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REVIEW

Intrapartum Oximetry of the Fetus

Christine E. East, PhD, and Paul B. Colditz, DPhil (Oxon), FRCPCH

From the Perinatal Research Centre, The University of Queensland, Royal Brisbane and Women’s Hospital, Brisbane, Australia.

Address correspondence to Christine E. East, PhD, Perinatal Research Centre, The University of Queensland, Royal Brisbane and Women’s Hospital, Herston, Queensland 4122, Australia. Address e-mail to c.east{at}uq.edu.au.

Abstract

Fetal monitoring during labor aims to identify fetal problems which, if uncorrected, may result in morbidity or death. A nonreassuring or abnormal fetal heart rate trace by cardiotocography (CTG) does not necessarily equate with fetal hypoxia and/or acidosis. However, in the absence of more objective data, the use of CTG often results in variable, but inappropriately high, operative delivery rates (forceps, vacuum, or cesarean delivery) for nonreassuring fetal status in many hospitals. The addition of fetal pulse oximetry (FPO) has the potential to improve the assessment of fetal well-being during labor. In this review we consider several aspects of FPO. Several factors, such as sensor to skin contact, uterine contractions, fetal hair, and caput succedaneum, influence the performance and use of FPO. Issues such as clinicians’ perspectives of FPO sensor placement, maternal perspectives of FPO during labor, and an economic analysis have all favored FPO. Several randomized controlled trials (RCTs) of FPO reported a reduction in cesarean delivery for nonreassuring fetal status when FPO was added to conventional CTG monitoring, with no difference in overall cesarean delivery rates. One large RCT reported no difference in mode of birth for any indication. Several issues relevant to the future of FPO have been addressed by these RCTs, the major issue being that it makes no difference to cesarean rates. It may be argued that FPO has a valid clinical use in monitoring the fetus with congenital heart block. Additionally, in situations of nonreassuring fetal status and dystocia, FPO may provide the necessary reassurance until adequate resources for cesarean delivery are available.

The aim of fetal monitoring during labor is to identify fetal problems which, if uncorrected, may result in morbidity or death. The fetal heart rate and uterine contractions may be recorded by using cardiotocography (CTG). Although there is no universal agreement on the interpretation of these patterns, several groups have published guidelines [for example, the American College of Obstetricians and Gynecologists (1); The Royal Australia and New Zealand College of Obstetricians and Gynaecologists (2); and Royal College of Obstetricians and Gynaecologists (3)]. Reassuring patterns require no specific action. Nonreassuring patterns occur in approximately 15% of monitored labors (4,5), and may prompt clinical actions ranging from simple maneuvers, such as a change of maternal position, to expedited birth of the baby. Ominous patterns usually prompt expedited birth with the aim of preventing or minimizing hypoxia in the fetus. The positive predictive value of CTG for adverse outcome is low and the negative predictive value is high (4). Thus, while a normal CTG usually indicates reassuring fetal status, a nonreassuring or abnormal CTG does not necessarily equate with fetal hypoxia and/or acidosis. These features, combined with marked interobserver variation in CTG interpretation (6), result in variable, but inappropriately, high operative delivery rates (forceps, vacuum, or cesarean delivery) for nonreassuring fetal status in many hospitals.

Once a nonreassuring fetal heart rate pattern has been identified during labor, a number of additional assessments of fetal well-being may be considered. These aim to improve the intrapartum assessment of fetal well-being and thereby safely reduce operative delivery rates for nonreassuring fetal status associated with conventional monitoring by CTG. They do not replace the CTG, but are usually complementary to it, either intermittently or continuously. One such test is fetal pulse oximetry (FPO or FSpo2).

Pulse oximetry of functional oxyhemoglobin and deoxyhemoglobin saturation is widely used in the adult and pediatric clinical setting (7). FPO has developed to accommodate the issues specific to the fetal environment, with the use of reflectance pulse oximetry in some systems: the light-emitting diodes and photodetector are housed in the same plane of the sensor, thus relying on backscattered or reflected light, rather than the transmitted light used in conventional oximeters (8,9).

