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Case report| Volume 198, ISSUE 6, P717-724, June 2008

Signal ambiguity resulting in unexpected outcome with external fetal heart rate monitoring

Published:March 31, 2008DOI:https://doi.org/10.1016/j.ajog.2008.02.030
Signal ambiguity resulting in unexpected outcome with external fetal heart rate monitoring
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      We report cases of unexpected adverse fetal outcome from monitored labors in which the fetal heart rate tracing was interpreted as reassuring. In these cases, portions from another signal source, usually maternal, were imperceptibly substituted into the fetal tracing in a way that masked the evidence of fetal compromise.

      Key words

      Electronic fetal monitors have evolved greatly in the last several decades, allowing accurate recording of the fetal heart rate (FHR) with an external Doppler transducer with much less signal loss than was present in the earlier products. This improved sensitivity has resulted in decreased use of the internal fetal scalp electrode and greater reliance on the external transducers.
      For Editors' Commentary, see Table of Contents
      In this article, we report a confusing situation which may occur during external fetal monitoring in which the fetal signal is replaced by an alternative signal from the mother or a second fetus without the usual recognizable transition associated with such signal source shifting. This masks the condition of the fetus without the attending staff being alerted to the loss of fetal signal. In approximately 10,000 deliveries, we have encountered 5 examples of unexpected adverse fetal outcome attributable to this signal ambiguity. We have also seen several cases without adverse outcome.
      This report consists of 2 cases illustrating the clinical problem, 2 cases illustrating the probable mechanism, and a table summarizing our adverse neonatal outcomes attributable to this process.
      Figure 1 illustrates the monitor tracing immediately prior to delivery of a stillborn infant. This neonate had no response to resuscitation efforts, indicating death at least some minutes before the actual delivery, implying that the heart rate tracing in the minutes before birth is entirely maternal, although the staff had interpreted it as fetal. This tracing demonstrates regular persistent heart rate accelerations during second-stage bearing-down efforts. FIGURE 2, FIGURE 3, FIGURE 4, FIGURE 5 show representative tracings from the earlier parts of the labor demonstrating the expected FHR reactivity of a healthy fetus at the start and typical responses to changing conditions during the labor. The transition to the pure maternal tracing at the end of labor (Figure 1) is not evident at any point in the record, although Figure 5 demonstrates a possible point, evidenced by the shifting baseline. The mother was tachycardic during the last several hours of labor, ultimately with a rate between contractions of 120 beats per minute.
      Figure thumbnail gr1
      FIGURE 1Tracing immediately prior to delivery of stillborn infant
      Neilson. Signal ambiguity and unexpected outcome with external fetal heart rate monitoring. Am J Obstet Gynecol 2008.
      Figure thumbnail gr2
      FIGURE 2Early labor, reassuring FHR tracing
      Neilson. Signal ambiguity and unexpected outcome with external fetal heart rate monitoring. Am J Obstet Gynecol 2008.
      Figure thumbnail gr3
      FIGURE 3Diminished variability and late decelerations
      Neilson. Signal ambiguity and unexpected outcome with external fetal heart rate monitoring. Am J Obstet Gynecol 2008.
      Figure thumbnail gr4
      FIGURE 4Improved variability 45 minutes later
      Neilson. Signal ambiguity and unexpected outcome with external fetal heart rate monitoring. Am J Obstet Gynecol 2008.
      Figure thumbnail gr5
      FIGURE 5Shifting baseline with accelerations during pushing
      Neilson. Signal ambiguity and unexpected outcome with external fetal heart rate monitoring. Am J Obstet Gynecol 2008.
      FIGURE 6, FIGURE 7, FIGURE 8, FIGURE 9, FIGURE 10, FIGURE 11 illustrate a case in which the shift was detected and steps were taken that resulted in a normal fetal outcome. The tracing is from a healthy multipara laboring spontaneously at term. Examples of the tracing are provided for early labor (Figure 6) and late first stage (Figure 7).
      Figure thumbnail gr6
      FIGURE 6Active labor, reassuring FHR tracing (11:30 am)
      Neilson. Signal ambiguity and unexpected outcome with external fetal heart rate monitoring. Am J Obstet Gynecol 2008.
      Figure thumbnail gr7
      FIGURE 7Continued reassuring FHR tracing (12:30 pm)
      Neilson. Signal ambiguity and unexpected outcome with external fetal heart rate monitoring. Am J Obstet Gynecol 2008.
      Figure thumbnail gr8
      FIGURE 8Start of second stage
      Beginning acceleration pattern with pushing and suggestion of discontinuity of tracing (arrows) are shown (2:45 pm).
      Neilson. Signal ambiguity and unexpected outcome with external fetal heart rate monitoring. Am J Obstet Gynecol 2008.
      Figure thumbnail gr9
      FIGURE 9Continued acceleration pattern (3:05 pm)
      Neilson. Signal ambiguity and unexpected outcome with external fetal heart rate monitoring. Am J Obstet Gynecol 2008.
