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Is intrapartum fetal blood sampling a gold standard diagnostic tool for fetal distress?
European Journal of Obstetrics & Gynecology and Reproductive Biology 156 (2011) 137–139
Contents lists available at ScienceDirect
European Journal of Obstetrics & Gynecology and Reproductive Biology journal homepage: www.elsevier.com/locate/ejogrb
Expert opinion
Is intrapartum fetal blood sampling a gold standard diagnostic tool for fetal distress? Amita A. Mahendru a, Christoph C. Lees b,* a
Clinical Research Fellow in Fetal Medicine, Rosie Maternity-Addenbrookes Hospital, Cambridge University Hospitals NHS Foundation Trust, Hills Road, Cambridge CB2 2QQ, UK Consultant in Obstetrics and Fetal-Maternal Medicine, Rosie Maternity-Addenbrookes Hospital, Cambridge University Hospitals NHS Foundation Trust, Hills Road, Cambridge CB2 2QQ, UK b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 December 2010 Received in revised form 16 December 2010 Accepted 23 December 2010
Developed in 1960s, cardiotocography is a screening test and fetal blood sampling (FBS) is an adjunctive, diagnostic technique to detect fetal hypoxia. A fetal blood sample pH value of less than 7.20 has a higher specificity than a pathological CTG to predict low Apgar score at 1 min. Though with a pathological CTG and despite a normal FBS pH value the risk of delivering a hypoxic infant is 30–50%, FBS has assumed considerable importance in purportedly reducing unnecessary obstetric intervention. The evidence for this is weak: the use of FBS with CTG has been shown to reduce operative vaginal deliveries though not Caesarean sections due to fetal distress. There is no difference in the umbilical artery pH at delivery with the use of intermittent FBS with CTG compared to CTG alone. FBS is an invasive procedure: obtaining an adequate blood sample is often difficult and the pH results are affected by handling of the sample, aerobic contamination and processing. Validation of intrapartum FBS requires that the pH and other values obtained are compared to a ‘gold standard’ technique. Although FBS has been compared to other tests such as scalp lactate, pulse oximetry, fetal ECG waveform analysis, and central haemodynamics in labouring rhesus monkeys, none of these can be considered as ‘gold standard’. In the light of the existing evidence, the role of intrapartum FBS as a gold standard diagnostic technique is unproven. ß 2011 Elsevier Ireland Ltd. All rights reserved.
Keywords: Fetal blood sampling Cardiotocography Hypoxia Validation Intrapartum Diagnostic technique
Contents 1. 2. 3.
Introduction . The evidence Conclusion . . References . .
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1. Introduction The aim of fetal monitoring in labour is to detect hypoxia, which can lead to acidaemia and adverse sequelae such as hypoxic– ischaemic encephalopathy and fetal death. A test used to detect hypoxia/acidaemia should have a high sensitivity so that a timely intervention can prevent any adverse outcome while at the same
* Corresponding author at: Department of Fetal Medicine, Cambridge University Hospitals NHS Foundation Trust, Cambridge CB2 2QQ, UK. Tel.: +44 01223 217972; fax: +44 01223 216185. E-mail address: [email protected] (C.C. Lees). 0301-2115/$ – see front matter ß 2011 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.ejogrb.2010.12.044
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time having a low false positive rate to prevent unnecessary intervention. The cardiotocograph (CTG) and fetal scalp blood pH assessment were both developed in the 1960s. Though the CTG has high sensitivity for intra-partum hypoxia/acidaemia [1–8], it has a high false positive rate and interpretation of CTG traces has well documented inter- and intra-observer variation [9]. The CTG is therefore useful as a screening though not diagnostic test for fetal hypoxia. Fetal blood sampling (FBS) is widely used as an adjunctive diagnostic test to quantify fetal hypoxia/acidaemia when indicated by an abnormal intrapartum CTG. The sensitivity of CTG in predicting FBS-pH of less than 7.20 is 0.50–0.90 and the specificity is 0.40–0.90 [1]. The use of FBS varies widely internationally: in the UK 40% obstetric units used FBS in 1979 [10]; in Germany fetal
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A.A. Mahendru, C.C. Lees / European Journal of Obstetrics & Gynecology and Reproductive Biology 156 (2011) 137–139
