CONTINUOUS INTRAPARTUM pH, pO2, pCO2, and SpO2 MONITORING

ANTEPARTUM AND INTRAPARTUM FETAL ASSESSMENT

0889-8545/99 $8.00

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CONTINUOUS INTRAPARTUM pH, PO, pco, and s p a MONITORING Helen M. Mc Namara, MD, MSc, and Gary A. Dildy 111, MD

Much controversy exists regarding the value of intraparturn fetal monitoring for the prediction of brain damage. The literature has been reviewed60 to identify specific fetal heart rate (FHR) patterns that may predict this condition. A total of 10 infant studies were identified. No pattern or cluster of patterns of FHR could predict adverse outcome. It was concluded that current monitoring techniques do not provide protection against the development of cerebral palsy. A recent National Institutes of Health workshop convened to develop standardized and unambiguous definitions for FHR tracings49reached no consensus regarding strict guidelines for clinical management using FHR patterns. There was good agreement as to the definition of a normal FHR tracing. There was also consensus as to patterns suggestive of potential neurologic damage and death, such as recurrent late or variable decelerations or substantial bradycardia with absent variability. Many fetuses exhibit FHR tracings that are intermediate between these two patterns, and their management is controversial. In this group, more research is urgently needed to detect fetal hypoxia and to minimize unnecessary interventions in the absence of fetal compromise. FHR patterns alone are insufficient to manage fetuses in labor, and a supplementary indicator of fetal well-being is required to facilitate appropriate decision making regarding the timing of delivery for fetal From the Department of Obstetrics and Gynecology, McGill University and Royal Victoria Hospital (HMN), Montreal, Quebec, Canada; and the Department of Obstetrics and Gynecology, University of Utah School of Medicine (GAD), Salt Lake City; and the Department of Maternal-Fetal Medicine, Utah Valley Regional Medical Center (GAD), Provo, Utah

OBSTETRICS AND GYNECOLOGY CLINICS OF NORTH AMERICA VOLUME 26 * NUMBER 4 * DECEMBER 1999

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compromise. In theory, this supplementary information is gained by fetal scalp blood sampling and acid-base measurement. In practice, this technique is rarely used owing to technical difficulties and lack of experience, and, in most cases, decisions regarding delivery are made based on FHR patterns alone. Even in centers where the technique is used, information is available only for the point in time when the test is performed. Because a minimum of two samples is required to establish any trend, multiple fetal incisions are required throughout labor. These issues have led to attempts to develop a user-friendly technique to monitor fetal oxygenation and acid-base status continuously throughout labor in an attempt to understand the physiology and pathophysiology of fetal acid-base status and to aid in the interpretation of intraparturn FHR patterns, leading to timely delivery of compromised infants while reducing inappropriate interventions for healthy infants. This article reviews experimental methods of intrapartum fetal acidbase analysis, with a focus on the attempts at continuous fetal pH and continuous fetal pOz monitoring in the 1970s, continuous fetal pC02 monitoring in the 1980s, simultaneous monitoring of fetal PO, and pC0, simultaneous monitoring of fetaI pH/ pC0, and the development of fetal oxygen saturation monitoring using pulse oximetry in the 1990s, which continues to be used today. CONTINUOUS pH MONITORING

Both acute and chronic hypoxemia may contribute to a decrease in fetal pH. The origin of blood supplying the fetal brain and scalp is the same, and therefore arterial scalp blood reflects oxygen delivery to the brain. Because fetal scalp blood sampling reflects acid-base status only at one moment in time, attempts have been made to develop methods to monitor intraparturn fetal blood parameters continuously. Continuous fetal pH measurement was introduced because of the limitations of intermittent scalp sampling, such as technical difficulty, operator inexperience, discomfort to the patient, and the reluctance to repeat the test, which limits the ability to observe a trend in fetal pH measurement throughout labor. It was thought that continuous recording of the fetal pH would result in fewer scalp incisions and minimum discomfort to the patient while obtaining continuous information regarding the fetal acid-base status, which could lead to improved diagnosis and management of fetal acidosis. In 1974 Stamm and c o - ~ o r k e r sfound ~ ~ that, in normal fetuses, tissue pH was similar to capillary pH, whereas in acidemic fetuses, there was a time lag of 1 to 2 minutes. Another study found that the time lag in electronic FHR monitoring may be 10 to 15 minutes during acute a ~ i d e m i a In . ~ ~a study of 229 measurements, the correlation between tissue pH and umbilical artery pH was 0.71.52 Two years later, Stamm introduced a miniature glass electrode to measure tissue pH in the fetal scalp." In this design, a glass membrane

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separated an internal electrolyte solution and scalp subcutaneous tissue fluid. To apply the electrode, a 2 X 2 mm incision was made in the fetal scalp after a large double-helix spiral electrode had been attached. The pH-sensitive glass tip had to penetrate the scalp to a depth of 3 mm and had to remain perpendicular to the scalp. The tissue pH was measured by calculating the difference in electrochemical potential across the glass membrane between the scalp subcutaneous tissue fluid and the internal electrolyte solution in the Stamm electrode. Use of this technique was limited because the device had to be well applied to the scalp to eliminate contamination from air or amniotic fluid and because it needed to remain perpendicular to the skin, penetrating to the correct tissue depth. In 1978 Wood and co-workersm noted that tissue pH sometimes changed with uterine contractions in the absence of any change in FHR. The most comm.on change was a fall in tissue pH with uterine contractions, particularly when contractions were strong. This group also noted that if the scalp tissue was thin, or if fluid from the vagina or outside of the scalp was carried into the tissue as the probe was inserted, a low pH reading could result. Other problems with this technique included sterilization, calibration, and fixation of the electrode on the fetal head.29The normal values for intraparturn scalp tissue pH levels were similar to those obtained by intermittent sampling75;however, as the fetal head descended and rotated during labor, there was a risk of electrode displacement or breakage of the glass tip, leading to scalp laceration, abscess formation, or both. In 1978 Lichtenegger and BurghardPo reported that storage of the continuous pH electrode in a reference buffer considerably improved the stability of the electrode. These investigators found that only after placing the pH electrode through an incision in the fetal scalp could its point enter deep enough into the tissue to obtain accurate measurements of fetal tissue pH. In the same year, Boos and colleagues7 studied 20 fetuses and one nonviable newborn using the Roche-modified pH electrode (originally the Stamm electrode). This group also reported that proper fixation of the electrode was difficult. They found that, in the physiologic pH range, good correlation was obtained between tissue pH and capillary blood pH, but the situation was less clear in the acidotic range. The application of the probe to the fetal scalp required technical improvements. To attach the probe to the fetus, the subject was placed in stirrups, and an amnioscope was introduced into the vagina. A fetal blood sample was taken, followed by the attachment of a double-helix spiral electrode designed to hold the pH electrode to the fetal scalp (Fig. l).”A small incision was made in the scalp through a central opening in the spiral electrode, and the tissue pH electrode was introduced using a special tool. The Roche-modified system was used in the study of 40 patients. The mean monitoring time was 2.5 hours. In the initial cases, the correlation between tissue pH and capillary blood pH was 0.74, in the latter 23 cases, the correlation was 0.82. The improvement in correlation was attributed to modification of the application technique and the

