Fetal Distress

Definition

Fetal distress is an imprecise term that has been used progressively less in the medical record since the American College of Obstetricians and Gynecologists (ACOG) published a committee opinion in 1998 that suggested replacing the term fetal distress with nonreassuring fetal heart rate tracing. Despite the ominous implications associated with the term fetal distress, it was frequently associated with delivery of an infant in good condition. In accordance with the ACOG recommendations, the medical record usually describes nonreassuring fetal heart rate (FHR) tracing with additional descriptions of the findings such as fetal bradycardia, repetitive late or variable decelerations, or loss of variability. Nonetheless, the term fetal distress is still often used when communicating patient condition and carries the implication of great urgency. When called to help with the treatment of fetal distress, part of the initial communication with the obstetrician should be to get clarification of the maternal and fetal condition and the true urgency of delivery because these will largely determine the anesthetic options.

Recognition

One of the great challenges to obstetricians managing patients in labor is deciding when the risk of injury to the mother or child is greater by continuing in labor than it would be by proceeding to an operative delivery. The obstetrician has a limited number of tools for assessing the condition of the fetus and predicting the future course of labor. Electronic FHR monitoring is the primary means of assessing fetal oxygenation because it is usually simple, continuous in real time, minimally invasive or noninvasive, and widely available. It is a monitor of the fetus’ own measurement and response to oxygenation and perfusion, which makes it at the same time both richly informative and subject to many confounders. Factors that alter the fetal central nervous system and autonomic responses, such as drugs (e.g., opioids, anesthetics, sympathetic or parasympathetic modulators such as β-agonists or atropine), neurologic abnormalities (e.g., anencephaly), or conditions that alter the measured response such as fetal cardiac arrhythmias are examples of influences that will make interpretation of the FHR more complicated. On the other hand, decades of widespread use and interpretation have produced a large library of observed FHR patterns and associated fetal outcomes that make it the most useful and trusted primary monitor of fetal well-being in labor.

Other measures of fetal well-being, such as the presence or absence of meconium in the amniotic fluid; the FHR response to acoustic or scalp stimulation; measurement of plasma pH in a blood sample obtained from a fetal scalp; and umbilical artery Doppler velocimetry looking for evidence of reduced, absent, or reversed umbilical artery diastolic blood flow can all be used to add information about fetal status, but none has the utility or value of FHR monitoring.

FHR monitoring is based on the premise that increasing degrees of hypoxia or asphyxia produce characteristic changes in the FHR pattern. Regulation of the FHR involves mechanisms that are still incompletely understood. At its most basic level, input from baroreceptors and chemoreceptors for oxygen and carbon dioxide are processed through brainstem nuclei and centers in the hypothalamus and cerebral cortex to produce sympathetic, parasympathetic, and humoral signals that modulate heart rate.

Electronic FHR monitors record the FHR and uterine contractions. The baseline FHR and changes in the heart rate can be analyzed in relation to uterine contractions. The main features of the FHR pattern that are measured ( Box 56.1 ) include baseline heart rate, heart rate variability, accelerations, and decelerations (three types: early, late, and variable). In 2008 a three-tier system for FHR tracing interpretation was introduced in the United States to standardize the overall interpretation and communication of FHR tracings ( Box 56.2 ). Tracings are rated Category I (Normal), Category II (Indeterminate), or Category III (Abnormal) based on the features observed in the tracing and their prognostic value.