blood sampling is performed in 26% of deliveries with pathological fetal heart rate trace [11] and its use varies from 1.43% (1986) to 0.03% (1992) of deliveries in California [12]. In an effort to reduce unnecessary intervention, particularly Caesarean section, FBS has assumed considerable importance as a tool to ‘diagnose’ fetal distress, based primarily on the pH following studies in the 1960s and 1970s [1,2,11,13]. The UK’s National Institute for Clinical Excellence (NICE) Intrapartum Care Guidelines recommend the use of FBS in the presence of a pathological fetal heart rate trace unless there is clear evidence of acute compromise, and in cases of suspected fetal acidosis prior to contemplating an assisted birth for an abnormal FHR trace [14]. Further, NICE guidance does not support use of FBS due to limited research evidence [4], but then confusingly suggests from ‘clinical experience’ that it may in fact avoid some instrumental births and Caesarean sections. A prospective cohort study quoted by NICE compared continuous tissue pH measurements from a glass electrode applied to the fetal scalp in 72 patients to 72 historic controls where only CTG was performed [3]. This technique is not analogous to FBS measurement, as it does not require blood sampling and gives a continuous pH assessment. A reduction in instrumental deliveries, but not Caesarean section, was noted and the author stated that the technique required ‘further development and clinical evaluation’ which to our knowledge has not happened in the intervening three decades [3]. FBS is an invasive procedure, is cumbersome, uncomfortable for both the woman and the obstetrician [4,15] and an adequate sample is obtained successfully in 79.4% of attempted cases [5]. Its results are affected by the sampling technique, handling of the sample and processing these results using a blood gas analyser machine [16]. Even under well-controlled experimental conditions, FBS can result in inaccurate results from aerobic contamination [17]. Though the use of FBS with CTG reduced the number of Caesarean sections, instrumental deliveries and episiotomies compared to CTG alone, it is noteworthy that the reduction in the number of Caesarean sections due to fetal distress was not statistically significantly different [2,4]. There was no statistically significant difference in the umbilical artery pH after birth with the use of FBS in a randomized controlled trial of 695 labouring women in 1970s [2], nor in a prospective cohort study of continuous tissue pH monitoring in 96 labouring women in 1979–1980 [3,4]. Although the risk of operative deliveries of infants who are not hypoxic is diminished by the use of FBS on patients with a pathological CTG, the risk of delivering a hypoxic infant despite normal FBS (>7.20) is 30–50% and with a normal CTG 20–50% [1]. This study cautioned the use of FBS due to its inability to identify a hypoxic infant. Over the last three decades several commentators have questioned the usefulness of intrapartum fetal blood sampling [12,18,19]. There is in fact no evidence to show that FBS reduces the Caesarean section rate for fetal distress, or improves fetal outcome. 2. The evidence It is instructive in this context to briefly consider the science and evidence underlying the use of FBS. Nearly a century ago it was first reported that by adult standards the cord blood of the fetus was acidotic [20]. Postpartum cord blood analysis remained the only method to investigate the intrapartum fetal state and acid– base status for the subsequent 40 years. In 1963, Dr. Erich Saling in Berlin introduced the technique of FBS to modern obstetric practice [21]. Bretscher and Saling devised the technique of obtaining capillary blood from the presenting fetal part so that the pH, oxygen and carbondioxide (CO2) content of the blood could be determined in utero. This technique was reported to reduce the
perinatal mortality rate and operative delivery rate in various studies carried out in England and Scotland in the 1960s and 1970s but the contribution of FBS was uncertain [22,23]. The acid–base balance of the fetus is determined by Astrup micro method [24] where the actual pH of the capillary sample is initially measured, followed by pCO2, base excess or deficit and standard bicarbonate by the normogram of Siggaard Andersen and Engel [25]. Studies of umbilical venous and arterial blood at birth have shown that the pH corresponds more closely with the clinical condition of the infant than pO2 or the pCO2 [26]. During fetal hypoxia, pCO2 rises and pH falls with a build-up of H+ ions causing metabolic acidosis secondary to anaerobic metabolism, as fetal lungs and kidneys cannot eliminate the excess CO2. Nonvolatile bases or acids produced by anaerobic metabolism increase the base excess. The pH and base excess are reliable as they reflect the fetal dependence on anaerobic metabolism due to lack of respiratory exchange in utero. In cases of maternal acidosis due to prolonged