Several investigators have reported clinical studies with prototype oximetry systems (10–20). At least three systems have been developed to commercial status. OB Scientific (Germantown, WI) markets a sensor, shaped like a tongue depressor, which slides along the fetal torso during a vaginal examination in labor, with or without ruptured amniotic membranes. Nonin Medical Inc. (North Plymouth, MN) incorporates pulse oximetry into a fetal scalp electrode. The Nellcor OxiFirstTM system (sometimes referred to as the Nellcor N-400 system) has a sensor that is directed to lie against the fetal temple or cheek during a vaginal examination in labor, after rupture of the amniotic membranes (TYCO Inc., Pleasanton, CA).

Numerous reviews have considered the adaptation of conventional pulse oximetry to the fetal environment, including sensor design, calibration issues, and clinical applications (21–26). This review provides a brief overview of factors influencing the performance and use of FPO and discusses several randomized controlled trials (RCTs) of FPO, along with issues such as clinicians’ perspectives, maternal perspectives, and economic analysis. The future of FPO is also considered.

FACTORS INFLUENCING THE PERFORMANCE AND USE OF FPO

Potential Artifacts in FPO
Sensor to Skin Contact and Uterine Contractions
A report that FSpo2 decreased after uterine contractions, using early prototype sensors (27), may have been confounded by artifact from inadequate contact between the sensor and fetal skin (28). The Nellcor N-400/FS14 system incorporated a complex algorithm for assessing signal quality and suspended FSpo2 display when the sensor lost contact with fetal skin (Nellcor internal data). East et al. (29) demonstrated a significant difference in mean signal quality of only five signal quality units (of a possible 100) with this oximetry system in the 30 s before intrauterine pressure catheter-monitored contractions and during noncontraction periods. This was noted in conjunction with a mean decline of 2.3% FSpo2 during contractions to 45% after contractions, a magnitude that is unlikely to be of any clinical importance (29). Episodes of excessive force on the sensor, such as may occur during uterine hyperstimulation (30) and reported with tightly affixed neonatal oximetry (31), were not identified in this study. Such conditions require further evaluation to determine their potential influence on detection and accuracy of FSpo2 values. While the use of the Nonin system’s spiral electrode on the fetal head may not be susceptible to direct pressure artifact from uterine contractions, stasis of scalp blood may be another source of artifact.

Fetal Hair
The presence of dark, thick, curly hair may make sensor attachment difficult (32), or may be a source of artifact, as it absorbs red light and affects the error/signal ratio (33). Optical shunting may also occur with lightly colored hair (34), although not all investigators have found a difference in FSpo2 values recorded from the scalp of babies with fair or dark hair (10). Nijland et al. (35) found no difference between FSpo2 values obtained from shaved and unshaved skin of fetal lambs. The Nellcor and OB Scientific FPO systems generally avoid areas of fetal hair, thereby avoiding this source of artifact. Reports of how the Nonin system deals with this issue are yet to emerge in the literature.

Caput Succedaneum
Early reports of FPO recorded using modified adult oximeters, not calibrated for the fetal environment, suggested that when FPO values could be recorded from caput, they were significantly higher than those recorded where no caput was present (36,37). One consideration was that the plethysmograph registered by the oximeter over caput may have come from venous pulsation, or from artifact as the arterial signal passed through the congested or edematous scalp (37). These findings contrasted with those of Knitza et al. (10), who noted no difference in FSpo2 values in babies born with or without caput. The Nonin system evolved from the sensor initially used by Knitza et al. (10). Details of the potential effect of caput on FSpo2 values when using this system are not currently available. By placing the FPO sensor on the fetal temple or cheek (Nellcor system) or back (OB Scientific system), this potential for artifact has been avoided by other manufacturers.

Critical Threshold Fetal Oxygen Saturation Values
Animal and human research suggests that when using sensors with 735 and 890 nm wavelengths, preductal FPO values ≥30% are considered reassuring (38–40). When FSpo2 values are reassuring, even in the presence of a nonreassuring CTG, it is considered appropriate to continue labor unless otherwise indicated (Fig. 1).