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THE pH ELECTRODE

Reference Double Spiral Electrode

pH Sensitive Surface

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Skin Surface

Subcutaneous Tissue

Figure 1. Continuous intraparturn fetal scalp pH electrode, as described by Lauersen and colleagues. (From Lauersen NH, Miller FC, Paul RH: Continuous intrapartum monitoring of fetal scalp pH. Am J Obstet Gynecol 13345, 1979; with permission.)

introduction of a new calibration method at 37°C. A good correlation was obtained between fetal continuous pH and immediate neonatal outcome. In this series, no maternal complications were reported, and there were no infant complications in 38 of 40 cases. One infant had a broken electrode tip within the fetal scalp during application, and another infant had inflammation of the electrode site, which required treatment with antibi0tics.3~ In 1980 Kellner and colleagues36studied 21 patients using the continuous pH system and reported success in 13 (61.9%). This group found a significant correlation between tissue pH and scalp pH (r = 0.94, P <.0001) and between tissue pH and umbilical artery blood pH (r = 0.92, P <.01). In the same year, Weber” studied continuous fetal tissue pH in 152 cases with a reported success rate of 53%. In this series, the average duration of monitoring was 151 minutes (range, 0 to 609), and the correlation between tissue pH at delivery and cord arterial pH was 0.78. In the presence of an abnormal FHR tracing, if the tissue pH was greater than 7.2, the Apgar scores were always 8 or greater at 1 minute. Normal tissue pH almost always indicated that the fetus was not asphyxiated, but low tissue pH could have been caused by factors other than fetal acidosis.” In 1982 the same investigator reported on 96 cases with acceptable readings in 72 cases (75%).” In that study, the tissue pH studies were used in labor management. In the absence of fetal bradycardia of less than 70 beats per minute (bpm), a pH cutoff of 7.20 was used for obstetric intervention. The data from this series were compared with information on 72 cases from an earlier series in which tissue pH was not used in the management of patients. When fetal

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continuous tissue pH readings were used in the management of labor, there was a significant reduction of operative vaginal delivery compared with the rate in a matched group in whom tissue pH was not used (9y0 versus 26%, P = .007) and a decrease in total interventions from 36% to 15%, with no difference in neonatal status between the two groups.R Young and colleaguesg2examined the relationship between continuous tissue pH monitoring and FHR patterns. This group found normal tissue pH in cases of accelerations and moderate bradycardia. Tachycardia was associated with a decrease in pH in 60% of fetuses, whereas reduced variability was associated with decreased pH in 36% of cases. Early and variable decelerations were followed by decreasing pH (20% to 25%), and late decelerations were associated with decreased pH in 93% of cases. This group concluded that use of the Roche electrode for continuous fetal pH measurement had several drawbacks. The application of the electrode was difficult, and the electrode was expensive. The sterilization procedure was associated with breakage of the electrode, and the mean lifetime of the electrode was 25 cases. The procedure required a scalp incision, with a risk of infection from mother to fetus and between patients because the electrode was not disposable.8z In 1980 a fiberoptic pH probe was developed by Peterson and ~olleagues.5~ This electrode worked on the concept of monitoring the color of a pH-sensitive dye. It involved a less invasive attachment procedure in that a scalp incision was no longer required. The sensor consisted of a spiral electrode with a sensing window near the tip that penetrated the tissue to a depth of 4 mm. In 1988 Hochberg and coworkersz7reported an 80% success rate using a fiberoptic pH electrode. Another attempt at a new electrode design was the polyvinyl chloride (PVC) electrode. This electrode worked on a similar principle as the glass electrode; however, the PVC electrode was potentially disposable, was stable, and did not require calibration. It was incorporated into a spiral needle, and it was suggested that a PVC electrode could be constructed to measure pC02 in the other spiral, thus allowing measurement of the acid-base profile continuously during labor.= In 1989 Small and colleaguesz0studied continuous fetal pH measurement in 59 patients using a spiral needle electrode. The probe was part of a reusable system that required sterilization with glutaraldehyde for 10 hours before each use. Before application, the probe required calibration for 30 minutes in pH 7.00 and pH 7.40 buffer solutions kept at 37°C in a heating block. The spiral needle electrode was developed to overcome previous problems with the potential for breakage associated with glass electrodes while attached to the scalp. Smith's group reported a poor correlation between tissue pH and capillary scalp pH in 19 cases (r = 0.487) and no correlation with either Apgar scores or cord gases.63 Another fiberoptic probe based on monitoring the color of a pHsensitive dye was developed in the same year. At the distal end of the probe, a 22-gauge needle formed into a spiral penetrated 3.5 to 4 mm into the fetal scalp. At the end of the needle was a window covered by a semipermeable membrane through which tissue fluids could pass. The

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end of the needle under the window was packed with pH-sensitive dye particles of phenol red polyacrylamide. A change in tissue pH caused a color change in the dye, and this color change was interpreted by the fiberoptic probe as a change in pH. The probe required sterilization for 10 hours before use, and an additional 30 minutes were required to calibrate it. In one study, the probe was used to obtain continuous pH readings at term in 59 ~atients.6~ There was poor correlation between tissue and capillary pH in 19 cases, and there was no significant correlation between the continuous pH reading and outcome measures such as Apgar scores or umbilical cord blood gas parameters at delivery. In 1991 Chatterjee and Hochberg16used the fiberoptic probe with a dye indicator to study continuous pH measurement in 124 fetuses. A comparison was made of the reliability of acidotic fetal tissue pH versus fetal capillary blood pH in the identification and prediction of obstetric problems. In this series, acidotic tissue pH was a far more powerful indicator of fetal, neonatal, and maternal problems than was acidotic capillary blood pH.