labour, equilibration of fixed acids across the placenta over a period of time [27] can lead to lowering of the fetal capillary pH. The results from fetal blood scalp sampling are assumed to reflect the synchronous fetal central pH and acid–base status. This has been investigated in only one study comparing simultaneous pH and acid–base values from fetal scalp sampling samples and chronic indwelling carotid and jugular catheters in rhesus monkey fetuses, which reported a positive correlation between pH and acid–base values of intrapartum fetal scalp blood samples compared to arterial and venous blood [28]. In exteriorised fetal lamb preparations using compression of the fetal skull, umbilical vein and/or artery to stress the fetus, blood taken from the fetal scalp closely approximated the fetal arterial blood values of pH, pO2 and pCO2 [29]. The metabolic effects of stressing the fetus in true labour, however, were not simulated. Though separated in time, samples taken just before delivery when compared to umbilical artery and venous blood taken at delivery showed significant correlation with the parameters of acid–base balance [30,31] and with pH, glucose, lactate and pyruvate and values in cord blood both at vaginal delivery and Caesarean section [32]. Evidence on the association between cord pH and adverse neonatal outcome is conflicting [33] and the predictive value of fetal scalp pH on neonatal outcome remains unproven. Differences in the technique, the analyser and the maternal– fetal interaction can also affect the results of the fetal capillary sample [34]. The common source of error is allowing exposure to air during sampling, resulting in an increased pO2, decreased pCO2 and artificially elevated pH. Under clinical circumstances, the error is inevitably greater than in strictly controlled experimental conditions. Insufficient hyperaemia of scalp can result in a decrease of oxygen tension. Measurement errors can also occur with incorrect machine calibration. During labour, various maternal and fetal interactions can affect the fetal capillary blood results [34,35]. Maternal low oxygenation secondary to anaemia, hypoxemia, general uteroplacental ischemia, supine hypotension, maternal alkalosis during labour or maternal lactic academia can alter fetal pH and can only be differentiated if maternal pH is measured simultaneously with fetal pH [35] Formation of caput succedaneum in the fetus can lead to dilution of blood by tissue fluid and local scalp stasis [27]. Stasis of scalp circulation can lead to significant drop in the local pH and falsely low pH results in cases of prolonged head compression of the head against a rigid cervix or perineum. In cases of established fetal hypoxaemia, the fetal peripheral circulation is decreased and the acid/base status of scalp blood will not necessarily represent cerebral blood. Apart from hypoxia and acidosis in labour, the fetus may be adversely affected by infection, drugs administered to
A.A. Mahendru, C.C. Lees / European Journal of Obstetrics & Gynecology and Reproductive Biology 156 (2011) 137–139
mother, haemorrhage, and intrapartum trauma, all of which cannot be predicted by FBS [15]. Validation of FBS in labour requires that the pH, BE, pO2 and pCO2 values obtained are compared to those obtained by a ‘gold standard’ technique. In fact, this widely performed technique, being itself assumed to be a ‘gold standard’, has never been validated in humans. The closest that FBS has come to validation is the comparison of central arterial and venous acid/base status in eleven labouring rhesus monkeys in 1968. Most studies have compared the correlation between scalp and other – almost exclusively post delivery – blood samples. When comparing two different techniques for measurement, this is not an appropriate statistical method as two methods may be closely correlated but give quite different absolute values. It would be practically difficult and quite possibly ethically unacceptable to conduct such studies in humans; hence FBS may never be subject to true validation. Despite many false dawns, techniques to reduce unnecessary interventions and improve neonatal outcomes such as fetal scalp lactate [5], fetal pulse oximetry [6] and fetal ECG [7] in labour still remain unproven. In a retrospective study of deliveries from a Swedish maternity unit in which fetal scalp blood sampling was undertaken due to ominous CTG trace, the predictive value of both pH and lactate for hypoxic ischemic encephalopathy were poor, but better for lactate (which requires one seventh of the amount of blood and is hence easier to obtain). The authors concluded that ‘‘The evaluation of the predictive values of a diagnostic test, with the use of any measure of morbidity and mortality is complicated when the procedure being assessed affects the outcome’’ implying that difficulty in obtaining a sample, time taken and inaccuracies ascribed to it could have a negative effect on pregnancy outcome [36]. Notwithstanding the absence of evidence for its validation, multiple potential sources for error with FBS exist whose impact on the results have been poorly, if at all, defined. The average time taken to obtain the results from the decision time to perform fetal blood sampling has been reported to be about 18 min [37], which could delay delivery, leading to a greater risk of fetal hypoxia and its sequelae. 