Opinion: New FHR monitoring standards: Something old and something new

December 12, 2008

The NIH/NICHD has released new guidelines on electronic fetal heart rate assessment. Two experts summarize the new standards and explain their value.

Obstetricians have been trying to link the fetal heart rate (FHR) with fetal health since the invention of the modified stethoscope by Pinard in the 1870s. Practical application of intrapartum FHR assessment, however, required the development of an electronic fetal monitoring (EFM) system based on the fetal ECG by Drs. Edward Hon and Orvan Hess at Yale in the late 1950s. Their discovery allowed for precise estimations of FHR baselines and periodic changes in a continuous fashion. Within a decade, clinicians throughout the United States had adopted EFM.

Unfortunately, the inconvenient truth about EFM is that it was introduced before its efficacy could be confirmed by hard-core research. Developed primarily to reduce the occurrence of cerebral palsy (CP) and intrapartum stillbirth, its value in preventing these outcomes has never been proven. In fact, it’s the secondary benefit of fetal monitoring-providing a convenient way to supervise several patients at one time-and our fear of litigation that are the primary reasons it survives. So EFM remains perhaps the most prominent example of universal adoption of a medical technology before its verification.

The shortcomings of electronic FHR monitoring

The available data suggest that continuous intrapartum EFM does not perform much better than the intermittent auscultation of Pinard. A Cochrane meta-analysis that included over 37,000 patients, for instance, concluded that while its application cut neonatal seizures in half (RR 0.50; 95% CI, 0.31–0.80), it failed to affect perinatal mortality or CP rates. The same meta-analysis also revealed that continuous EFM increased cesarean (RR 1.16; 95% CI, 1.01–1.32) and operative vaginal (RR 1.16; 95% CI, 1.01–1.32) deliveries. In fact, as ACOG reports, the false-positive rate of EFM for CP exceeds 99%, with as few as 1 to 2/1,000 fetuses with nonreassuring patterns developing CP.

The specificity of EFM may be improved with fetal scalp stimulation and scalp pH. A heart rate acceleration stimulated by digital pressure on the fetal scalp is highly correlated with a normal fetal pH (negative likelihood ratio 0.08; 95% CI, 0.02–0.41), although its absence is not always associated with acidosis in the setting of a worrisome tracing. Scalp pH testing can be employed at this point with a value of <7.20 (mixed venous) typically used as an indication for delivery, although its use appears to be rapidly decreasing across the country.

Because of the false-positive rates of these approaches, additional methods of assessing fetal status in labor have been introduced under more rigorous testing, with mixed results. Fetal pulse oximetry has been compared to EFM in several clinical trials but the research to date shows no difference in overall cesarean delivery rates, although fewer of them were performed for nonreassuring fetal status (RR 0.68; 95% CI, 0.68–0.99). These studies also didn’t show differences in any maternal or neonatal outcomes. More promising is the STAN fetal heart monitor, an FDA-approved device that takes Dr. Hon’s original technology of monitoring the fetal ECG and uses specialized software to examine specific changes in the ECG related to fetal hypoxemia. Its use is suggested only in higher-risk patients who are likely to develop metabolic acidosis, rather than as a substitute for standard EFM. The most recent Cochrane review of STAN, which incorporated nearly 10,000 patients, showed reduced rates of metabolic acidosis and neonatal encephalopathy as well as operative vaginal deliveries without influencing cesarean rates.

Finding a better way to communicate

Unfortunately, none of these tests has proven to be universally applicable and traditional EFM is here to stay. With this in mind, we are pressed to improve EFM interpretation to increase its usefulness and accuracy. The NIH/NICHD recognized this in 1997 and convened a workshop to provide a consensus on definitions related to EFM interpretation.This publication standardized terms for baseline rates (normal, tachycardia, bradycardia), variability, accelerations, decelerations, and contraction patterns. In our opinion, these guidelines provided a common interpretation paradigm-a way to speak a common language. Many institutions, including our own, mandated the use of the NICHD criteria on their L&D floors and through credentialing examinations for all providers involved in inpatient care of pregnant patients. However, the 1997 criteria had its limitations; for instance, it did not clarify the terms “reassuring” or “nonreassuring” or clearly explain the appropriate management for abnormal patterns.