CONTINUOUS PO,MONITORING In 1956 Clark devised an electrode to measure the partial pressure of oxygen (PO,) transcutaneously This electrode consisted of an anode, a cathode, a heating element, thermistors, and an electrolyte solution between the electrode and a covering membrane.@In 1976 Staisch and colleaguesffireported that, in sheep, scalp PO, levels closely reflected PO, levels in the central fetal circulation as demonstrated by comparisons between fetal scalp PO, and carotid artery PO, values. This group also found that administration of 95% O2 to the mother resulted in a 27% increase in fetal scalp PO, and a 29% increase in fetal carotid artery PO, when compared with fetal PO, levels on room air. The electrode was modified by Huch in 1977 to monitor fetal PO, during labor.@ Oxygen diffused across the skin into the electrode and was reduced at the cathode, producing an electrical potential difference. Because good contact was essential, the area had to be shaved and cleansed. Fixation was achieved by glue or vacuum. The electrode required calibration and had to be heated to 43 to 44°C to cause tissue hyperemia. This heated electrode could burn fetal skin at 45°C. In 1977 Lofgren and Jacobson41studied continuous fetal PO, transcutaneously in 19 patients. The majority of the group was primiparous, and all of the fetuses were in vertex presentations. The electrode was applied using glue ( n = 12) or a suction device (n = 7) at 4 to 6 cm dilatation, and an average of 1 hour of data was obtained. The mean transcutaneous p02 was 20 mm Hg at the beginning of registration and declined to 14 mm Hg at the end of registration. The relevance of very low transcutaneous p02 values was unclear, and it was not possible to determine to what extent these tracings reflected true physiologic changes as opposed to abnormalities owing to technical In 1979 Weber and S e ~ h e used r ~ ~ a modified Clark electrode (Radiometer,

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Copenhagen) to measure continuous transcutaneous PO, in 40 fetuses. Of these fetuses, 24 had good readings during the second stage of labor. The correlation between fetal continuous transcutaneous PO, and both umbilical artery PO, and O2saturation (SaOJ was 0.56 (EY.01). OConnor and Hytten= studied continuous fetal transcutaneous PO, in 39 patients using a commercially available skin electrode. This group found a weak correlation between transcutaneous p02 and umbilical blood PO,. They suggested that the weak correlation may be explained by scalp ischemia produced by ”head to cervix” pressure during labor. In the presence of caput formation, low transcutaneous p02 values were obtained in the absence of fetal acidosis. It was concluded that the ”tonsure effect” was a major obstacle to the use of surface electrodes on the presenting part for intrapartum blood gas monitoring. In 1979 Baxi and co-workers5examined continuous fetal transcutaneous p02 following paracervical block. Decreased variability was not uniformly associated with low transcutaneous PO, levels. On some occasions, fetal transcutaneous PO, declined, even in the absence of increased uterine activity, which was noted at times following paracervical block. The severity of the transcutaneous PO, decline seemed to be related to the analgesic effectiveness of the paracervical block. In 1981 Willcourt and colleagues79reported their findings in a study of continuous transcutaneous PO, measurements in 46 fetuses, 30 of whom had abnormal FHR patterns. An increase in transcutaneous PO, was associated with a decrease in FHR variability, and a decrease in transcutaneous PO, was associated with an increase in FHR variability. Willcourt and co-workers tried to explain this discrepancy by stating that the fetal transcutaneous PO, declined during a FHR deceleration and rose thereafter with corresponding decreased variability. They also found that incomplete recovery of the fetal transcutaneous p02 was associated with progressive acidosis. Repetitive and isolated late deceleration patterns showed markedly dissimilar fetal transcutaneous PO, changes, which suggested that different mechanisms may be involved in the production of these FHR patterns. Rooth and c o - ~ o r k e r classified s~~ the interaction among fetal transcutaneous PO, FHR, and intrauterine pressure into 10 different pattern types. In 8 of the 10 types, a reduction in transcutaneous PO, was observed as a result of FHR decelerations and uterine contractions. In the other two pattern types, the decreased transcutaneous PO, measurements were associated with pressure and stasis of blood in the presenting part. In 41 recordings, 1161 contractions were analyzed. In 155 of the contractions during the first stage of labor, transcutaneous p02 was affected by stasis or pressure, and this occurred in 48% of contractions in the second stage. It was concluded that transcutaneous PO, should be reliable in the first stage of labor, but that stasis and pressure may be falsely associated with decreased fetal transcutaneous p02 in the second stage of labor. In 1981 Aarnoudse and co-workers2studied subcutaneous PO, in 25 fetuses using a needle electrode. The p02was measured in subcutane-

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ous tissue using a small electrode combined with a spiral electrocardiographic (ECG) electrode. One hour after application of the electrode in early first-stage labor, a mean fetal subcutaneous PO, of 29.4 k 7.5 mm Hg was recorded. Over the course of labor, subcutaneous PO, gradually fell to a mean value of 22.4 k 5.6 mm Hg recorded just before pushing efforts commenced. A fetal subcutaneous PO, reading of less than 20 mm Hg was recorded in one case of severe fetal distress. The correlation between fetal subcutaneous PO, just before delivery and cord arterial PO, was 0.85. The fetal subcutaneous PO, seemed to be higher than fetal transcutaneous PO, levels previously described. This discrepancy could be accounted for by the different techniques of measurement used. The subcutaneous PO, needle electrode measures deeper layers of the scalp, and the transcutaneous PO, surface electrode measures oxygen from more superficial capillaries where blood flow is more likely to be affected by mechanical factors. In 1985 this group1 reported on continuous fetal subcutaneous PO, measurements using the combined fetal ECG / fetal subcutaneous PO, needle in 34 fetuses. The incidence of FHR abnormalities increased significantly with decreasing subcutaneous PO, from 0.8% of the 10-minute periods in which subcutaneous PO, was 25 mm Hg or greater to 53% of the periods in which subcutaneous PO, was less than 10 mm Hg. CONTINUOUS pCOz MONITORING