3. Conclusion Fetal blood sampling cannot be considered as a ‘gold standard’ test for fetal hypoxia/acidaemia. Relying entirely on an FBS result to qualify an abnormal CTG does not reduce Caesarean Section for fetal distress. Further, using FBS with CTG instead of CTG alone shows the same or indeed possibly a higher risk of babies being misclassified as not hypoxic. The American College of Obstetricians and Gynecologists Guidance on Intrapartum Fetal Heart rate Monitoring does not recommend use of fetal blood sampling as an adjunct to CTG: ‘‘Scalp stimulation which is less invasive, provides similar information about the likelihood of fetal acidaemia as does scalp pH’’ [38]. The role of fetal scalp lactate and fetal ECG remains unproven. In this context, in the absence of any large robust intrapartum studies of FBS as an adjunct to CTG, we should question its place as a recommended ‘diagnostic’ adjunct to the CTG. References [1] Weber T. The validity of discontinuous pH measurements on fetal blood and of cardiotocography in predicting neonatal Apgar score. Dan Med Bull 1979;26(4):186–91. [2] Haverkamp AD, Orleans M, Langendoerfer S, McFee J, Murphy J, Thompson HE. A controlled trial of the differential effect of intrapartum fetal monitoring. Am J Obstet Gynecol 1979;134:399–409. [3] Weber T. CTG supplemented with continuous fetal pH monitoring during labour. Acta Obstet Gynecol Scand 1982;61:351–5.
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[4] Alfirevic Z, Devane D, Gyte GML. Continuous CTG as a form of electronic fetal monitoring for fetal assessment during labour. Cochrane Database Syst Rev 2008;4. [5] East CE, Leader LR, Sheehan P, Henshall NE, Colditz PB. Intrapartum fetal scalp lactate sampling for fetal assessment in the presence of a non-reassuring fetal heart rate trace. Cochrane Database Syst Rev 2010;3. [6] East CE, Chan FY, Colditz PB, Begg LM. Fetal pulse oximetry for fetal assessment in labour. Cochrane Database Syst Rev 2007;2. [7] Neilson JP. Fetal ECG for fetal monitoring during labour. Cochrane Database Syst Rev 2006;3. [8] Nelson KB, Dambrosia JM, Ting TY, Grether JK. Uncertain value of electronic fetal monitoring in predicting cerebral palsy. N Engl J Med 1996;334(10): 613–8. [9] Devane D, Lalor J. Midwives’ visual interpretation of intrapartum cardiotocograph intra- and inter-observer agreement. J Adv Nurs 2005;52(2):133–41. [10] Gillmer MDG, Combe D. Intrapartum fetal monitoring practice in the UK. BJOG 1979;86:753–60. [11] Werner Stein, Hellmeyer L, Misselwitz B, Schmidt S. Impact of fetal blood sampling on vaginal delivery and neonatal outcome in deliveries complicated by pathologic fetal heart rate: a population based cohort study. J Perinat Med 2006;34:479–83. [12] Goodwin TM, Milner-Masterson L, Paul R. Elimination of fetal scalp blood sampling on a large clinical service. Obstet Gynecol 1994;83(6):971–3. [13] Zalar RW, Quilligan EJ. The influence of scalp sampling on the Caesarean section rate for fetal distress. Am J Obstet Gynecol 1979;135:239–46. [14] NICE guideline. Intrapartum care, care of healthy women and their babies during childbirth; 2007. p. 225–92 [15] Steer PJ. Is fetal blood sampling and pH estimation helpful or harmful? Arch Dis Child 1987;62:1097–8. [16] Sherman DJ, Arieli S, Raziel A, Bukovsky I. The effect of sampling technique on measurement of fetal blood pH and gases – an in vitro system. Am J Obstet Gynecol 1994;171(4):1125–8. [17] Morgan BLG, Chao CR, Iyer V, Ross MG. Correlation between fetal scalp blood samples and intravascular blood pH, pO2 and oxygen saturation measurements. J Matern Fetal Neonatal Med 2002;11:325–8. [18] Perkins RP. Perinatal observations in a high-risk population managed without intrapartum fetal pH studies. Am J Obstet Gynecol 1984;149(3):327–35. [19] Clark SL, Paul RH. Intrapartum fetal surveillance: the role of fetal scalp blood sampling. Am J Obstet Gynecol 1985;153(7):717–20. [20] Beard RW, Nathanielsz PW. Fetal physiology and medicine, the basis of perinatology. In: Maternal and fetal acid–base balance1st ed., W.B. Saunders Company