Figure 111
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  		Figure 1. Nonreassuring cardiotocograph with reassuring fetal oxygen saturation values. Fetal oxygen saturation (FSpo2) values are represented by the beaded line printed on the lower graph, (scale 0–100) and printed intermittently at the lower edge of the fetal heart rate graph. Paper speed 1 cm/min. Note that some values are absent at the beginning of this trace: FSpo2 values are not recorded if there is no contact between the FPO sensor and the fetal skin, or if the signal is of inadequate quality. A fetal scalp pH taken several minutes later was 7.32.

 

There are several recommendations as to how long to allow FSpo2 values <30% to continue before intervening. Proposed durations include between consecutive contractions (approximately 1 min) (41), 2 min (42), 10 min (43), or after cumulative episodes of low FSpo2 (44). Data from the FOREMOST randomized trial (45) suggest that an average FSpo2 <30% for 10 min may predict fetal scalp blood pH <7.20 (5). The fetus is usually not at risk from short periods of hypoxia, as there is preferential redistribution of cardiac output to the brain, heart, adrenals, and blood flow to the placenta (46). With prolonged hypoxia, however, compensatory mechanisms become inadequate, resulting in reduced cardiac output and blood pressure, with consequently reduced cerebral blood flow (47). This severely limits oxygen delivery to the brain and resultant neural damage (48). Appropriate interventions to avoid such sequelae may include maternal position change, to urgent birth via cesarean delivery (39,41,43). Clinical reports and research guidelines note that when the CTG is ominous, for example, in the presence of severe variable decelerations, the CTG is a better predictor of poor outcome than FSpo2 values that would usually be reassuring (41,49). Further exploration into the ability of the fetus to sustain reassuring FSpo2 values in such circumstances may not be feasible in the human setting. Rather, clinical expertise in CTG interpretation and initiating appropriate intervention are recommended in these circumstances (41). As for most clinical situations, assessment of fetal well-being during labor is best achieved through a combination of numerous considerations, rather than only the CTG or only FSpo2 values.

These data represent an action threshold for preductal FSpo2 values measured with the Nellcor system. An action value of 30% may be too conservative in a breech fetus, because postductal FSpo2 values are lower than those from the preductal regions (50,51). The Nonin system measures preductal saturation (fetal scalp), whereas postductal FSpo2 is measured from the torso when using the OB Scientific sensor.

The critical threshold for FPO using wavelengths typical of those used in neonates and adults is unclear, given the differences in FSpo2 values returned when different wavelengths are used (52). Knitza et al. (53) reported a pilot study using the Nonin system (wavelengths 661 and 905 nm). They suggested that FSpo2 of 25%–30% represented fetal hypoxia. The OB Scientific system uses wavelengths of 660 and 890 nm (9). Luttkus et al. (54) used this system to compare FSpo2 values <30% or that declined by >20%, with ST events recorded from a fetal electrocardiogram. Desaturations occurred more frequently in fetuses with ST segment changes (54).

RCTs OF FETAL OXIMETRY

Five published RCTs have examined the clinical effectiveness of FPO using the Nellcor system. Four studies randomized laboring women whose fetuses demonstrated nonreassuring fetal heart rate tracings by CTG into two groups: in one group, continuous conventional fetal monitoring by CTG was used (CTG-only group) and, in the other group, FPO was added to CTG monitoring (CTG + FPO group) (55–58). Prespecified primary outcomes included cesarean delivery or operative birth (cesarean, forceps, or vacuum) for nonreassuring fetal status. All studies considered overall cesarean rates and other maternal and fetal/neonatal outcomes.

Before randomization, all participants in the Kuhnert and Schmidt trial (56) had fetal scalp blood sampling. By implication, those with nonreassuring fetal scalp blood findings would have proceeded to operative delivery, rather than study entry. It could, therefore, be considered that these fetuses were not exhibiting evidence of nonreassuring fetal status at the time of study entry, in contrast to those who were, based on the nonreassuring CTG only, as for the remaining studies (55,57,58).

Entry criteria to the study by Klauser et al. (57) included gestation from 28 wk, in contrast to the other reported studies, where only women of gestation ≥36 wk were included (55,56,58). Fetal monitoring of premature fetuses is more challenging to interpret than in the term equivalent. The report did not provide outcomes by gestational age (57).