The transcutaneous pC0, electrode was developed essentially as a pH electrode placed in an electrolyte solution beneath a membrane permeable to carbon dioxide. When applied to the fetal head using glue or suction techniques, the electrode had to be heated to 39 to 44°C to increase capillary blood flow. It also had to be in close contact with the skin to allow CO, from the skin to diffuse through the membrane into the electrolyte solution. This diffusion of CO, caused a change in the pH, which was used as an indicator of the amount of CO, diffusing across the membrane.76The transcutaneous pC0, electrode worked on the same principle as the continuous transcutaneous PO, electrode but was less sensitive to skin thickness, temperature, and blood flow. Application did not require shaving of the fetal head. In 1984 Hansen and studied transcutaneous pC0, in 55 fetuses using a CO, electrode applied to the fetal scalp by a suction ring. The correlations between fetal transcutaneous pC0, and fetal blood pC0, were as follows: umbilical artery r = 0.6, P<.OOl; umbilical vein r = 0.69, P
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degree of maternal hyperventilation had no adverse effect on the status of the infant at delivery. In 1986 Nickelsen and Webelj' monitored fetal transcutaneous pC0, simultaneously using two electrodes fixed by both glue and suction in 10 fetuses. No difference was found between the two fixation methods with regard to transcutaneous pC0, readings, stabilization time, or success rate, but the glue fixation method was more time-consuming and caused more discomfort to patients, and reapplication of the electrode was not usually possible. The same question was addressed in a study by Schmidt and Salinf in 1987. These researchers found that the correlation between transcutaneous pC0, and fetal blood pC0, was similar for both fixation methods. The mean values of transcutaneous PO, were significantly higher when suction fixation was used in comparison with glue fixation. Another study examined transcutaneous pC0, as an atraumatic tool to identify or exclude fetal acidosis in 224 high-risk deliveries.62A heated electrochemical sensor was used with a measuring temperature of either 39°C (n = 105) or 44°C (n = 119). A statistically significant correlation was found between the transcutaneous pC0, and the fetal blood pH at both temperatures. It was concluded that the continuous measurement of fetal transcutaneous pC0, may be useful to exclude fetal acidosis in most cases in which fetal distress is suspected based on the FHR pattern. In a similar study by Nickelsen and Weber,53a total of 122 fetuses (80 fetuses with an electrode temperature of 44°C and 42 with an electrode temperature of 41°C) were monitored using transcutaneous pC0,. Significant correlations were found between transcutaneous pC0, and umbilical arterial blood pC0, at both temperatures, but a larger CO, contribution from skin metabolism was suspected at the lower temperature. Mean values of fetal transcutaneous pC0, rose during normal labor and especially in fetuses developing acidosis; however, only four of six infants in this series born with acidemia had transcutaneous pC0, values exceeding the normal range. In 1993 Braems and co-workersgstudied transcutaneous pC0, in 21 deliveries using an electrode applied to the presenting part after rupture of the membranes. The acid-base status of the fetus was determined in capillary blood (n = 33) and umbilical cord arterial samples. The fetal transcutaneous pC0, was correlated with pC0, of capillary blood ( Y = 0.56, n = 33, P<.OOl); pC0, of umbilical arterial blood (Y = 0.75, n = 15, P<.Ol); pH of capillary blood (Y = -0.56, n = 33, P<.OOl); and pH of umbilical arterial blood (r = -0.34, n = 15, P = NS). COMBINED CONTINUOUS PO, AND pC0, MONIT0RING

In 1984 Sykes and co-workers68used mass spectrometry to measure continuous transcutaneous PO, and transcutaneous pC0, simultaneously. The transducer consisted of a gas-collecting chamber heated to 43.5"C. Diffusion of the gases into the chamber occurred through the

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skin across a thin polymer membrane and then to the spectrometer via a cannula. During late decelerations, continuous transcutaneous PO, levels fell, whereas continuous transcutaneous pC0, levels increased. According to Huch,28the problems involved in transcutaneous p02 and transcutaneous pC0, measurements include diffusion, limitations owing to peripheral perfusion, fetal head compression, and the unpredictable effects of caput formation. The normal range for trancutaneous pC02 is wide, and this factor limits its use in the prediction of acidosis. HuchZ8 has concluded that transcutaneous pC0, and transcutaneous pC0, are not useful for continuous fetal monitoring in clinical practice, although these techniques may be valuable in the study of the physiology and pathophysiology of labor.28 COMBINED CONTINUOUS pH AND pC0, MONITORING

In 1985Nickelsen and co-workersMsimultaneouslymonitored tissue pH and transcutaneous pC0, in 30 fetuses and observed a positive correlation with cord blood parameters (pH, r = 0.69; pC0, r = 0.68). The Siggaard-Andersen nomogram (to derive tissue base excess values from the simultaneous continuous tissue pH and transcutaneous pC02 readings) was used in an attempt to detect metabolic acidosis. Measurements were successful in 13 fetuses; only infants with decreased umbilical arterial pH had decreasing tissue base excess. In a subsequent study using the same methods, the standard base excess (SBE) was calculated on line using a microcomputer.5° A statistically sigruficant correlation was obtained between the base excess in umbilical artery blood at delivery and the experimental continuous SBE during labor (r = 0.66, n = 13, K.05). In seven nonacidotic infants, the SBE slightly increased or remained constant during the intraparturn period.50In two cases in which acidemic infants were delivered, the SBE decreased during 1ab0r.s~ The main limitation of these studies was the lack of a user-friendly tissue pH electrode as previously discussed. CONTINUOUS SpO, MONITORING

Pulse oximetry has been used as a noninvasive means to measure oxygen saturation in adults and neonates for almost 2 decades. The ability to assess this parameter without the need for a blood sample was revolutionary when the technology was first introduced. For the purpose of consistency., oxygen saturation measured indirectly by pulse oximetry is designated SpO, whereas the designation Sa02refers to oxygen saturation measured directly by co-oximetry on a blood sample. Pulse oximeters can be found in most intensive care units and operating rooms. They have been accepted as part of routine anesthesia monitoring and are used extensively in the neonatal unit. Most emergency. rooms in the United States are equipped with a pulse oximeter to identify patients