Ltd.; 1976. p. 499–509. [21] Huntingford PJ. A direct approach to the study of the fetus. Lancet 1964;1:95– 6. [22] Garud MA, May DPL, Simmons SC. Fetal blood sampling in the regional hospital. BMJ 1969;1:346–8. [23] Simmons SC, Lieberman B. The combined use of cardiotocography and fetal blood sampling in monitoring the fetus in labour. J Obstet Gynecol Br Common 1972;79:816. [24] Astrup P, Jorgensen K, Siggaard Andersen O, Engel K. Lancet 1960;1:1035. [25] Morris ED, Beard RW. The rationale and technique of foetal blood sampling and amnioscopy. J Obstet Gynaecol Br Common 1965;4:489–95. [26] Weisbrot IM, James LS, Prince CE, Holaday DA, Apgar V. J Paediatr 1958;58:395. [27] Berg D, Saling E. The oxygen partial pressures in the human fetus during labour and delivery. In: Longo LD, Bartels H, editors. Respiratory gas exchange and blood flow in the placenta. Bethesda: US Dept HDW, MD; 1972. p. 441–57. [28] Adamsons K, Beard RW, Cosmi EV, Myers RE. The validity of capillary blood in the assessment of the acid–base state of the fetus. In: Adamsons K, editor. Diagnosis and treatment of fetal disorders. New York: Springer-Verlag; 1968. p. 175. [29] Gare DJ, Whetham JCG, Henry JD. The validity of scalp sampling. Am J Obstet Gynecol 1967;99:722–4. [30] Saling E. Amnioscopy and fetal blood sampling: observations on foetal acidosis. Arch Dis Child 1966;41:472–6. [31] Beard RW, Morris ED. Foetal and maternal acid–base balance during normal labour. J Obstet Gynecol Br Common 1965;72:496–506. [32] Kubli F. Influence of labour on fetal acid–base balance. Clin Obstet Gynecol 1968;11:168–91. [33] Malin GL, Morris RK, Khan KS. Strength of association between umbilical cord pH and perinatal and long term outcomes: systematic review and metaanalysis. BMJ 2010;340:c1471. [34] Parer JT. The current role of intrapartum fetal blood sampling. Clin Obstet Gynecol 1980;23(2):565–82. [35] Pearson JF. Fetal blood sampling and gas exchange. J Clin Path 1976;29(Suppl. 10):31–4. [36] Kruger K, Hallberg B, Blennow M, Kublickas M, Westgren M. Predictive value of fetal scalp blood lactate concentration and pH as markers of neurologic disability. Am J Obstet Gynecol 1999;181:1072–8. [37] Tuffnell D, Haw WL, Wilkinson K. How long does a fetal scalp blood sample take? BJOG 2006;113:332–4. [38] ACOG Practice Bulletin Number 106. Intrapartum fetal heart rate monitoring: nomenclature, interpretation, and general management principles. Obstet Gynecol 2009;114(1):192–202.
Contents lists available at ScienceDirect
European Journal of Obstetrics & Gynecology and Reproductive Biology journal homepage: www.elsevier.com/locate/ejogrb
Expert opinion
Is intrapartum fetal blood sampling a gold standard diagnostic tool for fetal distress? Amita A. Mahendru a, Christoph C. Lees b,* a
Clinical Research Fellow in Fetal Medicine, Rosie Maternity-Addenbrookes Hospital, Cambridge University Hospitals NHS Foundation Trust, Hills Road, Cambridge CB2 2QQ, UK Consultant in Obstetrics and Fetal-Maternal Medicine, Rosie Maternity-Addenbrookes Hospital, Cambridge University Hospitals NHS Foundation Trust, Hills Road, Cambridge CB2 2QQ, UK b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 December 2010 Received in revised form 16 December 2010 Accepted 23 December 2010
Developed in 1960s, cardiotocography is a screening test and fetal blood sampling (FBS) is an adjunctive, diagnostic technique to detect fetal hypoxia. A fetal blood sample pH value of less than 7.20 has a higher specificity than a pathological CTG to predict low Apgar score at 1 min. Though with a pathological CTG and despite a normal FBS pH value the risk of delivering a hypoxic infant is 30–50%, FBS has assumed considerable importance in purportedly reducing unnecessary obstetric intervention. The evidence for this is weak: the use of FBS with CTG has been shown to reduce operative vaginal deliveries though not Caesarean sections due to fetal distress. There is no difference in the umbilical artery pH at delivery with the use of intermittent FBS with CTG compared to CTG alone. FBS is an invasive procedure: obtaining an adequate blood sample is often difficult and the pH results are affected by handling of the sample, aerobic contamination and processing. Validation of intrapartum FBS requires that the pH and other values obtained are compared to a ‘gold standard’ technique. Although FBS has been compared to other tests such as scalp lactate, pulse oximetry, fetal ECG waveform analysis, and central haemodynamics in labouring rhesus monkeys, none of these can be considered as ‘gold standard’. In the light of the existing evidence, the role of intrapartum FBS as a gold standard diagnostic technique is unproven. ß 2011 Elsevier Ireland Ltd. All rights reserved.