      Figure thumbnail gr10
      FIGURE 10Fetal scalp electrode applied midway through the second contraction on this record
      This reveals underlying deep deceleration pattern previously masked (3:15 pm).
      Neilson. Signal ambiguity and unexpected outcome with external fetal heart rate monitoring. Am J Obstet Gynecol 2008.
      Figure thumbnail gr11
      FIGURE 11Magnified portion of paper tracing at time of scalp electrode placement (arrow)
      This shows the pulse oximetry tracing superimposed on the fetal monitor tracing. The maternal pulse rate from the pulse oximeter shows as a nearly straight line superimposed on the fetal heart rate tracing.
      Neilson. Signal ambiguity and unexpected outcome with external fetal heart rate monitoring. Am J Obstet Gynecol 2008.
      In the second stage, a pattern of repetitive heart rate accelerations began to appear (Figure 8) with some brief discontinuity (data loss) at the start of the accelerations, and this pattern became more obvious and sustained shortly thereafter (Figure 9).
      Because of maternal tachycardia, a pulse oximeter was applied and its tracing of the maternal heart rate superimposed precisely on the “fetal” tracing, suggesting that the “fetal” record was actually maternal. A scalp electrode was then placed (Figure 10) and demonstrated that the fetus was experiencing deep variable decelerations where the monitor had previously recorded accelerations. Figure 11 shows a magnified part of the paper FHR tracing showing the maternal pulse oximetry record superimposed on the fetal monitor tracing. The pulse oximetry record revealing the maternal pulse rate does not appear on the electronically stored version of the tracing shown in Figure 10. The abnormal fetal pattern persisted, prompting a cesarean section delivery, resulting in a healthy newborn.
      Expanded use of maternal pulse oximetry has provided additional insight into this phenomenon, as shown in the next 2 cases (FIGURE 12, FIGURE 13, FIGURE 14 and FIGURE 15, FIGURE 16, FIGURE 17). These illustrate what may be the transition event in patients in which the fetus was experiencing variable decelerations that later became replaced by accelerations of probable maternal origin, based on the coincidence with the pulse oximetry tracing. Note that the tracing may shift repeatedly between maternal and fetal signal sources in a way that keeps the final readout in or near the expected physiologic range for a healthy fetus.
      Figure thumbnail gr12
      FIGURE 12Case showing variable decelerations just prior to second stage
      Neilson. Signal ambiguity and unexpected outcome with external fetal heart rate monitoring. Am J Obstet Gynecol 2008.
      Figure thumbnail gr13
      FIGURE 13Variable decelerations were replaced by confusing pattern with some data loss
      Same case as in .
      Neilson. Signal ambiguity and unexpected outcome with external fetal heart rate monitoring. Am J Obstet Gynecol. 2008.
      Figure thumbnail gr14
      FIGURE 14This shows replacement of prior variable deceleration pattern with apparent maternal pattern during pushing
      Same case with pulse oximeter added.
      Neilson. Signal ambiguity and unexpected outcome with external fetal heart rate monitoring. Am J Obstet Gynecol 2008.
      Figure thumbnail gr15
      FIGURE 15Another case showing variable deceleration pattern prior to second stage with some data loss
      Maternal pulse oximeter was placed.
      Neilson. Signal ambiguity and unexpected outcome with external fetal heart rate monitoring. Am J Obstet Gynecol 2008.
      Figure thumbnail gr16
      FIGURE 16Pulse oximetry tracing shows apparent transition to maternal pattern during pushing
      Same case as above approximately 45 minutes later.
      Neilson. Signal ambiguity and unexpected outcome with external fetal heart rate monitoring. Am J Obstet Gynecol 2008.
      Figure thumbnail gr17
      FIGURE 17Continuation of same case showing more complete transition to maternal pulse with some data loss
      Neilson. Signal ambiguity and unexpected outcome with external fetal heart rate monitoring. Am J Obstet Gynecol 2008.
      The display of maternal heart rate on the fetal tracing is relatively common
      • Nageotte M.P.
      Avoiding 5 common mistakes in FHR monitoring.
      Contemp Ob Gyn. 2007; 52: 50-55
      and normally presents no problem because it is easily recognizable due to the very different rate and the usual loss of data preceding the transition. However, in these and the other cases we have encountered, transition from the fetal to the maternal signal occurred imperceptibly, in some cases probably several times with minimal to no data interruption. Experienced obstetricians reviewing these cases have been unable to determine the transition points. Therefore, the staff is not alerted to the possibility of misleading data, and the potential for unrecognized fetal distress occurs.
      In singleton pregnancies, the most characteristic indication of this problem is the pattern of repeated heart rate accelerations during second stage (Table). Sherman et al
      • Sherman D.J.
      • Frenkel E.
      • Kurzweil Y.
      • Padua A.
      • Arieli S.
      • Bahar M.
      Characteristics of maternal heart rate patterns during labor and delivery.
      Obstet Gynecol. 2002; 99: 542-547
