Clinical significance of fetal heart rate patterns during labor. II. Late decelerations

Increased variability of fetal heart rate during labour: a review of preclinical and clinical studies

Increased fetal heart rate variability (FHRV) in intrapartum cardiotocographic recording has been variably defined and poorly understood, limiting its clinical utility. Both preclinical (animal) and clinical (human) evidence support that increased FHRV is observed in the early stage of intrapartum fetal hypoxaemia but can also be observed in a subset of fetuses during the preterminal stage of repeated hypoxaemia. This review of available evidence provides data and expert opinion on the pathophysiology of increased FHRV, its clinical significance and a stepwise approach regarding the management of this pattern, and propose recommendations for standardisation of related terminology.

Uterine activity: implications for the condition of the fetus

AIM

The aim of this review is to provide more insight in the fetal mechanisms as a response to uterine contractions and to emphasize the importance of correct assessment of uterine activity (UA) patterns during labor.

STUDY RESULTS

UA causes a decreased flow through the uterine artery. In the healthy uncompromised fetus, this will not cause fetal acidemia. The fetus has developed certain protection mechanisms to survive labor; (1) During a contraction, fetal preload increases and enables the fetus to maintain a constant blood flow through the umbilical artery and (2) UA increases the blood flow in the fetal middle cerebral artery, i.e., a brain sparing effect. The shortcoming of those protection mechanisms in the compromised fetus and in case of excessive UA increases the risk of adverse fetal outcome. The brain sparing effect will become more pronounced to compensate for the decreased umbilical artery blood flow and fetal oxygen saturation. Maintenance of normal UA, especially a sufficiently long relaxation time, is essential so that the supply of well oxygenated maternal blood to the intervillous space will be restored and the fetal cerebral oxygen saturation can remain stable.

CONCLUSION

Adequate UA monitoring is a prerequisite for proper reading and interpretation of cardiotocograms. It alarms in cases of excessive UA and can help to prevent fetal acidemia. Uterine contraction monitoring deserves full attention in daily obstetric practice.

[Fetal heart rate during labour: definitions and interpretation]

Continuous fetal heart rate monitoring is widely used during labor even in low risk pregnancies. Consensus is necessary to define and interpret accurately the different FHR patterns. The normal FHR tracing include baseline rate between 110-160 beats per minute (bpm), moderate variability (6-25 bpm), presence of accelerations and no decelerations. Uterine activity is monitored simultaneously: contractions frequency, duration, amplitude and relaxation time must be also normal. Abnormal baseline heart rate during 10 minutes or more is termed tachycardia above 160 bpm (except for FIGO above 150) and bradycardia below 110 bpm. Variability is minimal below 6 bpm and absent when non visible. Decelerations are classified as early, variable, late, and prolonged. Early and late decelerations have an onset gradual decrease of FHR, in contrast variable decelerations have an abrupt onset. Early deceleration is coincident in timing with uterine contraction. Variable deceleration is variable in onset, duration and timing, and may be described as typical or non reassuring. Late deceleration is associated with uterine contraction; the onset, nadir, and recovery occur after onset, peak and end of the contraction. Prolonged deceleration is lasting more than two but less 10 minutes, with almost onset abrupt and no repetition. Electronic fetal monitoring is a method to detect risk of fetal asphyxia; analysis and interpretation of FHR patterns are difficult with a high false positive rate, increasing operative deliveries. The patterns who are predictive of severe fetal acidosis include recurrent late or variable or prolonged decelerations or bradycardia, with absent FHR variability, and sudden severe bradycardia. The other FHR patterns are not conclusive and defined as non reassuring; obstetrical risk factors must be considered and other method (like scalp sampling for pH) utilised to evaluate fetal state.

Perinatal asphyxia: assessing its causal role and timing

Perinatal asphyxia, whether prenatal, intrapartum, or neonatal is thought to be a significant contributor to newborn morbidity and mortality as well as long-term neurological deficits. Development of an intrapartum tool/test that can reliably identify and discriminate between varying degrees of fetal acidemia and suggest whether it is respiratory or metabolic in nature would be highly desirable. This article critically reviews the available experience with the currently available monitoring techniques and the significance of abnormalities of fetal and intrapartum measurements with respect to the predictive value of the observations available to the clinician.