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who are seriously ill. The oximeter is especially helpful when assessing the response to oxygen therapy.= It has been stated that any clinical situation associated with respiratory or circulatory compromise may benefit from oxygen monitoring.8 Pulse oximetry is defined as the measurement of oxygen saturation using optical means. The differential absorption characteristics of oxygenated hemoglobin (0,Hb) and deoxygenated hemoglobin (Hb) at two different wavelengths of light allow a determination of the oxygen saturation. Pulse oximetry is used to measure hemoglobin oxygen saturation in arterial pulsatile blood by determining the color of the blood between a light source and a photodete~tor.~~ The amount of light absorbed is calculated by subtracting the light reflected from the total light emitted according to a modification of the Beer-Lambert law. The difference between oxygenated and deoxygenated hemoglobin allows a calculation of the oxygen saturation. Oxygen saturation was first defined as oxygen content expressed as a percentage of oxygen capacity: Functional SaOz%=

OZHb x o&b + Hb

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Arterial hemoglobin oxygenation can be determined directly and continuously in vivo using spectrophotometry. The difference in light absorption between oxygenated and deoxygenated hemoglobin is evident from the prominent color differences in spectral light absorbance of red oxygenated hemoglobin and blue deoxygenated hemoglobin.8*Oxygenated blood absorbs more infrared light at 940 nm, and deoxygenated blood absorbs more red light at 660 nm.6 Oxygen in blood is either dissolved in plasma or bound to hemoglobin. The portion of oxygen in plasma is expressed as the PO,. The SaO, refers to the proportion of oxygen bound to hemoglobin in arterial blood. The PO, accounts for approximately 1%to 2% of the total oxygen in the blood, whereas the SaO, accounts for 98%; therefore, oxygen saturation is more clinically significant when determining oxygen content of the blood. The SaO, is influenced by the affinity of hemoglobin for oxygen. This affinity is represented by the oxyhemoglobin dissociation curve (Fig. 2). The curve demonstrates the relationship between SaO, and PO,. At the flat upper portion of the curve, PO, may vary greatly with little change in oxygen saturation. At the steep part of the curve, with a small decrease in PO, there is a sharp drop in oxygen saturation. In practice, a light source (light-emitting diode) shines light through tissue such as the finger. A photodetector on the other side of the finger detects how much light passes through without being absorbed (Fig. 3). The difference between the incident light and the amount of light transmitted at each wavelength allows a calculation of oxygen saturation. The physics of pulse oximetry are explained by the Beer-Lambert law. The Beer-Lambert law relates the concentration of a solute to the

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Figure 2. Oxyhemoglobin dissociation curve illustrating the relationship between oxygen dissolved in blood plasma (PaO,) and oxygen bound to hemoglobin (SaO,). (Courtesy of Nellcor Puritan Bennett, Pleasanton, CA.)

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Figure 3. Transmission (A) and reflectance (6)methods of pulse oximetry. Transmission pulse oximeters are typically used in adult and pediatric monitoring; the reflectance technique was developed specifically for fetal application. LEDs, light-emittingdiodes. (Courtesy of Nellcor Puritan Bennett, Pleasanton, CA.)

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intensity of light transmitted through a solution. The differences in absorption between oxyhemoglobin and deoxyhemoglobin are described quantitatively by the extinction coefficients in this law as follows: I, = Ih e-A[A = DCe] where I,,,, is the intensity of transmitted light, Ii, is the intensity of incident light, e is the extinction coefficient of solute, A is the absorption, D is the path length, and C is the concentration of solute. Solute concentration can be calculated from measurements of incident light and transmitted light intensity at a known wavelength, path length, and extinction coefficient.7"For Beer's law to be valid, the solvent must be transparent, the light path length must be known exactly, and no absorbing species can be present in solution other than in known solute. It is difficult to fulfill these conditions in clinical practice, and therefore instruments require empirical corrections to Beer's law to improve a~curacy.~" In neonates, oximeters are unable to distinguish between HbF and HbA. Pologe and R a l e have ~ ~ ~compared the extinction curves for both types of hemoglobin and have found that the light absorption properties of hemoglobin lie primarily in the "heme" portion, whereas the functional difference between HbF and HbA lies in the globin chain. Therefore, HbF should not have an impact on pulse oximetry readings; however, there has been no published study examining the accuracy of oximetry in the presence of high HbF levels.58Fetal hemoglobin has a theoretical significance because the extinction curve for HbF is slightly different from that for HbA. Nevertheless, the pulse oximeter should read accurately because it is measuring the percentage of saturated hemoglobin.26In a study of 18 critically ill newborns, changes in fetal hemoglobin did not affect pulse oximeter accuracy, leading to the conclusion that the oximeters used in the study accurately measured oxyhemoglobin independent of HbF levels? CONTINUOUS FETAL SpO, MONITORING Fetal oxygen saturation monitoring using pulse oximetry technology was first explored in the late 1980s. Apart from two letters, the first publication describing pulse oximetry in the fetus was a case report on the use of early technology in a fetus with cord compression?" Fetal pulse oximetry was a new technology that had its own unique set of problems during development. First, the fetal pulses were much smaller than adult pulses and therefore required amplification to obtain useful signals. Also, it was obvious that the transmission principle previously referred to was not applicable to the fetus in utero. This observation led to the development of a reflectance sensor that housed the light-emitting diodes and the photodetector side by side (see Fig. 3). Early work also revealed a problem in monitoring fetuses with thick dark curly hair.33In other ways, monitoring the fetus with pulse oximetry had fewer limita-

