David A. Miller, MD

we have reviewed standard electronic fetal monitoring (EFM) nomenclature, challenged common notions about fetal heart rate (FHR) decelerations, and exposed a specious theory concerning intrapartum fetal head compression. This month's article will examine the evidence underlying the common perception that EFM increases the cesarean delivery rate. 

When EFM replaced the traditional practice of intermittent auscultation in the 1970s, a series of studies reported significantly lower perinatal mortality rates in electronically monitored patients.

  These studies were nonrandomized, and employed nonconcurrent controls, leading some to cite unrelated and simultaneous improvements in neonatal care and falling perinatal mortality rates as possible sources of bias.  

Randomized controlled trials of EFM versus auscultation

In 1976, Haverkamp and colleagues reported the first prospective randomized study of 483 high-risk obstetric patients, comparing EFM with intermittent FHR auscultation in labor.

  There were no significant differences in perinatal mortality, Apgar scores, cord blood pH values, or neonatal morbidity between the EFM and control groups. The monitored group, however, had a significantly higher rate of cesarean delivery compared with the auscultated group (16.5% vs 6.6%, respectively).  

A second study by Renou and colleagues in 1976 randomized 350 high-risk patients to EFM or intermittent auscultation during labor.

  There were no significant differences between the groups with respect to perinatal mortality, Apgar scores, or maternal or neonatal infection. Patients in the monitored group, however, had significantly higher umbilical cord blood pH values, significantly lower rates of admission to the neonatal intensive care unit (NICU), fewer neonatal neurologic signs or symptoms, and fewer cases of neonatal brain damage (not further defined). The cesarean delivery rate was again significantly higher in the monitored group than in the control group (22.3% vs 13.7%, respectively).  

In 1978, Kelso and associates published a randomized controlled trial comparing EFM with intermittent auscultation in 504 low-risk patients.

  There were no significant differences between the groups with respect to perinatal mortality, low Apgar scores, cord blood pH values, NICU admissions or lengths of stay, neonatal or maternal infections, or abnormal neonatal neurologic findings. The only significant difference between the groups was an increase in incidence of cesarean birth in monitored patients compared with auscultated patients (9.5% vs 4.4%, respectively).  

In 1979, Haverkamp and colleagues published another randomized controlled trial in high-risk patients. The design was similar to their first study, but additional measures of infant status were included as well as the option to perform fetal scalp pH determination during labor.

  A total of 690 high-risk patients were randomized into 3 groups. In the first group, fetal assessment during labor was accomplished by intermittent auscultation. The second group had continuous EFM alone, and the third group had continuous EFM with the option to measure scalp blood pH as needed. Among the 3 groups, no significant differences were seen in perinatal mortality, Apgar scores, cord blood pH values, maternal or neonatal infectious morbidity, NICU admissions, or neonatal neurologic abnormalities. A significantly increased incidence of cesarean birth was demonstrated in the group with EFM alone (18%) compared with auscultation alone (6%). The option to perform scalp blood sampling resulted in an intermediate cesarean delivery rate (11%) that was not significantly different from either of the other groups.  

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A look at the claim that EFM leads to an increase in cesarean deliveries.

Fetal heart rate monitoring and the cesarean delivery rate

This fifth article in the Electronic Fetal Monitoring Mythbusters series reviews the scientific evidence behind the notion that late decelerations are more ominous than variable decelerations. 

In this series, we have reviewed standard electronic fetal monitoring (EFM) nomenclature, explored various systems of classifying fetal heart rate (FHR) decelerations, and refuted an obscure myth concerning the consequences of intrapartum fetal head compression. 

This column will examine the evidence underlying the common misconception that late decelerations are "ominous," while variable decelerations are "benign."

As early as the 19th century, investigators using intermittent auscultation described episodes of slowing FHR that later became known as decelerations.

 The advent of EFM permitted visual analysis and objective quantitation of decelerations. Hon, Kubli, and others classified decelerations as early, late, variable, or prolonged, based upon visual appearance and temporal relationship to uterine contractions.

 Early decelerations were ascribed to fetal head compression, and were considered innocuous.

 Variable decelerations were attributed to umbilical cord compression, and late decelerations were thought to be indicative of interference with maternal-fetal gas exchange during uterine contractions (ie, "uteroplacental insufficiency"). 

In 1969, Kubli, et al reported that late decelerations were associated with lower fetal scalp pH values than were mild or moderate variable decelerations.

 This study did not attempt to distinguish between benign respiratory acidemia, caused by elevated pCO

, and potentially significant metabolic acidemia, caused by tissue hypoxia, anaerobic metabolism, and accumulation of lactic acid. In addition, the study did not address neonatal outcome and did not control for such confounding factors as baseline rate, variability, accelerations, or frequency of decelerations. Nevertheless, the authors stated that late deceleration of the FHR is "more ominous than moderate or mild variable deceleration" and that "the mere presence of late deceleration is ominous."

 this line of reasoning set the stage for one of the most pervasive myths in contemporary fetal monitoring: the misconception that "late decelerations are ominous, and variable decelerations are benign." 

In 1997 and 2008, the National Institute of Child Health and Human Development published standardized definitions of FHR patterns, including late and variable decelerations.

 A late deceleration is defined as a gradual (onset to nadir ≥30 seconds) decrease in FHR, with the onset, nadir, and recovery of the deceleration occurring after the beginning, peak, and ending of the contraction, respectively. 			

In 1979, Martin, et al used autonomic blocking agents to demonstrate the physiologic mechanism of late deceleration in fetal lambs.

 Transient interruption of fetal oxygenation resulted in a biphasic autonomic reflex response. Fetal hypoxemia, detected by chemoreceptors, triggered sympathetic outflow and fetal hypertension, which was prevented by alpha-adrenergic blockade. Transient fetal hypertension resulted in baroreceptor stimulation and re-flex parasympathetic slowing of the heart rate, which was blocked by atropine. 

Subsequent studies in animals have demonstrated that fetal hypoxemia results in increased vascular resistance and decreased blood flow to nonvital vascular beds of the intestine, limbs, and kidneys, while blood flow to the vital vascular beds of the brain, heart, and adrenal glands is preserved or increased.

 When oxygen deprivation is sufficient to result in metabolic acidemia, a late deceleration can reflect transient hypoxic myocardial depression.

In humans, Low, et al reported a 45% incidence of metabolic acidemia when late decelerations accompanied at least 10% of uterine contractions in a 2-hour period. However, even in the absence of decelerations, metabolic acidemia was observed in 10% of newborns.

A new assessment of what late and variable decelerations mean.

David A. Miller, MD

Articles

Fetal heart rate monitoring and the cesarean delivery rate

Information of late or variable decelerations