Maternal positioning affects fetal heart rate changes after epidural analgesia for labour

Adverse fetal heart rate (FHR) changes suggestive of fetal hypoxia are seen in patients with normal term pregnancies after initiation of epidural block for labour analgesia. It was our hypothesis that, in some parturients, these changes were a consequence of concealed aortocaval compression resulting in decreased uterine blood flow. We expected that the full lateral position compared with the wedged supine position would provide more effective prophylaxis against aortocaval compression. To test our hypothesis we studied the role of maternal positioning on FHR changes during onset of epidural analgesia for labour. Eighty-eight ASA Class I or II term parturients were randomized into two groups: those to be nursed in the wedged supine position and those to be nursed in the full lateral position during induction of an epidural block. External FHR monitoring was employed to assess the fetal response to initiation of labour epidural analgesia. Epidural catheters were sited with the parturients in the sitting position and the patients then assumed the study position. After a negative test dose, a standardized regimen of bupivacaine 0.25% was employed to provide labour analgesia. The quality and efficacy of the block were assessed using VAS pain scores, motor block scores and sensory levels. The results demonstrated that there was no difference in the quality of analgesia provided nor in the incidence of asymmetric blocks. There was no difference in the observed incidence of FHR changes occurring during the initiation of the epidural block.(ABSTRACT TRUNCATED AT 250 WORDS)

Clinical significance of fetal heart rate patterns during labor. IX: Prolonged pregnancy

The intrapartum FHR and UC were recorded in a population of 707 consecutive cases of prolonged gestation looking for a characteristic pattern. There is no typical pattern for prolonged pregnancy. However, there are differences from preterm and term populations. There is a very high incidence of variable (55%) and late (17%) decelerations. These were seen more frequently among cases receiving oxytocin. Nearly 50% of C-sections had lates. Baseline alterations (tachycardia 26%, fixed 8%, and saltatory 17%) were often associated. Apgar scores < or = 6 at 1 minute were correlated with variable and late decelerations. However, acidemia (UA pH < or = 7.20) could not be predicted from FHR patterns because there was no correlation between low pH and clinical depression. Alarmingly a few fetuses in agonal state presented normal appearing tracings, or deteriorated rapidly without the usual indicative changes by FHR. All intrapartum or NND had this misleading pattern. In view of this unpredictable aberrant pattern as the only certain means to prevent these deaths, it is suggested that no pregnancy should be allowed to reach 294 days post LMP.

Fetal heart rate monitoring and neonatal mortality in the very preterm infant

A retrospective study was performed to determine the usefulness of intrapartum fetal heart rate patterns in managing infants of 26 to 30 weeks' gestational age by a comparison of intrapartum tracings with neonatal outcome. Fetal heart rate patterns of 26 infants who died were matched for gestational age with those of infants who did not die or demonstrate developmental abnormalities after a 1-year follow-up were analyzed. A normal fetal heart rate pattern was associated with a good outcome (p less than 0.05), the only deaths (three) being secondary to unrelated factors. An abnormal fetal heart rate tracing predicted 90% of deaths; however, an abnormal fetal heart rate tracing was also found in 15 of 31 infants with no mortality or morbidity. Evidence would thus suggest that the very preterm infant can tolerate the stress associated with normal labor and that a normal fetal heart rate pattern predicts good fetal outcome in the absence of unrelated perinatal abnormality. With significantly abnormal patterns, however, further parameters must be evaluated before the diagnosis of fetal distress associated with subsequent mortality can be made with certainty.

A study of variable decelerations in association with other heart rate patterns during monitored labor

A review of 1,011 consecutive intrapartum heart rate tracings yielded 37.3% with some degree of variable deceleration pattern. No differences in Apgar score distribution were observed in the presence of uncomplicated variable deceleration pattern when compared to those tracings marked normal. However, the presence of variable decelerations in association with other heart rate patterns resulted in lower mean Apgar scores at 1 and 5 minutes, which were significantly different from those of the fetal heart rate (FHR) normal group. Mean Apgar scores at 1 and 5 minutes were significantly different from normal when variable decelerations were noted in the presence of tachycardia and loss of variability. Mean Apgar scores were lower when bradycardia (prolonged episodes of heart rate less than 120 bpm) was present in the record when compared to normal, but the presence of variable decelerations with bradycardia did not result in different mean scores. The presence of baseline changes with loss of variability and variable decelerations appeared to result in the lowest mean scores. When bradycardia or tachycardia occurred in exclusive association with variable decelerations, the percentage of depressed newborn infants was relatively high.

Antepartum fetal testing. I. The oxytocin challenge test

Two hundred seventy-eight high-risk patients were managed by a comprehensive assessment of the anatomic, biochemical, and functional environment of the fetoplacental unit utilizing predetermined guidelines. Decisions to terminate pregnancy were reserved for patients who demonstrated a positive OCT. Perinatal outcome in patients with positive OCT's was significantly worse than in patients who did not have a positive OCT. Patients with suspicious OCT's frequently had positive OCT's and were more likely to bear growth-retarded infants, whereas negative OCT's in general were associated with a favorable outcome. Correlation of estriol excretion with the OCT and perinatal outcome was inconsistent. There were a total of four prenatal deaths, all of which were considered unpreventable.

