Interpretative aspects on the acid-base composition and its variation in fetal scalp blood and maternal blood during labour

The second stage of labor

The second stage of labor extends from complete cervical dilatation to delivery. During this stage, descent and rotation of the presenting part occur as the fetus passively negotiates its passage through the birth canal. Generally, descent begins during the deceleration phase of dilatation as the cervix is drawn upward around the fetal presenting part. The most common means of assessing the normality of the second stage of labor is to measure its duration, but progress can be more meaningfully gauged by measuring the change in fetal station as a function of time. Accurate clinical identification and evaluation of differences in patterns of fetal descent are necessary to assess second stage of labor progress and to make reasoned judgments about the need for intervention. Three distinct graphic abnormalities of the second stage of labor can be identified: protracted descent, arrest of descent, and failure of descent. All abnormalities have a strong association with cephalopelvic disproportion but may also occur in the presence of maternal obesity, uterine infection, excessive sedation, and fetal malpositions. Interpretation of the progress of fetal descent must be made in the context of other clinically discernable events and observations. These include fetal size, position, attitude, and degree of cranial molding and related evaluations of pelvic architecture and capacity to accommodate the fetus, uterine contractility, and fetal well-being. Oxytocin infusion can often resolve an arrest or failure of descent or a protracted descent caused by an inhibitory factor, such as a dense neuraxial block. It should be used only if thorough assessment of fetopelvic relationships reveals a low probability of cephalopelvic disproportion. The value of forced Valsalva pushing, fundal pressure, and routine episiotomy has been questioned. They should be used selectively and where indicated.

Evaluating the value of intrapartum fetal scalp blood sampling to predict adverse neonatal outcomes: A UK multicentre observational study

OBJECTIVE

To evaluate the value of fetal scalp blood sampling (FBS) as an adjunct test to cardiotocography, to predict adverse neonatal outcomes.

STUDY DESIGN

A multicentre service evaluation observational study in forty-four maternity units in the UK. We collected data retrospectively on pregnant women with singleton pregnancy who received FBS in labour using a standardised data collection tool. The primary outcome was prediction of neonatal acidaemia diagnosed as umbilical cord arterial pH < 7.05, the secondary outcomes were the prediction of Apgar scores<7 at 1st and 5th minutes and admission to the neonatal intensive care unit (NICU). We evaluated the correlation between the last FBS blood gas before birth and the umbilical cord blood and adjusted for time intervals. We constructed 2 × 2 tables to calculate the sensitivity, specificity, positive (PPV) and negative predictive value (NPV) and generated receiver operating curves to report on the Area Under the Curve (AUC).

RESULTS

In total, 1422 samples were included in the analysis; pH values showed no correlation (r = 0.001, p = 0.9) in samples obtained within an hour (n = 314), or within half an hour from birth (n = 115) (r=-0.003, p = 0.9). A suboptimal FBS pH value (<7.25) had a poor sensitivity (22%) and PPV (4.9%) to predict neonatal acidaemia with high specificity (87.3%) and NPV (97.4%). Similar performance was noted to predict Apgar scores <7 at 1st (sensitivity 14.5%, specificity 87.5%, PPV 23.4%, NPV 79.6%) and 5th minute (sensitivity 20.3%, specificity 87.4%, PPV 7.6%, NPV 95.6%), and admission to NICU (sensitivity 20.3%, specificity 87.5%, PPV 13.3%, NPV 92.1%). The AUC for FBS pH to predict neonatal acidaemia was 0.59 (95%CI 0.59-0.68, p = 0.3) with similar performance to predict Apgar scores<7 at 1st minute (AUC 0.55, 95%CI 0.51-0.59, p = 0.004), 5th minute (AUC 0.55, 95%CI 0.48-0.62, p = 0.13), and admission to NICU (AUC 0.58, 95%CI 0.52-0.64, p = 0.002). Forty-one neonates had acidaemia (2.8%, 41/1422) at birth. There was no significant correlation in pH values between the FBS and the umbilical cord blood in this subgroup adjusted for sampling time intervals (r = 0.03, p = 0.83).

CONCLUSIONS

As an adjunct tool to cardiotocography, FBS offered limited value to predict neonatal acidaemia, low Apgar Scores and admission to NICU.

Fetal monitoring with lactate

Lactate is a metabolite that can safely and easily can be determined in fetal scalp blood using new microvolume (5-20 microliters) lactate meters. However, new lactate analysing methods need their own reference values. There are factors other than hypoxia that might increase fetal lactate levels, although this does not disqualify this parameter for intrapartum surveillance. Available data on fetal lactate determination give support that it can simplify FBS in labour and is likely to predict fetal tissue hypoxia at least as well as is pH determination. Prospective randomized studies are needed before the method can be introduced into clinical practice. As a predictor of neonatal outcome, lactate can substitute pH in routine assessment of cord artery blood at delivery.

Intrauterine growth retardation: fluctuation of fetal pH measured between beginning and at the completion of labor

The aim of this study was to evaluate the fluctuation of fetal pH in blood samples taken at the beginning of labor and at the moment of birth as related to intrauterine growth retardation syndrome. This is a prospective follow-up of term gestations, of which 41 were diagnosed as intrauterine growth retardation (IUGR) and 61 as normal ones. pH was measured in scalp blood sample at the beginning of the labor and in umbilical artery right after birth in both groups respectively. The rate of decrease of pH value in relation to duration of labor was determined for each case. Our results are: 1) Lower baseline pH were found in the IUGR group (pH 7.32 vs 7.34, p < 0.01), lower arterial blood pH at birth/.23 +/- 0.08 vs 7.27 +/- 0.08, p < 0.05). 2) Faster decrease of the pH during the labor (0.019 unit/hour vs 0.012, p < 0.05) as related to IUGR.

CONCLUSION

IUGR fetuses are more acidotic at the beginning of the labor, and the rate of decrease of the fetal pH per unit of time during the labor is faster at least in a theoretical situation nevertheless it will require more studies if any practical applications are thought of.

Rapid whole blood lactate measurement in the fetus and mother during labour

Using a new rapid method, fetal and maternal whole blood lactate was measured before the onset of labour at elective Caesarean section in 8 patients, during labour in 34 normal patients, and in a further 28 patients whose babies showed varying degrees of clinical depression and/or acid base abnormality at birth. The mean (+/- SEM) umbilical venous and arterial and maternal venous lactate values in the 8 cases delivered by elective Caesarean section were 1.20 (+/- 0.16), 1.46 (+/- 0.22) and 1.14 (+/- 0.46) mmol/l, respectively. For the normal group the mean fetal lactates (+/- SEM) in the latent and active phases of labour, and in the umbilical vein and artery, were 1.91 (+/- 0.25), 2.42 (+/- 0.46), 2.71 (+/- 0.19) and 3.09 (+/- 0.20) mmol/l, respectively. The mean maternal venous lactate (+/- SEM) in the latent and active phases of labour and at delivery were 1.07 (+/- 0.09), 1.45 (+/- 0.12) and 2.69 (+/- 0.24) mmol/l. the rise in fetal lactate throughout labour was due in part to the rise in maternal lactate. Increasing neonatal depression was associated with increasing fetal lacticacidaemia. This associationachieved statistical significance at delivery.