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tions than adult or neonatal monitoring. There were no false readings owing to ambient lighting because the sensor was placed internally. The fetus does not usually have significant levels of methemoglobin or sulfhemoglobin, although carboxyhemoglobin may be present in small amounts if the gravida is a smoker. Carboxyhemoglobin is associated with an artificially high SpO, reading only at levels greater than 3%26and this level is unlikely in fetal blood even if the gravida is a heavy smoker. Other technical problems arose in regards to the method of sensor application to the fetal presenting part. Early methods of contact included the use of glue or suction. The effect of pressure from the effacing and dilating maternal cervix on the presenting part (and therefore readings taken from the presenting part) was a concern, and it was established that areas of caput formation should be avoided as a monitoring site.32In 1991 Gardosi and colleagues24reported their experience with a ”fetal invasive” system (i.e., an invasive attachment to fetal scalp) to measure fetal oxygen saturation using a standard pulse oximeter. Other investigators subsequently developed and studied other fetal invasive pulse oximetry systems. The discussion herein is confined to the development and evolution of a “fetal noninvasive” system (i.e., no attachment to fetus), which thus far has been the most extensively used. In 1989 the first fetal prototype sensor was designed in conjunction with engineers at Nellcor (FS-10 Sensor, Nellcor, Pleasanton, CA) and was initially tested at St. James’s Hospital in Leeds, in the United Kingdom. The lights and the detector were housed in a modified version of the Copeland electrode, which was used in Europe for internal FHR m~nitoring.~~ The “clip” sensor was placed on the fetal presenting part, and the signals were poor. This observation led to the development of the fetal noninvasive pulse oximetry system by the same group of researchers who studied 113 fetuses of low-risk women in normal lab0r.4~ The next sensor design had no means of active attachment to the fetus (Fig. 4) and was introduced past the cervix beyond the fetal presenting part. The sensor was inserted as far as possible then gradually pulled back 1 cm at a time until a strong pulsatile signal was observed. These early sensor designs were followed by modifications of the size of the sensor head and the development of a standard method of fetal sensor insertion, which continues to be used in current ongoing studies. During the development phase, the design progressed from a stiff sensor handle to a stylet introducer, which could be removed when the sensor was in place. Centimeter markings in different colors were added on either side of the handle to allow identification of maternal and fetal surfaces of the sensor external to the vagina. The placement method involved placing the sensor at right angles to the sagittal suture to reach the optimum fetal monitoring site, which is the fetal cheek.47Further modifications to the sensor design were associated with improved performance. Multiple sensor designs were explored in the initial development phase, leading to other sensor modifications that resulted in the development of the FS-14 sensor (see Fig. 4). The most critical development was a change in the wavelengths of the light-emitting diodes used in fetal surveillance,

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Figure 4. The Nellcor FS-10 (A) and FS-14 (B) sensors. Each sensor contains two LEDs, one photodetector, and three impedance contact electrodes. The FS-10 prototype used a 2-part liquid injection silicone sensor body, a 10 mm center-to-centerdistance between the LEDs and photodetector, with red (660 nrn) and infrared (890 nrn) LEDs. The FS-14 sensor body is constructed of a thermoplastic elastomer (Santoprene), has a 14-mm center-tocenter distance between the LEDs and photodetector, and uses different wavelength red (735 nm) and infrared (867 nm) LEDs. The FS-10 sensor (series 2419) was used in many observational clinical studies performed circa 1993. The FS-14 sensor (series 5002) was used in studies conducted circa 1998, including the US multicenter randomized clinical trial. (Courtesy of Gary A. Dildy, MD.)

which resulted in significantly improved signal quality and was a major advance in fetal noninvasive pulse oximetry. CLINICAL STUDIES OF FETAL NONINVASIVE SpO, Fetal Oxygen Saturation and Cord Blood Gas Analysis

In 1992 during the development phase, the authors used one of the early prototype FSlO sensors to compare fetal oxygen saturation levels within 10 minutes of delivery with cord blood analysis and Apgar scores in 37 fetuses.%Data of sufficient quality for analysis were obtained from 28 fetuses. There was g significant correlation between the fetal SpO, at delivery and cord blood parameters: cord arterial pH (r = 0.63, P = .001), cord venous pH (r = 0.57, P = .002), and cord venous SaO, (r = 0.59, P<.OOl). There was no correlation between fetal SpO, at delivery and Apgar scores in this early study. In 1994 further research was conducted on 39 fetuses using the new FS14 sensor design.', These fetuses had FHR abnormalities in the first stage of labor. A significant correlation between the fetal SpO, immediately prior to delivery and both umbilical vein pH and 1-minute Apgar score was observed, but no correIation was found between fetal SpO, and umbilical vein oxygen saturation. In 1996 Langer and ~ o - w o r k e r sused ~ ~ the FS14 sensor in their observational study of 62 fetuses. Fetal Sp02 levels during the

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second stage of labor showed a significant correlation with both cord arterial and venous pH and scalp pH. In 1997 Alshimmiri and coworkers3reported an observational study of 54 patients, including highrisk mothers, using the FS-14 Sp02sensor. They examined the prediction of umbilical artery base excess by intrapartum fetal SpO, monitoring. A weak correlation was found between fetal SpO, levels at delivery and both umbilical artery pH and base excess. In the same year, East and colleaguesz reported their observational study using the FS-14 sensor in 118 cases. They found no correlation between fetal SpO, at delivery and umbilical cord blood gases, xanthine, and hypoxanthine. Thus, conflicting data remain regarding the correlation between fetal SpO, at delivery and cord blood acid-base parameters. Fetal Oxygen Saturation and Fetal Scalp pH

Using data obtained from the prototype FS-10 sensor, Johnson and co-workersH reported the results of a retrospective examination of the correlation between fetal SpO, levels and scalp pH in 13 fetuses. No significant correlation was found in this early study; however, the number of subjects available for study was small. Subsequently, Luttkus and co-workersQ performed a prospective study using the FS-14 sensor at the time of 51 scalp blood samples. They found a significant correlation (Y = 0.67) between fetal Sp02and scalp blood analysis. In 1997 Carbonne and co-~orkers'~ also used the FS-14 sensor to examine the predictive value of fetal SpO, for adverse neonatal outcome and found that the predictive value was similar to that of fetal scalp pH. Another study reported in 1998 examined the correlation between fetal SpO, and scalp pH in 46 term fetuses. Low fetal SpO, (530% for 210 minutes) was significantly correlated with low scalp pH (<7.2).37 Fetal Oxygen Saturation and Maternal Oxygen Administration