Keywords: Fetal blood sampling Cardiotocography Hypoxia Validation Intrapartum Diagnostic technique
Contents 1. 2. 3.
Introduction . The evidence Conclusion . . References . .
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1. Introduction The aim of fetal monitoring in labour is to detect hypoxia, which can lead to acidaemia and adverse sequelae such as hypoxic– ischaemic encephalopathy and fetal death. A test used to detect hypoxia/acidaemia should have a high sensitivity so that a timely intervention can prevent any adverse outcome while at the same
* Corresponding author at: Department of Fetal Medicine, Cambridge University Hospitals NHS Foundation Trust, Cambridge CB2 2QQ, UK. Tel.: +44 01223 217972; fax: +44 01223 216185. E-mail address: [email protected] (C.C. Lees). 0301-2115/$ – see front matter ß 2011 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.ejogrb.2010.12.044
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time having a low false positive rate to prevent unnecessary intervention. The cardiotocograph (CTG) and fetal scalp blood pH assessment were both developed in the 1960s. Though the CTG has high sensitivity for intra-partum hypoxia/acidaemia [1–8], it has a high false positive rate and interpretation of CTG traces has well documented inter- and intra-observer variation [9]. The CTG is therefore useful as a screening though not diagnostic test for fetal hypoxia. Fetal blood sampling (FBS) is widely used as an adjunctive diagnostic test to quantify fetal hypoxia/acidaemia when indicated by an abnormal intrapartum CTG. The sensitivity of CTG in predicting FBS-pH of less than 7.20 is 0.50–0.90 and the specificity is 0.40–0.90 [1]. The use of FBS varies widely internationally: in the UK 40% obstetric units used FBS in 1979 [10]; in Germany fetal
138
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blood sampling is performed in 26% of deliveries with pathological fetal heart rate trace [11] and its use varies from 1.43% (1986) to 0.03% (1992) of deliveries in California [12]. In an effort to reduce unnecessary intervention, particularly Caesarean section, FBS has assumed considerable importance as a tool to ‘diagnose’ fetal distress, based primarily on the pH following studies in the 1960s and 1970s [1,2,11,13]. The UK’s National Institute for Clinical Excellence (NICE) Intrapartum Care Guidelines recommend the use of FBS in the presence of a pathological fetal heart rate trace unless there is clear evidence of acute compromise, and in cases of suspected fetal acidosis prior to contemplating an assisted birth for an abnormal FHR trace [14]. Further, NICE guidance does not support use of FBS due to limited research evidence [4], but then confusingly suggests from ‘clinical experience’ that it may in fact avoid some instrumental births and Caesarean sections. A prospective cohort study quoted by NICE compared continuous tissue pH measurements from a glass electrode applied to the fetal scalp in 72 patients to 72 historic controls where only CTG was performed [3]. This technique is not analogous to FBS measurement, as it does not require blood sampling and gives a continuous pH assessment. A reduction in instrumental deliveries, but not Caesarean section, was noted and the author stated that the technique required ‘further development and clinical evaluation’ which to our knowledge has not happened in the intervening three decades [3]. FBS is an invasive procedure, is cumbersome, uncomfortable for both the woman and the obstetrician [4,15] and an adequate sample is obtained successfully in 79.4% of attempted cases [5]. Its results are affected by the sampling technique, handling of the sample and processing these results using a blood gas analyser machine [16]. Even under well-controlled experimental conditions, FBS can result in inaccurate results from aerobic contamination [17]. Though the use of FBS with CTG reduced the number of Caesarean sections, instrumental deliveries and episiotomies compared to CTG alone, it is noteworthy that the reduction in the number of Caesarean sections due to fetal distress was not statistically significantly different [2,4]. There was no statistically significant difference in the umbilical artery pH after birth with the use of FBS in a randomized controlled trial of 695 labouring women in 1970s [2], nor in a prospective cohort study of continuous tissue pH monitoring in 96 labouring women in 1979–1980 [3,4]. Although the risk of operative deliveries of infants who are not hypoxic is diminished by the use of FBS on patients with a pathological CTG, the risk of delivering a hypoxic infant despite normal FBS (>7.20) is 30–50% and with a normal CTG 20–50% [1]. This study cautioned the use of FBS due to its inability to identify a hypoxic infant. Over the last three decades several commentators have questioned the usefulness of intrapartum fetal blood sampling [12,18,19]. There is in fact no evidence to show that FBS reduces the Caesarean section rate for fetal distress, or improves fetal outcome. 