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Adequate fetal monitoring is essential in the optimal treatment of both expectant mother and her fetus during pregnancy and labour. In the ideal situation, the technique for fetal monitoring should be reliable, safe and without any discomfort for the mother and child. Nowadays, no fetal monitoring technique meets all requirements described above.

The currently available techniques are:

Cardiotocography (CTG): Since its introduction in the early 1970’s, CTG has been the most common used diagnostic tool during labour. Although almost all interventions are based on this form of fetal monitoring, it has a poor sensitivity and specificity for the identification of fetal distress and/or acidosis. A suspicious CTG claiming fetal distress has a very low positive predictive value for fetal hypoxia.[1,2] Besides, it has a very high intra-observer and inter-observer variability,[3] causing debate and uncertainty in delivery units. In obtaining good signals of fetal heart rate and/or uterine activity, the non-invasive traditional approach might be insufficient. Therefore, heart rate recording by a fetal scalp electrode and uterine contractions by an intra uterine pressure catheter might be required or preferred. However, these methods are causing patient and/or fetal discomfort with reported risks.[4,5] Mostly, by the absence of any technique that has a better performance in the determination of fetal distress, CTG is widely used and it provides the obstetrician or the midwife the feeling to be in charge during parturition.

Ultrasound: The usage of ultrasound in the examination and screening of fetal anatomical anomalies during pregnancy has increased enormously and has gained a very important position. However, the power to identify anomalies is strongly associated with the level of training, but even in the highest trained and well-equipped ultrasound centres anomalies are misdiagnosed or missed.[6] In later pregnancy, ultrasound is used to examine fetal wellbeing, by measuring fetal growth, Doppler velocity parameters of fetal or umbilical cord blood vessels, the amount of amniotic fluid, and fetal movement by a biophysical profile. However, its performance is still not very accurate.[7]

ST segment analysis (STAN): Since the beginning of the 21st century, STAN has been introduced in the delivery room. As changes in the ST segment in the ECG of adults reflect acute myocardial ischemia, it was expected to trace similar effects on the fetal heart ECG in case of fetal hypoxia in parturition. Although a non-significant trend in decreased rates of metabolic acidosis has been shown using STAN, no significant difference in fetal outcome or Caesarean section rate was observed as compared to CTG alone.[8] Recently, a first report on the value of relative ST events by correction for fetal heart axis restored some hope for the importance of fetal ECG in detecting fetal distress.[9]

Fetal blood sampling (FBS): Since FBS is the most direct measurement to define fetal status, its application is limited because it requires ruptured membranes, at least 2 centimeters of cervical dilatation, and expertise.[10,11] For it offers only a momentous result, and because it is traumatic, it can be used only very few times in one patient to determine the fetal condition, and therefore is only supplemental to another basic fetal monitoring principle.[12]

Therefore, in order to be able to supply the best kind of treatment to both pregnant woman and fetus an additional method for fetal monitoring is required.

Non-invasive electrophysiology: An alternative approach to obtain information about both fetus and uterus is non-invasive electrophysiology. Signals of depolarising cells of muscle tissue are passed along to surrounding cells causing depolarisation of the latter. These changes in electrical potential are conducted by surrounding tissues and can actually be recorded at the skin using electrodes. This way, the electrical activity of the fetal heart, the maternal heart and the uterine muscle can be recorded in a combined electrical measurement. As a result, information on fetal heart rate and uterine activity is obtained in a non-invasive, passive and therefore safe way. Discomfort for the pregnant woman can thus be minimalized.

Additional electrical information of contractions of uterine muscle tissue and of the fetal myocardium can be obtained and displayed. With this new technique important information can be obtained, that might contribute to better diagnosis of fetal wellbeing and fetal distress.

