- The value of this new technology might not be so much the prediction of acidosis but identification of the well-oxygenated fetus so that labor may be safely continued in the presence of a concerning—but not ominous—fetal heart rate tracing.
- The only randomized study published so far did not determine whether clinical decisions can be based solely on fetal pulse oximetry. The investigators did suggest that sensitivity and specificity for metabolic acidemia was improved in the intervention group—a promising appraisal, in contrast with previous observational data.
When a teenage nullipara underwent labor induction for preeclampsia at 37 weeks, she was given epidural analgesia and seizure prophylaxis with magnesium sulfate. Her electronic fetal heart rate (FHR) tracing was initially reassuring, with only occasional variable decelerations, but subsequently revealed a baseline of 140 beats per minute (bpm), minimal to absent variability, no accelerations, and variable decelerations to 90 bpm with rapid return to baseline.
The tracing was interpreted as nonreassuring, and a fetal pulse oximeter was inserted. It revealed a fetal oxygen saturation rate between 45% and 50%, and labor was allowed to continue. After 3.5 hours in the second stage, the patient was delivered by outlet forceps. Her infant had Apgar scores of 8 at 1 minute and 9 at 5 minutes. The umbilical arterial pH was 7.25, and base excess was–4.9.
Fetal pulse oximetry made it possible to manage this case without resorting to emergent cesarean. But is this noninvasive technology truly a step forward in intrapartum assessment of fetal well-being?
We describe what the evidence (a single randomized study and a number of observational studies) reveals about these questions:
- How accurately does fetal pulse oximetry reflect the fetal condition?
- What is the critical threshold for fetal oxygen desaturation?
- Is a single reading reliable?
- Does oximetry correlate with acid-base status?
- Does the combination of oximetry and electronic monitoring improve accuracy?
- Will fetal pulse oximetry improve neonatal outcomes?
- How precise is it?
- Is it easy to use?
Needed: Effective adjunct to electronic monitoring
Except in the chronically hypoxic fetus (which is affected by the time labor begins), the pathophysiology of acute intrapartum events is a continuum, from hypoxemia to respiratory acidosis to metabolic acidosis and, ultimately, clinical impairment. The goal of intrapartum surveillance is to detect fetal hypoxemia before it progresses to asphyxia and perinatal mortality or long-term morbidity.
Although it is approved as an adjunct to electronic fetal monitoring (EFM), fetal pulse oximetry has gained only sporadic use since it became available in the United States in 2000—even though EFM has proved disappointing as a tool for predicting fetal hypoxia. Only about 10% of US obstetrical units had fetal pulse oximetry technology as of 2002.1
Clinicians began questioning the reliability of subjective interpretation of fetal heart tracings soon after EFM went into general use. Thirty years later, a meta-analysis of 12 randomized clinical trials involving 58,855 gravidas cast doubt on the benefits of EFM,2 which is associated with an increase in operative deliveries as a result of high sensitivity but low specificity in predicting fetal hypoxia and acidosis.
FDA approval was based on sole randomized trial
The only commercially available fetal oximetry sensor, the Nellcor N-400 (Nellcor, Pleasanton, Calif), obtained US Food and Drug Administration (FDA) approval as an adjunct to EFM when the latter indicates a nonreassuring FHR pattern. That approval was based on the only randomized study3 of fetal pulse oximetry conducted, which involved 1,010 women with predefined nonreassuring FHR patterns in labor.
Goal: Reduced cesarean rate with comparable outcomes. Investigators hypothesized that adjunctive fetal oximetry would improve assessment and reduce the cesarean rate without altering neonatal outcome. Indeed, in the oximetry group, the rate of cesarean delivery performed for a nonreassuring FHR tracing (4.5% versus 10.2%; P = .007) was significantly reduced. Other findings:
- Same neonatal outcomes, with no significant differences between the 2 groups.
- Higher cesarean rate for dystocia in the intervention group, offsetting any advantage in the overall cesarean delivery rate (29% versus 26%). This unexpected increase in cesarean deliveries raises several possibilities:
- Given the unblinded design, it is possible that clinicians, circumspect of the pulse oximetry, continued to perform cesareans for nonreassuring FHR, but labeled the indication for surgery differently. The validity of the dystocia diagnosis was discredited by a subsequent partogram analysis that showed a similar rate of arrested labor in both groups.
- A nonreassuring FHR in conditions of normal fetal oxygenation is predictive of dystocia. Previous randomized studies of EFM have suggested the same thing.4
- Dystocia is the consequence of the device itself. Anecdotal observations suggest a higher rate of persistent occiput posterior positions with fetal oximetry.