The fifth and largest RCT (n = 5341) examined whether knowledge of FPO values influenced cesarean delivery rates in laboring women with CTG monitoring (59). All consenting women had a FPO sensor placed, then were randomized to have the FSpo2 values displayed (open group) or blinded (masked group). The primary outcome was cesarean delivery. Some outcomes from a subgroup of those with a nonreassuring CTG before randomization were provided.

Given the heterogeneity among the five RCTs and the pending update of a systematic review (60), meta-analysis of these RCTs has not been presented here. Relevant findings from each trial are presented in Table 1. The substantive finding of these trials is dominated by the largest RCT, which concluded that knowledge of FSpo2 values did not reduce overall cesarean delivery rates.


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  		Table 1. Selected Entry Criteria and Outcomes From Randomized Trials of Fetal Oximetry

 

After delivery, neonatal outcomes were not different between the CTG-only or masked FPO group and the CTG + FPO group (55–59). Very large sample sizes would be required to detect between-group differences in low-prevalence adverse outcomes, such as meconium aspiration, neonatal encephalopathy, or long-term neurodevelopmental delay. Such adverse events were closely monitored within the FOREMOST trial (45), with all adverse maternal and neonatal events in both study groups reported to the Data Monitoring Committee, regardless of their likelihood of resulting from trial participation. Very few events occurred, despite all participants requiring a nonreassuring fetal status before study entry. No events resulted in changes to the trial protocol (58). The lack of between-group differences for outcomes such as umbilical arterial pH <7.00, low Apgar scores, or admission to the neonatal intensive care unit, provides some reassurance that delaying or averting operative intervention during labor did not compromise these babies’ outcomes.

Compliance with trial protocol was not well reported in the RCTs. Any variable monitored during labor would, at best, form only part of the overall clinical picture. Although the documented indication for cesarean delivery may have been nonreassuring fetal status, it remains unclear whether such an indication was actually informed by FSpo2 values or by the CTG and other factors such as the degree of meconium staining of the amniotic fluid, for example. The conclusion of one study that knowledge of FSpo2 values did not reduce cesarean rates (59) may reflect only a part of the overall picture. The question may be: "Did clinicians trust the validity of the FSpo2 values?"

FURTHER CONSIDERATIONS OF FPO

If FPO makes no difference to cesarean rates, does it make any meaningful differences for other relevant clinical outcomes, or to clinicians, childbearing women, and the health dollar?

Further Clinical Outcomes
Several reports demonstrated that the addition of FPO in cases of complete congenital fetal heart block (CCHB) provided an important variable of fetal well-being (61–63). In the fetus with CCHB, the CTG usually records the fetal heart rate at approximately 70 bpm, without the typical heart rate fluctuations and responses to labor seen in other babies. Cesarean delivery is often offered, rather than labor with the uncertainty of fetal well-being. As an example, we monitored two fetuses with CCHB. FPO provided additional information about fetal condition, resulting in vaginal births of these babies (63).

Clinicians’ Perceptions of Sensor Placement
An important, although often overlooked, consideration of new technology is ease of use and utility for the relevant clinician: for fetal oximetry, this includes nurses, midwives, and obstetric medical personnel (64). Johnson (33) evaluated operators’ scores for fixation of early prototype sensors fixed to the fetal scalp by glue or suction and found that neither rated well. These findings prompted the development of new prototypes: neither method of fixation is currently used. Several investigators have reported that clinicians rated ease of Nellcor FPO sensor placement favorably (64–66).

Childbearing Women’s Perceptions of FPO
An equally important aspect of technology development is an evaluation by those to whom it is applied. In the case of FPO, childbearing women provide valuable feedback on factors such as level of comfort during sensor placement and ongoing use, in addition to the potential for this form of monitoring to limit their ability to move freely during labor (45,67–69). Women’s perceptions of their experience with labor and fetal monitoring were compared for the CTG-only and CTG + FPO groups in the FOREMOST randomized trial (45). The addition of FPO technology did not affect women’s perceptions of fetal monitoring or their labor when there was already some concern about fetal well-being. Put another way, the trade-off between additional technology during labor was acceptable when the option of avoiding a cesarean delivery was considered (45).