Electrohysterography (EHG): The uterus consists for 90 percent of muscle tissue. Contractions of these muscles can be registered on the abdominal skin. In comparison to registration of contractions by tocodynamometers, the EHG is superior.[13-17] The performance of EHG is independent of Body Mass Index, whereas the tocodynamometers perform increasingly worse at higher indices.[18-20] Besides, identification of the wave propagation of these contractions opens up the possibility to distinguish between contractions causing cervical dilatation and harmless Braxton Hicks contractions.[21]

Electrocardiography (ECG): The electrophysiological information of the fetal myocardium can be used in several ways. First, it can be used to reliable present the fetal heart rate.[22] In combination with the EHG it can therefore easily replace the regular CTG. Second, whereas the ECG is registered directly from a single myocardial contraction, the variability of the heart rate is available beat-to-beat,[23,24] in contrast to the information on fetal heart rate obtained by Doppler ultrasound, that is subject to averaging of several heart beats. Fetal heart rate variability can be used to monitor autonomic nervous system modulation and may provide an early diagnostic tool for detection of fetal distress.[25,26] Third, as the electrophysiological signals are obtained by multiple electrodes, a correction for the fetal heart axis can be made,[27] providing the presentation of the well-known 12-leads Einthoven ECG. With these ECG signals, it will be able to more accurately diagnose fetal heart anomalies and/or arrhythmias.[28]