In 1993 the authors reported their experience with the FS-10 prototype sensor at the development site in Leeds regarding the effect of maternal oxygen administration on fetal SPO,.~~ In the 12 fetuses monitored, maternal administration of 27% oxygen increased the average fetal SpO, by 7.5%, the effect being reversed when the oxygen was withdrawn. The maternal administration of 100% oxygen led to an increase in fetal SpO, of 11%.Using a quadratic regression model, it took 9 minutes for the fetal SpO, to reach its maximum value following maternal oxygen administration. In 1994 this question was addressed at the Utah development site in a study of 20 fetuses. The results confirmed that maternal administration of 100% oxygen increased fetal SpO, from 50% to 64%.19 In that study, although a trend toward increased fetal SpO, was noted, no significant effect on fetal SpO, was observed when

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40% oxygen was administered to the mother unless the initial fetal SpO, was low. In both studies, maternal administration of 100% oxygen resulted in a significant rise in fetal SpO,. Fetal Oxygen Saturation and Maternal Position Change

When FHR patterns become abnormal during labor, one of the first clinical responses is to change the maternal position. In 1996 the FS14 sensor was used in an observational study by Carbonne and co-~orkers'~ to examine the effects of maternal position during labor on fetal SpO, in 15 fetuses. Maternal supine position was associated with lower fetal SpO, and maternal left lateral position was associated with a significant increase in fetal SpO, in healthy fetuses. Fetal Oxygen Saturation and Epidural Analgesia

In an attempt to assess fetal SpO, during maternal epidural analgesia, the authors performed an observational study at the time of 27 epidural "top-ups" in 17 mothers using the prototype FSlO fetal sensor.35 There was no change in fetal SpO, following an uncomplicated epidural top-up. In this series, epidural analgesia in the absence of complications did not affect fetal SpO, in healthy fetuses. Fetal Oxygen Saturation and Labor

In 1993 an observational study was performed by the Provo, Utah, development group using the prototype FS-10 sensor to examine fetal SpO, trends during labor.ls In 73 fetuses, the mean fetal SpO, was 57.9% k 10%. A subsequent study by the same group2I in 1994 also using the FS-10 sensor was performed to examine trends in fetal SpO, during labor in a review of 291 records. The mean fetal SpO, in the first stage of labor was 59% k 10%. The mean fetal SpO, decreased to 53% k 10% in the second stage, which was statistically significant. In 1996 Maesel and co-workersg3also reported a decrease in fetal SpO, from the first stage of labor to the second stage in their cohort of 96 fetuses. This question was further addressed by Langer and colleagues38in their study of 62 fetuses using the FS-14 sensor in which no change in fetal SpO, was noted during labor. Overall, the evidence suggests that intraparturn fetal SpO, decreases throughout labor in the healthy fetus. FETAL OXYGEN SATURATION AND ABNORMAL FETAL HEART RATE

In 1993 the authors reported their observations using the FS-10 sensor on the relationship between fetal SpO, FHR patterns, and abnor-

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ma1 neonatal outcome.44Fetal SpO, dropped in the two cases requiring operative delivery for fetal distress. In one case, this drop preempted FHR changes. In 1994 Carbonne and co-WorkerP examined fetal SpO, in the presence of abnormaI FHR patterns in 55 fetuses using the FS-14 sensor. The negative predictive value of a fetal SpO, level of greater than 40% for a cord pH less than 7.2 was 100%. It was concluded that normal fetal SpO, could be reassuring in the presence of abnormal FHR patterns. In 1997 Butterwegge'O examined fetal pulse oximetry readings in the presence of abnormal FHR patterns using the FS-14 sensor in 34 fetuses. He concluded that if the fetal SpO, level remained greater than 40%, all cord arterial pH values were greater than 7.2. In the same year, a retrospective study was performed by van den Berg and co-workers7*to examine whether fetal SpO, measurement in association with FHR patterns would improve assessment of the fetal condition. A total of 119 medical records were reviewed by four experts. They first reviewed FHR patterns only then subsequently examined the same records with both FHR patterns and fetal SpO, data. They recorded their perceived need for intervention and estimated the umbilical artery pH in each situation. The addition of the fetal SpO, data led to a reduction of interventions in the nonacidotic group from 27 to 16 and in the acidotic group from six to four.

Fetal Oxygen Saturation and Meconium-StainedLiquor

Using the FS-14 sensor, Carbonne and co-w~rkers'~ examined fetal SpO, in 38 cases of meconium-stained liquor with abnormal FHR patterns. The fetal SpO, was significantly lower in cases with meconium aspiration syndrome, with a mean fetal SpO, level of 45% in the first stage of labor and 27% in the second stage. There was no difference in fetal scalp pH or cord blood analysis between the two groups. Intrapartum fetal Sp02 was decreased in cases of subsequent meconium aspiration syndrome.

Fetal Oxygen Saturation and Uterine Contractions

Using the prototype FS-10 sensor design, the authors conducted a prospective observational study of 18 fetuses in early labor to examine the effect of uterine contractions on fetal Sp02.&The effect of both intrauterine pressure and head-to-cervix force was examined using time series analysis and a regression model of 159 contractions. The average fetal SpO, decreased following a contraction. The fetal SpO, nadir occurred 92 seconds after the peak of a contraction and took 1.5 minutes to recover (P = .036). In this series, fetal SpO, was lower following uterine contractions, even in healthy fetuses.

Fetal Oxygen Saturation and Puerperal Morbidity

The authors examined the medical records of 112 consecutive patients monitored with the prototype FS-10 fetal pulse oximetry sensor and 122 matched controls delivered in the same unit. The median blood loss was 200 mL in both groups, and fetal SpO, monitoring was not associated with an increase in maximum recorded maternal postpartum t e m ~ e r a t u r e Dildy . ~ ~ and co-workersZoalso found no difference in puerperal morbidity in cases monitored by pulse oximetry and matched controls with FHR monitoring alone. SUMMARY

