in Figure 10.1. For example, a women who has preterm delivery secondary to PPROM at 27 weeks gestation, may have had weeks of “silent”
or painless contractions or cervical dilation prior to developing ruptured membranes and delivery. Using this broader conceptual framework, this chapter will review the epidemiology, etiology, prevention, and treatment of women with preterm labor.
FIG. 10.1. Overview of spontaneous preterm birth.
MECHANISMS OF LABOR ONSET
Labor occurs when mechanisms are present that convert the uterus from a state of containment to an environment that attempts to expel the fetus. In humans, the average gestational period is 280 days ± 14 days. Therefore, term labor is defined as labor that occurs between 37 and 42 weeks gestation. Preterm labor is defined as labor that occurs between 20 and 37 weeks gestation. In theory, pathologic activation of the normal parturition process results in preterm labor and delivery.
In both term and preterm labor, following an unknown stimulus, the mechanisms that produce labor override those that maintain the pregnancy. Activation of the parturition process results in membrane activation, cervical ripening, and an increase in myometrial responsiveness to endogenous and exogenous signals.
Subsequently, labor progresses along a common pathway that results in uterine contractions that are sufficient to cause progressive cervical dilation to allow for expulsion of the fetus. A number of inciting events have been implicated in premature births. These events include decidual hemorrhage (abruption), mechanical
factors (overdistension of the uterus, cervical incompetence), hormonal changes (fetal or maternal stress) or subclinical/clinical infection. Infection is associated with as many as one third of preterm deliveries, particularly those occurring at the earliest gestational ages. The role of infection in preterm labor will be reviewed separately in this chapter.
Animal models have helped in understanding labor. Important findings in animal labor models include an increase in oxytocin receptors present in the myometrium, gap junctions developing between myometrial cells, an increased response to agents capable of producing contractions in the uterus, and physical and biochemical
changes of the cervix resulting in a softened consistency. Uterine smooth muscle contractility is produced by the actin–myosin interaction, following myosin light chain phosphorylation, which is controlled by myosin light chain kinase. Myosin light chain kinase is activated by calcium as a calmodulin–calcium complex. Cyclic adenosine monophosphate (cAMP) also regulates kinase by inhibiting phosphorylation. Many factors are involved in this control. Some of the proposed theories of labor will be discussed in the following sections.
Hormonal
Alteration in systemic or local levels of steroid hormones is an initiating factor of labor in some animals. The understanding of their possible role in human labor has been continuously evolving. The withdrawal of the uterine inhibitor hormone progesterone has been shown to play a major role in many animals (e.g., sheep, rats, rabbits). In sheep, this withdrawal seems to be caused by an increased responsiveness of fetal adrenal cells to adrenocorticotropic hormone (ACTH) that results in increased production of cortisol. Through several steps, cortisol redirects placental steroid biosynthesis and decreases progesterone secretion. The decreased circulating progesterone in the sheep permits increased myometrial gap junction formation, an increase in prostaglandin formation, and increased response of the uterus to agents capable of producing contractions. In this sheep model, fetal ACTH secretion has control of the onset of labor.