In April 2008, the NICHD directed a second workshop to revisit its 1997 recommendations; updated definitions and interpretation standards were published in September 2008.Here are some of the key points:

  • GENERAL CONSIDERATIONS included highlighting the dynamic and contextual nature of EFM interpretation. The authors stressed that a particular assessment is only limited to its specific time period. The workshop also emphasized the poor positive predictive value of EFM: While the presence of accelerations and/or variability reliably predicts the absence of fetal metabolic acidemia, the absence of accelerations and/or variability does not predict acidemia.
  • UTERINE CONTRACTIONS are now quantified as the number of contractions present in a 10-minute window, averaged over 30 minutes, with normal representing five or fewer contractions in 10 minutes and tachysystole meaning five or more contractions in 10 minutes. The terms hyperstimulation and hypertonus, which are referred to in ACOG Practice Bulletin No. 70, are discouraged.
  • DECELERATIONS are now defined as recurrent if they occur with 50% or more of uterine contractions in any 20-minute window and as intermittent if they occur with less than 50% of uterine contractions in any 20-minute window. Otherwise the same definitions for early, late, and variable decelerations as well as normal FHR baseline and variability remain.
  • A THREE-TIER INTERPRETATION SYSTEM was introduced in 2008 to aid in reading, communicating, and managing FHR tracings:

    a. CATEGORY I (NORMAL): Strongly predictive of normal fetal acid-base status; may be followed in a routine manner. These tracings include all of the following:

▶ baseline rate 110–160 bpm ▶ moderate variability ▶ absent, late, or variable decelerations ▶ early decelerations may or may not be present ▶accelerations may or may not be present   b. CATEGORY II (INDETERMINATE): Not predictive of abnormal fetal acid-base status but require evaluation, continued surveillance, and reevaluation, taking into account the clinical context.

c. CATEGORY III (ABNORMAL): Predictive of abnormal fetal acid-base status; require prompt evaluation and management. Include either:

▶ absent baseline FHR variability with:

  • recurrent late decelerations
  • recurrent variable decelerations, or
  • bradycardia

▶ sinusoidal FHR

The best way to learn these categories is to learn the definitions of I and III; all other tracing types fall into category II’s grey zone. In reality, a large proportion of tracings are neither ‘reassuring’ nor ‘nonreassuring.’ In fact, the time may have arrived for us to abandon these terms. In the first place, using two systems and five terms can only make interpretation and communication confusing. Moreover, the new tripartite system allows more ‘wiggle room’ by creating a category that neither connotes normality nor mandates immediate delivery.

It is left to the caregiver to decide upon appropriate management of category II tracings but category III tracings warrant prompt delivery. It is also unclear how the legal profession will view the change in language given their frequent misuse of the terms ‘reassuring’ and ‘nonreassuring.’

We hope that these new guidelines will improve communication on L&D and applaud the efforts of the workshop to continue the discourse on EFM. We encourage you to learn these new definitions and incorporate them into your practice as they become a standard of care. It’s clear, however, that we need more technology and research to improve our assessments of fetal health in labor. While improving the way we talk about fetal assessment is good, finding a better tool for such assessment would be better. ◀

DR. PETTKER is Assistant Professor, Department of Obstetrics, Gynecology and Reproductive Sciences, Yale University School of Medicine, and Medical Director of the Labor and Birth Unit, Yale-New Haven. DR. LOCKWOOD, Editor in chief, is Anita O’Keefe Young Professor and Chair, Department of Obstetrics, Gynecology and Reproductive Sciences, Yale University School of Medicine, New Haven, CT.

 

REFERENCS

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  • American College of Obstetricians and Gynecologists. ACOG Practice Bulletin No. 70. Intrapartum fetal heart rate monitoring. Obstet Gynecol. 2005;106:1453-1460.
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