The goal of intraparturn surveillance and its further development is better patient care for both the fetus and the gravida. A normal FHR pattern is usually associated with the delivery of a normal well-oxygenated infant; however, a nonreassuring FHR is not always associated with the delivery of a compromised infant. This situation has led to an increase in unnecessary obstetric interventions in the form of a rising cesarean section rate. Fetal scalp sampling was developed in an attempt to improve the predictive value of electronic FHR monitoring, but because this technique is not widely used, management decisions are frequently made using FHR patterns alone. Much research has been performed in the search for a continuous biochemical measurement of fetal status, including continuous pH, PO, or pC0, and various combinations of these methodologies. None of these measurements are used in current clinical practice, mainly owing to technical problems and difficulties associated with the continuous direct measurement of these parameters in fetal blood throughout labor. The promising new field of fetal pulse oximetry has the potential to provide reliable, meaningful, and reproducible data as shown in early cross-sectional studies and more recent longitudinal studies. By identifying developing hypoxia, this technology may reduce the uncertainty associated with electronic FHR monitoring. Fetal pulse oximetry may also provide critical information relating to the detection and management of the hypoxic fetus. Any new method of intraparturn fetal monitoring requires careful evaluation to assess its potential value before its introduction into clinical practice. The use of fetal SpOzmonitoring in the presence of a nonreassuring FHR pattern is being examined in a multicenter randomized controlled trial.I7This study will address the question of whether supplementary monitoring of fetal SpO, levels can lead to a reduction in the cesarean section rate for fetal distress. The available data on fetal noninvasive pulse oximetry have been obtained from a combination of well-designed cohort studies (level 11-2 evidence)” or from earlier multiple time series (level 11-3 evidence). The results from the US Multicenter Trial (level I evidence) should provide a significant addition to current

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evidence. A continuous fetal noninvasive monitor measuring fetal oxygenation directly could lead to an improvement in the sensitivity and specificity of fetal surveillance. This improvement could ultimately result in a reduction in unnecessary interventions by differentiating hypoxic fetuses from nonhypoxic fetuses and, more importantly, may lead to earlier intervention for fetuses in danger of serious compromise.

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47. McNamara H, Chung DC, Ritsen C, et al: The development of a fetal pulse oximetry sensor and placement method for the measurement of intraparturn F-Sp02 using fetal pulse oximetry. J Perinatol 17(3):258, 1997 48. McNamara H, Johnson N, Lilford R The effect on fetal arteriolar oxygen saturation resulting from giving oxygen to the mother measured by pulse oximetry. Br J Obstet Gynaecol 100446449, 1993 49. National Institute of Child Health and Human Development Research Planning Workshop: Electronic FHR monitoring: Research guidelines for interpretation. Am J Obstet Gynecol 17713851390, 1997 50. Nickelsen C, Weber T: Continuous standard base excess monitoring during human labor. Eur J Obstet Gynecol Reprod Biol 21:7-14, 1986 51. Nickelsen C, Weber T: Fetal transcutaneous carbon dioxide monitoring-the effect of different fixation methods. Br J Obstet Gynaecol931268-1271, 1986 52. Nickelsen C, Weber T: The current status of intrapartum continuous fqtal tissue pH measurements. J Perinat Med 19237-92, 1991 53. Nickelsen C, Weber T: Normal range of fetal transcutaneous carbon dioxide tension during labor. Br J Obstet Gynaecol95:257-264, 1988 54. Nickelsen C, Thomsen SG, Weber T: Continuous acid-base assessment of the human fetus during labor by tissue pH and transcutaneous carbon dioxide monitoring. Br J Obstet Gynaecol92220-225, 1985 55. Nickelsen C, Thomsen SG, Weber T Continuous simultaneous tissue pH and transcutaneous carbon dioxide monitoring during labor. In Rolfe P (ed): Fetal Physiological Measurements. London, Buttenvorths, 1986, p 164 56. OConnor MC, Hytten FE: Measurement of fetal transcutaneous oxygen tensionproblems and potential. Br J Obstet Gynaecol 86:948-953, 1979 57. Peterson JI, Goldstein SR, Fitzgerald RV Fiberoptic pH probe for physiological use. Anal Chem 52864-869,1980 58. Pologe J, Raley D: Effects of fetal hemoglobin on pulse oximetry. J Perinatol 7324326, 1987 59. Rooth G, Fall 0, Huch A, et al: Distribution of observed patterns in fetal transcutaneous oxygen tension. Am J Obstet Gynecol 140:693-698, 1981 60. Rosen MG, Dickinson J C The paradox of electronic fetal monitoring: More data may not enable us to predict or prevent infant neurologicar morbidity. Am J Obstet Gynecol 168~745-751,1993 61. Schmidt S, Saling E: Comparative study of application techniques for the tcPCO, measurement in the fetus. Gynecol Obstet Invest 2316-22, 1987 62. Schmidt S, Saling E: The continuous measurement of transcutaneous carbon dioxide tension (TcPC02), an atraumatic tool to verlfy fetal acidosis? Br J Obstet Gynaecol 94963-966, 1979 63. Small ML, Beall M, Platt LD, et al: Continuous tissue pH monitoring in the term fetus. Am J Obstet Gynecol 161:323-329,1989 64. Smith N Assessment of fetal acid-base status. Baillieres Clin Obstet Gynaecol 1(1):97, 1987 65. Staisch K, Nuwayhid B, Bauer R, et al: Continuous fetal scalp and carotid artery oxygen tension monitoring in the sheep. Obstet Gynecol47587-592, 1976 66. Stamm 0, Latscha U, Janecek P, et al: Development of a special electrode for continuous subcutaneous pH measurement in the infant scalp. Am J Obstet Gynecol 124193-195, 1976 67. Stamm 0, Latscha U, Janecek P, et al: Kontinuerliche pH Messung am kindlichen Kopf post partum und sub partu. Z Geburtshilfe Perinatol 178:53%576, 1974 68. Sykes GS, MoIloy I'M, Wollner JC, et al: Continuous, noninvasive measurement of fetal oxygen and carbon dioxide levels in labor by use of mass spectrometry. Am J Obstet Gynecol 124193-195, 1976 69. Thomsen SG, Weber T: Fetal transcutaneous carbon dioxide tension during the second stage of labor. Br J Obstet Gynaecol 91:1103-1106, 1984 70. Tremper K, Barker S: Pulse oximetry. Anaethesiology 70:98-108, 1989 71. van den Berg PP, Dildy GA, Luttkus A, et al: The efficacy of intrapartum fetal

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Address reprint requests to Helen Mc Namara, MD, MSc Perinatal Research, F440 Women’s Pavilion Royal Victoria Hospital 687 Pine Avenue West Montreal, H3A 1Al Quebec, Canada