Intrauterine Asphyxia: Clinical Implications for Providers of Intrapartum Care

Jenifer Fahey, CNM, MSN, MPH; Tekoa L. King, CNM, MPH

Disclosures

J Midwifery Womens Health. 2005;50(6):498-506. 

In This Article

Intrapartum Assessment of Fetal Oxygenation

The criteria outlined by the task forces and presented in Table 3 offer no guidance for the clinician in predicting or detecting asphyxia in labor. Clinicians continue to be limited by use of technology with an imprecise ability to identify the development of asphyxia in the fetus.

Theoretically, variant fetal heart rate patterns either lead to fetal hypoxia or are responses to fetal hypoxia.[30] Therefore, electronic fetal heart rate monitoring (EFM) should enable clinicians to detect the presence or potential development of asphyxia and allow intervention in time to prevent or reduce neurologic damage. However, the use of fetal heart rate patterns to predict subsequent neurologic damage, such as cerebral palsy, results in a 99% false-positive rate.[31] This is due in part to the fact that permanent neurologic damage is a rare event, and variant fetal heart rate patterns are extremely common. Furthermore, interpretation of a fetal heart rate tracing is a subjective process. Many of the published studies assessing the impact of EFM were conducted at a time when EFM was a new technology and there were few, if any, standard clinical definitions for what constituted an abnormal fetal heart rate tracing. Researchers have found that individuals considered experts in interpretation of fetal heart rate patterns will agree on approximately 60% of normal patterns but only 25% of pathologic patterns.[32]

In addition, even with standard definitions for fetal heart rate pattern type, the positive predictive value for acidemia in fetuses demonstrating "abnormal" fetal heart rate patterns is less than 50%.[33] A meta-analysis of nine early studies of EFM showed that in a population monitored with EFM, as opposed to intermittent auscultation, there was a substantial increase in the rate of cesarean birth (OR 1.53; 95% CI 1.17-2.01) with no significant reduction in overall perinatal mortality (OR 0.87; 95% CI 0.57-1.33).[34] Current evidence suggests that continuous EFM and intensive auscultation (one-on-one nurse listening through a contraction every 15 minutes in the first stage of labor and every 5 minutes in the second stage) are equivalent for the prevention of fetal death but that neither is an effective tool for the prevention of cerebral palsy.[21,35]

Thirty years of experience with EFM, coupled with increased understanding of fetal heart rate physiology, has increased our understanding of fetal heart rate response to labor. More recent studies on fetal heart rate patterns and their relationship to neonatal outcomes have been able to detect an association between specific fetal heart rate patterns and an increase in the risk of compromised neonatal outcomes. Although they have not been able to prove that the association is etiologic, the remarkable consistency in findings from this work has provided evidence that the following specific patterns correlate with an increased incidence of metabolic acidemia and of adverse fetal outcomes: 1) minimal or absent variability for an hour or more as a solitary finding not due to a known, benign cause such as maternal medication; 2) recurrent late decelerations or repetitive moderate to severe variables and minimal or absent variability; 3) persistent tachycardia or bradycardia and minimal/absent variability; or 4) persistent or progressive bradycardia, particularly bradycardia below 80 beats per minute.[35,36,37] Although many of the fetuses who exhibit these variant heart rate patterns will have no evidence of compromise at birth, the presence of these patterns warrants further investigation to determine fetal well-being (see below) and, when appropriate, intrauterine resuscitation and expedited delivery.[35]

Despite the poor predictive value of variant fetal heart rate patterns, EFM does provide us with the ability to establish fetal well-being. A fetal heart rate pattern with normal rate, moderate variability, presence of accelerations, and absence of periodic decelerations correlates highly with absence of fetal acidemia.[38,39]

In the term fetus, the presence of accelerations of 15 beats per minute lasting 15 seconds or more is an excellent indicator of the absence of acidemia.[40] This is true whether the accelerations are spontaneous or induced through scalp stimulation or vibroacoustic stimulation. It must be kept in mind, however, that approximately 50% of fetuses without accelerations after stimulation will have a fetal scalp pH of >7.20.[41]

Fetal scalp blood sampling is another method that has been used during labor to identify fetuses with acidemia, but this technique is technically difficult and invasive, and it requires personnel and equipment not available in many birth settings. Because it only determines the presence or absence of fetal acidemia at one moment in time, it is not useful in subsequent clinical management beyond a short period of time. Fetal scalp sampling is, therefore, becoming less common as a method to evaluate fetal oxygenation in labor.

In 2000, the Food and Drug Administration approved the marketing of a fetal oxygen saturation-monitoring system. This system has an optical sensor that is applied to the fetal cheek and connected to a fetal cardiac monitor. Proponents of pulse oximetry argue that it can be used to reduce the false-positive rate of nonreassuring fetal heart tracing and, therefore, reduce the rate of operative and instrumental interventions.[42] ACOG, citing concerns about increasing medical costs without demonstrated improvement in outcomes, currently does not recommend the use of fetal pulse oximetry.[43] This monitoring modality, they argue, is not currently more effective than other monitoring and, thus, represents an unnecessary expense. Further studies on fetal pulse oximetry are underway, and it is possible that in the future, this testing modality may be used as an adjunct to EFM and fetal scalp or vibroacoustic stimulation in the assessment of fetal well-being in labor.

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