2. The evidence It is instructive in this context to briefly consider the science and evidence underlying the use of FBS. Nearly a century ago it was first reported that by adult standards the cord blood of the fetus was acidotic [20]. Postpartum cord blood analysis remained the only method to investigate the intrapartum fetal state and acid– base status for the subsequent 40 years. In 1963, Dr. Erich Saling in Berlin introduced the technique of FBS to modern obstetric practice [21]. Bretscher and Saling devised the technique of obtaining capillary blood from the presenting fetal part so that the pH, oxygen and carbondioxide (CO2) content of the blood could be determined in utero. This technique was reported to reduce the
perinatal mortality rate and operative delivery rate in various studies carried out in England and Scotland in the 1960s and 1970s but the contribution of FBS was uncertain [22,23]. The acid–base balance of the fetus is determined by Astrup micro method [24] where the actual pH of the capillary sample is initially measured, followed by pCO2, base excess or deficit and standard bicarbonate by the normogram of Siggaard Andersen and Engel [25]. Studies of umbilical venous and arterial blood at birth have shown that the pH corresponds more closely with the clinical condition of the infant than pO2 or the pCO2 [26]. During fetal hypoxia, pCO2 rises and pH falls with a build-up of H+ ions causing metabolic acidosis secondary to anaerobic metabolism, as fetal lungs and kidneys cannot eliminate the excess CO2. Nonvolatile bases or acids produced by anaerobic metabolism increase the base excess. The pH and base excess are reliable as they reflect the fetal dependence on anaerobic metabolism due to lack of respiratory exchange in utero. In cases of maternal acidosis due to prolonged labour, equilibration of fixed acids across the placenta over a period of time [27] can lead to lowering of the fetal capillary pH. The results from fetal blood scalp sampling are assumed to reflect the synchronous fetal central pH and acid–base status. This has been investigated in only one study comparing simultaneous pH and acid–base values from fetal scalp sampling samples and chronic indwelling carotid and jugular catheters in rhesus monkey fetuses, which reported a positive correlation between pH and acid–base values of intrapartum fetal scalp blood samples compared to arterial and venous blood [28]. In exteriorised fetal lamb preparations using compression of the fetal skull, umbilical vein and/or artery to stress the fetus, blood taken from the fetal scalp closely approximated the fetal arterial blood values of pH, pO2 and pCO2 [29]. The metabolic effects of stressing the fetus in true labour, however, were not simulated. Though separated in time, samples taken just before delivery when compared to umbilical artery and venous blood taken at delivery showed significant correlation with the parameters of acid–base balance [30,31] and with pH, glucose, lactate and pyruvate and values in cord blood both at vaginal delivery and Caesarean section [32]. Evidence on the association between cord pH and adverse neonatal outcome is conflicting [33] and the predictive value of fetal scalp pH on neonatal outcome remains unproven. Differences in the technique, the analyser and the maternal– fetal interaction can also affect the results of the fetal capillary sample [34]. The common source of error is allowing exposure to air during sampling, resulting in an increased pO2, decreased pCO2 and artificially elevated pH. Under clinical circumstances, the error is inevitably greater than in strictly controlled experimental conditions. Insufficient hyperaemia of scalp can result in a decrease of oxygen tension. Measurement errors can also occur with incorrect machine calibration. During labour, various maternal and fetal interactions can affect the fetal capillary blood results [34,35]. Maternal low oxygenation secondary to anaemia, hypoxemia, general uteroplacental ischemia, supine hypotension, maternal alkalosis during labour or