      have published an analysis of maternal and fetal heart rate patterns in labor and report that mothers typically show accelerations in heart rate while pushing and do not show decelerations. In contrast, fetuses commonly show decelerations with pushing, and almost never show repeated accelerations. Thus, the pattern of repeated accelerations demonstrated by our cases will usually be maternal.
      TABLECase summaries for unexpected adverse outcomes
      CaseClinical settingEFM findingsOutcome
      1Term twins Induction of laborAlternated between fetuses during late first stage, remaining in normal range until bradycardia recognized in 1 fetusEmergency cesarean section. One normal newborn and 1 severely acidotic fetus with neonatal death
      2Postdate induction (case presented in text)Shifting baseline in early second stage with appearance of fetal heart rate accelerations during pushingStillborn, no response to resuscitation
      3Term, normal pregnancy with spontaneous laborFetal heart rate accelerations during contractions appeared during long second stage
      • Spontaneous delivery
      • Apgar scores 2, 6, 6, 7
      • pH 6.9
      • Did well in neonatal intensive care unit
      4Term, elective induction laborGradual transition from fetal heart rate decelerations in second stage to fetal heart rate accelerations
      • Spontaneous delivery
      • Apgar scores 2, 6, 7
      • Did well in neonatal intensive care unit
      5Term, spontaneous labor, planning VBACEarly second-stage bradycardia transitioned to accelerations during contractions
      • Spontaneous delivery
      • Apgar scores 2, 6, 8
      • pH 7.1
      • Cerebral edema with signs of CNS injury
      CNS, central nervous system; EFM, electronic fetal monitor; VBAC, vaginal birth after cesarean.
      Neilson. Signal ambiguity and unexpected outcome with external fetal heart rate monitoring. Am J Obstet Gynecol 2008.
      The conditions under which this phenomenon occurs are maternal tachycardia with excursions into the fetal range and simultaneous fetal bradycardia or tachycardia outside the normal fetal range. It appears that when the maternal rate meets typical fetal characteristics and the fetal rate is not typical, the switch may occur. This could be a consequence of the evolution of fetal monitoring transducers, which are now highly sensitive and capable of picking up signals from both mother and fetus simultaneously. Internal discrimination circuitry (logic) determines which signal is displayed as fetal, and when the mother looks more like a fetus than the fetus, the logic may cause a switch in the display.
      Sensitivity to these conditions and recognition of the characteristic pattern of recurring heart rate accelerations during second-stage bearing-down efforts are the key factors that should lead to suspicion of this situation, and fetal scalp electrode placement will resolve the question. In twin (or higher-order) pregnancies, the monitor has not only the maternal rate to discriminate but also signals from another fetus. Thus, we believe that fetal scalp electrode placement on the leading twin should be considered to minimize this potential confusion.
      Maternal pulse oximetry, especially in second stage, should help eliminate the risk of this confusion, but the pulse rate printout from the pulse oximeter is affected by maternal movement and other factors, limiting its reliability for this purpose. A more precise way to differentiate the recording of maternal heart rate (MHR) from FHR would be to compare the MHR obtained from maternal electrocardiogram (ECG), which is available on some monitors. Ideally, a modification of current monitors would be to obtain the maternal ECG from electrodes mounted on the Doppler and toco transducers. Ongoing comparison could be made electronically as is available on some twin monitors, which alarm when the same twin is being monitored twice. The maternal and fetal signals could then be compared using the same process as is on the twin monitors, and appropriate alarms could indicate when the MHR is being recorded on the fetal tracing.
      This problem is not confined to a single manufacturer or model and can be expected from any of the fetal monitors currently being used in hospital settings. We have discussed this with manufacturers and have notified the Food and Drug Administration of this problem. However, until a better technical solution appears, the scalp electrode is the most accurate way to assess the fetal status.

      References

        • Nageotte M.P.
        Avoiding 5 common mistakes in FHR monitoring.
        Contemp Ob Gyn. 2007; 52: 50-55
        View in Article
        • Sherman D.J.
        • Frenkel E.
        • Kurzweil Y.
        • Padua A.
        • Arieli S.
        • Bahar M.
        Characteristics of maternal heart rate patterns during labor and delivery.
        Obstet Gynecol. 2002; 99: 542-547
        View in Article

      Article info

      Publication history

      Published online: March 31, 2008
      Accepted: February 14, 2008
      Received in revised form: January 29, 2008
      Received: September 25, 2007

      Identification

      DOI: https://doi.org/10.1016/j.ajog.2008.02.030

      Copyright

      © 2008 Mosby, Inc. Published by Elsevier Inc. All rights reserved.

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