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References:
[1] Alfirevic A, Devane D, Gyte GML. Continuous cardiotocography (CTG) as a form of electronic fetal monitoring (EFM) for fetal assessment during labour. Cochrane Database Syst Rev 2006;3(3):CD006066.
[2] Sultana J, Chowdhury TA, Begum K, Khan MH. Comparison of normal and abnormal cardiotocography with pregnancy outcomes and early neonatal outcomes. Mymensingh Med J. 2009;18(1 Suppl):S103-7.
[3] Schiermeier S, Westhof G, Leven A, Hatzmann H, Reinhard J. Intra- and interobserver variability of intrapartum cardiotocography: a multicenter study comparing the FIGO classification with computer analysis software. Gynecol Obstet Invest. 2011;72(3):169-73.
[4] S. L. Bloom, C. Y. Spong, E. Thom et al. Fetal pulse oximetry and cesarean delivery. The New England Journal of Medicine. 2006:355(21);2195–2202.
[5] Harper LM, Shanks AL, Tuuli MG, Roehl KA, Cahill AG. The risks and benefits of internal monitors in laboring patients. Am J Obstet Gynecol 2013;209(1):38.
[6] Rydberg C, Tunón K. Detection of fetal abnormalities by second-trimester ultrasound screening in a non-selected population. Acta Obstet Gynecol Scand. 2017 Feb;96(2):176-182.
[7] Miranda J, Rodriguez-Lopez M, Triunfo S, Sairanen M, Kouru H, Parra-Saavedra M, Crovetto F, Figueras F, Crispi F, Gratacos E. Prediction of fetal growth restriction using estimated fetal weight versus a combined screening model at 32-36 weeks of gestation. Ultrasound Obstet Gynecol. 2016 Dec 22.
[8] Blix E, Brurberg KG, Reierth E, Reinar LM, Øian P. ST waveform analysis versus cardiotocography alone for intrapartum fetal monitoring: a systematic review and meta-analysis of randomized trials. Acta Obstet Gynecol Scand [internet]. 2016;95(1):16-27.
[9] Vullings R, Verdurmen KMJ, Hulsenboom ADJ, Scheffer S, de Lau H, Kwee A, Wijn PFF, Amer-Wåhlin I, van Laar JOEH, Oei SG. The electrical heart axis and ST events in fetal monitoring: A post-hoc analysis following a multicentre randomised controlled trial. PLoS One. 2017;12(4):e0175823.
[10] Van den Berg P, Schmidt S, Geche J, Saling E. Fetal distress and the condition of the newborn using cardiotocography and fetal blood analysis during labour. Br J Ostet Gynaecol. 1987;94(1):72-5.
[11] Tsikouras P, Koukouli Z, Niesigk B, Manav B, Farmakides G, Csorba R, Galazios G, Teichmann AT. Predictive value of fetal scalp pH and base excess for fetal acidosis and poor neonatal outcome. J Matern Fetal Neonatal Med. 2017 Aug 17:1-6.
[12] Westgate J, green K. How well is fetal blood sampling used in clinical practice? Br J Obstet Gynaecol. 1994;101(3):250-1.
[13] Vlemminx MW, de Lau H, Vullings R, Peters CH, Oei SG. Electrohysterography. a promising alternative for monitoring contractions. Ned Tijdschr Geneeskd. 2015;159:A8535.
[14] Euliano TY, Nguyen MT, Darmanjian S, McGorray SP, Euliano N, Onkala A, et al. Monitoring uterine activity during labor: a comparison of 3 methods. Am J Obstet Gynecol. 2013;208:66 e1-6.
[15] Euliano TY, Nguyen MT, Darmanjian S, Busowski JD, Euliano N, Gregg AR. Monitoring Uterine Activity during Labor: Clinician Interpretation of Electrohysterography versus Intrauterine Pressure Catheter and Tocodynamometry. Am J Perinatol 2016;33(9):831-8.
[16] Hadar E, Biron-Shental T, Gavish O, Raban O, Yogev Y. A comparison between electrical uterine monitor, tocodynamometer and intra uterine pressure catheter for uterine activity in labor. J Matern Fetal Neonatal Med. 2014;10:1-8.
[17] Jacod BC, Graatsma EM, Van Hagen E, Visser GH. A validation of electrohysterography for uterine activity monitoring during labour. J Matern Fetal Neonatal Med. 2010;23(1):17-22.
[18] Euliano TY, Nguyen MT, Marossero D, Edwards RK. Monitoring contractions in obese parturients: electrohysterography compared with traditional monitoring. Obstet Gynecol. 2007;109(5):1136-40.
[19] Cohen WR, Hayes-Gill B. Influence of maternal body mass index on accuracy and reliability of external fetal monitoring techniques. Acta Obstet Gynecol Scand. 2014;93(6):590-5.
[20] Graatsma EM, Miller J, Mulder EJ, Harman C, Baschat AA, Visser GH. Maternal Body Mass Index Does Not Affect Performance of Fetal Electrocardiography. Am J Perinatol. 2010;27(7):573-7.
[21] Mischi M, Rabotti C, Vosters LJ, Oei SG, Bergmans JM. Electrohysterographic conduction velocity estimation. Conf Proc IEEE Eng Med Biol Soc. 2009;2009:6934-7.
[22] Sänger N, Hayes-Gill BR, Schiermeier S, Hatzmann W, Yuan J, Herrmann E, Louwen F, Reinhard J. Prenatal Foetal Non-invasive ECG instead of Doppler CTG – A Better Alternative? Geburtsh Frauenheilk 2012;72:630–633.
[23] Peters CH, van Laar JO, Vullings R, Oei SG, Wijn PF. Beat-to-beat heart rate detection in multi-lead abdominal fetal ECG recordings. Med Eng Phys. 2012;34(3):333-8.
[24] Van Laar JO, Warmerdam GJ, Verdurmen KM, Vullings R, Peters CH, Houterman S, Wijn PF, Andriessen P, Van Pul C, Guid Oei S. Fetal heart rate variability during pregnancy, obtained from non-invasive electrocardiogram recordings. Acta Obstet Gynecol Scand. 2014;93(1):93-101.
[25] Siira SM, Ojala TH, Vahlberg TJ, Jalonen JO, Välimäki IA, Rosen KG, Ekholm EM. Marked fetal acidosis and specific changes in power spectrum analysis of fetal heart rate variability recorded during the last hour of labour. BJOG 2005;112:418-23.
[26] Warmerdam GJ, Vullings R, Van Laar JO, Van der Hout-Van der Jagt MB, Bergmans JW, Schmitt L, Oei SG. Selective heart rate variability analysis to account for uterine activity during labor and improve classification of fetal distress. Conf Proc IEEE Eng Med Biol Soc. 2016;2016:2950-3.
[27] Verdurmen KM, Hulsenboom AD, van Laar JO, Wijn PF, Vullings R, Oei SG. Orientation of the electrical heart axis in mid-term pregnancy. Eur J Obstet Gynecol Reprod Biol. 2016 Dec;207:243-6.
[28] Verdurmen KM, Eijsvoogel NB, Lempersz C, Vullings R, Schroer C, van Laar JO, Oei SG. A systematic review of prenatal screening for congenital heart disease by fetal electrocardiography. Int J Gynaecol Obstet. 2016;135(2):129-34.