maternal lactic academia can alter fetal pH and can only be differentiated if maternal pH is measured simultaneously with fetal pH [35] Formation of caput succedaneum in the fetus can lead to dilution of blood by tissue fluid and local scalp stasis [27]. Stasis of scalp circulation can lead to significant drop in the local pH and falsely low pH results in cases of prolonged head compression of the head against a rigid cervix or perineum. In cases of established fetal hypoxaemia, the fetal peripheral circulation is decreased and the acid/base status of scalp blood will not necessarily represent cerebral blood. Apart from hypoxia and acidosis in labour, the fetus may be adversely affected by infection, drugs administered to
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mother, haemorrhage, and intrapartum trauma, all of which cannot be predicted by FBS [15]. Validation of FBS in labour requires that the pH, BE, pO2 and pCO2 values obtained are compared to those obtained by a ‘gold standard’ technique. In fact, this widely performed technique, being itself assumed to be a ‘gold standard’, has never been validated in humans. The closest that FBS has come to validation is the comparison of central arterial and venous acid/base status in eleven labouring rhesus monkeys in 1968. Most studies have compared the correlation between scalp and other – almost exclusively post delivery – blood samples. When comparing two different techniques for measurement, this is not an appropriate statistical method as two methods may be closely correlated but give quite different absolute values. It would be practically difficult and quite possibly ethically unacceptable to conduct such studies in humans; hence FBS may never be subject to true validation. Despite many false dawns, techniques to reduce unnecessary interventions and improve neonatal outcomes such as fetal scalp lactate [5], fetal pulse oximetry [6] and fetal ECG [7] in labour still remain unproven. In a retrospective study of deliveries from a Swedish maternity unit in which fetal scalp blood sampling was undertaken due to ominous CTG trace, the predictive value of both pH and lactate for hypoxic ischemic encephalopathy were poor, but better for lactate (which requires one seventh of the amount of blood and is hence easier to obtain). The authors concluded that ‘‘The evaluation of the predictive values of a diagnostic test, with the use of any measure of morbidity and mortality is complicated when the procedure being assessed affects the outcome’’ implying that difficulty in obtaining a sample, time taken and inaccuracies ascribed to it could have a negative effect on pregnancy outcome [36]. Notwithstanding the absence of evidence for its validation, multiple potential sources for error with FBS exist whose impact on the results have been poorly, if at all, defined. The average time taken to obtain the results from the decision time to perform fetal blood sampling has been reported to be about 18 min [37], which could delay delivery, leading to a greater risk of fetal hypoxia and its sequelae. 3. Conclusion Fetal blood sampling cannot be considered as a ‘gold standard’ test for fetal hypoxia/acidaemia. Relying entirely on an FBS result to qualify an abnormal CTG does not reduce Caesarean Section for fetal distress. Further, using FBS with CTG instead of CTG alone shows the same or indeed possibly a higher risk of babies being misclassified as not hypoxic. The American College of Obstetricians and Gynecologists Guidance on Intrapartum Fetal Heart rate Monitoring does not recommend use of fetal blood sampling as an adjunct to CTG: ‘‘Scalp stimulation which is less invasive, provides similar information about the likelihood of fetal acidaemia as does scalp pH’’ [38]. The role of fetal scalp lactate and fetal ECG remains unproven. In this context, in the absence of any large robust intrapartum studies of FBS as an adjunct to CTG, we should question its place as a recommended ‘diagnostic’ adjunct to the CTG. References [1] Weber T. The validity of discontinuous pH measurements on fetal blood and of cardiotocography in predicting neonatal Apgar score. Dan Med Bull 1979;26(4):186–91. [2] Haverkamp AD, Orleans M, Langendoerfer S, McFee J, Murphy J, Thompson HE. A controlled trial of the differential effect of intrapartum fetal monitoring. Am J Obstet Gynecol 1979;134:399–409. [3] Weber T. CTG supplemented with continuous fetal pH monitoring during labour. Acta Obstet Gynecol Scand 1982;61:351–5.
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