Intrapartum fetal heart rate and sudden infant death syndrome

The intrapartum fetal heart rate (FHR) tracings of 20 infants who died of sudden infant death syndrome (SIDS) and 20 matched control infants, both drawn from a population at increased obstetrical risk, were compared. This is a blind retrospective study aimed at quantifying (a) the incidence of FHR patterns, (b) the range of FHR and variability levels, and (c) the episodes of variable and late decelerations occurring in conjunction with abnormal FHR levels. Tracings of SIDS infants were similar to those of control infants. Although the three infants with bradycardic FHR levels were restricted to the SIDS group, infants in this group could not be reliably differentiated from control infants on the basis of intrapartum FHR tracings.

Clinical significance of fetal heart rate patterns during labor. V. Variable decelerations

Among a large series of high-risk patients who had direct electronic monitoring, 330 fetuses presented "variable" decelerations and 598 had no decelerations. Among those with variable decelerations, there were 18 who presented such associated fetal heart rate (FHR) alterations as tachycardia, fixed baseline, and slow recovery ("hypoxic components"). The fetal outcome, as well as the characteristics of FHR patterns, were compared among the three groups. Baseline changes were significantly higher among pure variable decelerations and still higher among those having variable decelerations with hypoxic components. Likewise, one- and five-minute Apgar scores were significantly lower in that order. Only 4 per cent with no decelerations had clinical fetal distress while 23 per cent with variable decelerations and 78 per cent of those with variable and hypoxic components had distress. Neonatal death was higher among groups with variable decelerations. Premature rupture of membranes seemed to be the only clearly different (and probably influencing) clinical condition, being higher among the group with variable decelerations. Variable decelerations ("cord patterns") are associated with a relatively high rate of depressed infants. This problem is much worse when elements characteristic of hypoxia are identified in FHR tracings. The degree of drop in FHR frequency is not among those elements so identified. It is thus possible to prevent significant fetal deterioration by meticulous analysis of FHR tracings and active management when "late" components are identified on variable decelerations.

The prediction of fetal oxygenation by an on-line computer analysis of fetal monitor output

The deceleration delay index (DDI) is a statistical parameter computed by an on-line system which analyzes fetal monitor output for the temporal relationship between each uterine contraction and any associated deceleration of the fetal heart rate. A similar parameter describing this relationship is known to be quantitatively predictive of fetal oxygenation in the rhesus monkey, a species for which the level of fetal oxygenation is also known to be directly and progressively related to aberrations in the vital signs, to brain damage, and to fetal death. Thus, the DDI should be of significant, practical value in predicting fetal hypoxia at a time early enough to avert severe neonatal depression and/or hypoxic brain damage. A blind clinical trial of 106 parturients is presented which supports this concept by showing the DDI to be highly correlated with another oxygenation-associated parameter, the one-minute Apgar score.

Clinical significance of fetal heart rate patterns during labor. I. Baseline patterns

From a population of 2,774 high-risk patients monitored during labor, 1,304 single pregnancies in cephalic presentation and with direct monitoring for at least 1 hour before completion or cesarean-section were studied. The maternal and fetal clinical data and the tracings were hand reviewed, coded, and programmed for computer analysis. In the record were studied baseline, its changes (tachycardia, fixed, saltatory), the accelerations, and the decelerations (early, variable, late). Fifty-four per cent had some type of FHR deceleration. Accelerations were recorded in over 12 per cent of all cases and were associated with cord problems in 41 per cent. Subgrouping the patients by age of gestation (less than or equal to 36 weeks, 37 to 41 weeks, and greater than or equal to 42 weeks) revealed a 10 per cent prolonged gestation rate and only 6.8 per cent premature; these had a lower 5 minute Apgar score. Fetal weight and age were positively correlated with Apgar score. Baseline changes were much frequent among pre- and postmature infants, particularly tachycardia in the latter (40 per cent). The premature infants had a 25 per cent incidence of fetal distress and the postmature infants had 20 per cent. Neonatal morbidity and mortality rates were very high among premature infants and a mortality rate of 2.3 per cent was found among postmature infants. Saltatory pattern and particularly fixed baseline seem characteristic of prolonged gestation and placental insufficiency. With tachycardia, they constitute subtle symptoms of fetal distress.

The evaluation of continuous fetal heart rate monitoring in high-risk pregnancy

Intrapartum electronic fetal heart rate monitoring of the high-risk obstetric patient is thought to improve the perinatal outcome. A prospective randomized study of 483 high-risk obstetric patients in labor was carried out comparing the effectiveness of electronic fetal monitoring with auscultation of fetal heart tones. The infant outcome was measured by neonatal death, Apgar scores, cord blood gases, and neonatal nursery morbidity. There were no differences in the infant outcomes in any measured category between the electronically monitored group and the auscultated group. The cesarean section rate was markedly increased in the monitored group (16.5 vs. 6.8 per cent in the auscultated patients). The presumptive benefits of electronic fetal monitoring for improving fetal outcome were not found in this study.