Economic Considerations
The American College of Obstetricians and Gynecologists expressed concern that the addition of FPO would potentially increase costs without necessarily improving clinical outcomes (70). One RCT included a cost-effectiveness analysis of the prespecified primary outcome; operative delivery for nonreassuring fetal status (58). Rather than increasing costs, it demonstrated a saving of $813 (Australian dollars) for each operative delivery averted by the use of FPO (71). Although presented here with caution due to the inherent biases of post hoc analyses, reconstruction of the economic analysis to examine the small, nonsignificant difference in overall cesarean rates demonstrated an even greater saving for each cesarean delivery averted in the FPO group (71).

FPO: THE FUTURE

The future of FPO may have already been decided by two factors. The recent publication of a large RCT that demonstrated no difference in cesarean delivery rates is compelling (59) and the manufacturers of the oximetry system used in this and all other published RCTs have discontinued production of the system.

Several issues still warrant consideration before FPO are disregarded as a potentially useful adjunct to intrapartum fetal monitoring. The published RCTs required varying entry criteria and considered different action algorithms, with compliance not well reported, as well as considering different primary outcomes. Have the right questions been addressed by these RCTs? Has the right clinical management protocol been proposed and evaluated? Could pre- and postductal FPO measured by systems other than the Nellcor system, which have not yet been evaluated in RCTs, influence the clinical protocol and outcome(s)? Does FPO have a place, albeit limited, in clinical practice? Several high-quality RCTs demonstrated a reduction in cesarean delivery for nonreassuring fetal status in the CTG + FPO group compared with the CTG-only group with similar trial entry criteria (55,58). This finding is important, even though there was no difference in these trials for overall mode of birth, as a result of more cesarean deliveries, forceps, or vacuum births being conducted due to dystocia or inadequate progress. Avoidance of an emergency intervention for the sake of the baby has implications in terms of stress levels for the mother and resource implications for the health service providers: these outcomes have not yet been evaluated in the RCTs. An "inevitable" operative birth (that is, for dystocia or inadequate progress) may be conducted when the woman has had more time to consider her options and when staffing levels can be adjusted over a number of hours rather than providing immediate staffing for an emergency cesarean.

The incidence of cesarean delivery for dystocia almost doubled in the RCT reported by Garite et al. (55) when FPO was added to CTG monitoring. Concerns were raised that the presence of the FPO sensor may have contributed to this outcome. Rates of cesarean for dystocia in the study by Bloom et al. (59), for which all participants had an FPO sensor, approximated those in the FPO group of the Garite et al. trial (55). The Nellcor sensor used in these trials was advanced through the cervix to lie against the fetal temple or cheek. Use of the OB Scientific sensor, placed through the cervix to lie against the fetal torso, compared with the Nonin system that attaches to the fetal head, may inform the unresolved question of the potential effect of an intrauterine sensor on dystocia.

Further exploration is recommended into the effect of a change in FSpo2 in the individual fetus, in addition to a critical action threshold. Timing of the intervention (such as cesarean delivery) may be more important than currently realized and may only be considered in very large trials. Additionally, the clinical reports using the Nonin and OB Scientific FPO systems are still emerging. The different measurement locations and sensor designs may impact on clinical protocols in observational and randomized trials.

The value of intrapartum FPO for monitoring the fetus with CCHB may also be appropriate (63).

CONCLUSION

Intrapartum FPO initially emerged as a promising technology to assist in the assessment of fetal well-being. Well-conducted RCTs demonstrated no effect of the addition of this technology on overall modes of birth or neonatal outcomes, suggesting that this technology will not make a valid contribution to future fetal monitoring. That conclusion may be premature, given the remaining unanswered questions of its place in intrapartum fetal monitoring.

Footnotes

Accepted for publication April 17, 2007.

Reprints will not be available from the author.

The authors were Chief Investigators of the FOREMOST trial, which was supported by a research grant from TYCO Inc. (Nellcor).

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Lippincott, Williams & Wilkins Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins and Stanford University Libraries' HighWire Press®. Copyright 2007 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999 HighWire Press