Fetal pulse oximetry: 8 vital questions
Will this noninvasive technique improve assessment of fetal well-being? The authors analyze what the evidence to date does and does not tell.
- 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 study 3 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.
Other trials underway. The ongoing Fetal Oximetry (FOX) trial of the National Institute of Child Health and Human Development Maternal-Fetal Medicine Units Network, involving 10,000 nulliparous participants, is comparing cesarean delivery rates and safety outcomes in patients monitored for FHR plus pulse oximetry with a group in which the clinicians are blinded to the pulse oximetry readings. Another randomized controlled trial of fetal pulse oximetry is underway in Australia.
Potential for increased costs. The American College of Obstetricians and Gynecologists (ACOG) has raised concerns about the potential increase in costs without demonstrable improvement in outcome. 5 ACOG has not endorsed fetal pulse oximetry for general practice.
Question 1How accurately does pulse oximetry reflect the fetal condition?
It yields only indirect information on the partial pressure of oxygen in the blood and no data on perfusion or acid-base status.
In other clinical settings, oxygen saturation is not an acceptable substitute for arterial blood gas analysis. The pulse oximeter is not a hemoximeter—only that device directly and reliably determines blood oxygen saturation by spectrophotometry. 6 Even the calculated oxygen saturation values provided automatically by modern blood gas analyzers are inaccurate. 7
Studies report varying results. In a comparison 8 of fetal oxygen saturation by hemoximetry in a fetal scalp blood (FSB) sample and fetal arterial oxyhemoglobin saturation (FSpO2) by pulse oximetry immediately before the blood sampling, the FSpO2 medians were always higher than the FSB hemoximetry saturation—which led to false-negative results in hypoxic babies.
In animal studies, pulse oximetry correlated well with simultaneously measured arterial oxygen saturation (r = 0.98, P = .01), 9 but data from human studies are inconsistent. While McNamara et al 10 reported good correlation between FSpO2 measurements and umbilical artery blood oxygen saturation at birth (r = 0.59, P <.001), Langer et al 11 found no relationship between FSpO2 levels determined during pushing efforts and oxygen saturation in umbilical vein blood at birth.
Possible reasons for the ambiguous findings:
- differences in practice, such as use of umbilical venous versus arterial blood, or measurement during pushing versus between pushes,
- different intervals from FSpO2 reading to umbilical blood sampling, or
- incomparable groups, such as all women in labor versus those with abnormal FHR.
Limitations. Fetal pulse oximetry measures arterial oxygen saturation during the systolic pulse wave in the skin microcirculation at head level. In the fetus, this is part of the preductal circulation, with oxygen saturation levels somewhere between umbilical arterial and umbilical venous blood oxygen saturation.
Theoretically, FSpO2 should be closer to FSB than to umbilical blood. Although FSB samples consist of capillary blood, which is not exactly central arterial blood, the differences are small, at least in the neonate. 12 In the intrapartum period, however, several variables with unknown effect may weaken relationships:
- different intervals between the last oximetry signal and blood sampling after delivery
- differences in local tissue perfusion status13
- perfusion changes during fetal compromise, as the fetus centralizes its blood flow, with vasoconstriction in the skin circulation
Question 2What is the critical threshold for fetal oxygen desaturation?
Human studies indicate that an FSpO2 of 33% is approximately the 10th percentile on the normal distribution, and an FSpO2 of 29% to 30% represents the third to fifth percentiles in normal-outcome labor. 14 Studies in catheterized fetal sheep suggest that the level below which metabolic acidosis can be anticipated is an FSpO2 of about 30%. 15
The 30% threshold also is supported by prospective human data from a multicenter trial. 16 According to those data, an FSpO2 of less than 30% has 100% sensitivity in predicting an FSB pH below 7.20. FSpO2 of less than 30% also correlated with a lack of variability on the FHR tracing. 17
The cutoff of 30% should not be interpreted as an indication of fetal distress, however. Rather, it represents a threshold below which increasing fetal acidosis will be encountered ( FIGURE 1). Oxygen saturation is a dynamic biologic parameter with broad variation.
FIGURE 1 Tracking fetal arterial oxyhemoglobin saturation
Question 3Is a single reading reliable?
The normal fetus has a remarkable capacity to compensate for transient episodes of desaturation. Thus, a single reading cannot reflect the fetal condition; the trend in FSpO2 must be taken into account. Research indicates only FSpO2 levels below 30% for more than 2 minutes 18 or more than 10 minutes19 are likely to be associated with intrapartum acidosis.
ACOG has raised concerns about the potential increase in costs without demonstrable improvement in outcome.
Gorenberg et al20 retrospectively correlated FSpO2 with umbilical artery pH and found that neither the 30% threshold alone nor the duration of FSpO2 below 30% correlated with fetal acidemia (pH below 7.20). Rather, the repetition of such episodes was more predictive. The authors concluded that more than 10 episodes of FSpO2below 30% would overcome the ability of the fetus to compensate.
The study was underpowered to detect a significant difference in acidemia, and did not allow sufficient observation time to detect the natural progression of hypoxia to metabolic acidosis, a better indicator of fetal compromise. Additional research is needed.
Question 4Does oximetry correlate with acid-base status?
Many of the studies mentioned here assumed a correlation. Whenever oxygen saturation in the umbilical artery is 30% or more, acidosis (pH below 7.13) in the same blood is rare—only 1%. 21 However, the correlation between fetal pulse oximetry values and acid-base status is much weaker. 8.
Leszczynska-Gorzelak et al22 found no relationship between FSpO2 levels in the first or second stage of labor and pH or partial pressure of oxygen in umbilical vein blood at delivery. Other investigators concluded similarly, considering intrapartum FSpO2 of limited use for predicting acidosis at birth, irrespective of FSpO2 cutoff. 23,24
Rijnders et al24 found no significant correlation between fetal scalp or umbilical artery blood pH and mean FSpO2 for the last 30 minutes before sampling (r = 0.02, P = .9). Even the lowest FSpO2 level did not correlate with arterial pH (r = .04, P = .84). None of the study’s 3 cases of umbilical pH below 7.05 would have been detected using the mean FSpO2 before delivery, and only 1 would have been detected using the lowest FSpO2.
In another multicenter study involving the Nellcor system in 164 cases with abnormal FHR, a correlation between oximetry and FSB sampling (r = 0.29, P < .01) was noted in the first stage of labor, but second-stage FSpO2 readings did not correlate with oxygen saturation, partial pressure of oxygen, pH, or bicarbonate level in the umbilical artery at birth. 25
An observational series26 of 128 fetuses with nonreassuring FHR patterns concluded that fetal distress was insufficiently identified by oximetry. Only 2 of the 11 cases with umbilical artery pH below 7.20 were detected by pulse oximetry recordings below 30% during the last 30 minutes of the second stage, and out of 5 cases with hypoxic readings in the second stage, only 2 were acidotic at birth. The calculated sensitivity was 18%, specificity 92%, positive predictive value (PPV) 40%, and negative predictive value (NPV) 80%. A low Apgar score was never predicted by fetal pulse oximetry.