Acute increase of umbilical artery vascular flow resistance in compromised fetuses provoked by uterine contractions

Umbilical cord pH, blood gases, and lactate at birth: normal values, interpretation, and clinical utility

Normal birth is a eustress reaction, a beneficial hedonic stress with extremely high catecholamines that protects us from intrauterine hypoxia and assists in the rapid shift to extrauterine life. Occasionally the cellular O2 requirement becomes critical and an O2 deficit in blood (hypoxemia) may evolve to a tissue deficit (hypoxia) and finally a risk of organ damage (asphyxia). An increase in H+ concentration is reflected in a decrease in pH, which together with increased base deficit is a proxy for the level of fetal O2 deficit. Base deficit (or its negative value, base excess) was introduced to reflect the metabolic component of a low pH and to distinguish from the respiratory cause of a low pH, which is a high CO2 concentration. Base deficit is a theoretical estimate and not a measured parameter, calculated by the blood gas analyzer from values of pH, the partial pressure of CO2, and hemoglobin. Different brands of analyzers use different calculation equations, and base deficit values can thus differ by multiples. This could influence the diagnosis of metabolic acidosis, which is commonly defined as a pH <7.00 combined with a base deficit ≥12.0 mmol/L in umbilical cord arterial blood. Base deficit can be calculated as base deficit in blood (or actual base deficit) or base deficit in extracellular fluid (or standard base deficit). The extracellular fluid compartment represents the blood volume diluted with the interstitial fluid. Base deficit in extracellular fluid is advocated for fetal blood because a high partial pressure of CO2 (hypercapnia) is common in newborns without concomitant hypoxia, and hypercapnia has a strong influence on the pH value, then termed respiratory acidosis. An increase in partial pressure of CO2 causes less increase in base deficit in extracellular fluid than in base deficit in blood, thus base deficit in extracellular fluid better represents the metabolic component of acidosis. The different types of base deficit for defining metabolic acidosis in cord blood have unfortunately not been noticed by many obstetrical experts and organizations. In addition to an increase in H+ concentration, the lactate production is accelerated during hypoxia and anaerobic metabolism. There is no global consensus on definitions of normal cord blood gases and lactate, and different cutoff values for abnormality are used. At a pH <7.20, 7% to 9% of newborns are deemed academic; at <7.10, 1% to 3%; and at <7.00, 0.26% to 1.3%. From numerous studies of different eras and sizes, it can firmly be concluded that in the cord artery, the statistically defined lower pH limit (mean -2 standard deviations) is 7.10. Given that the pH for optimal enzyme activity differs between different cell types and organs, it seems difficult to establish a general biologically critical pH limit. The blood gases and lactate in cord blood change with the progression of pregnancy toward a mixed metabolic and respiratory acidemia because of increased metabolism and CO2 production in the growing fetus. Gestational age-adjusted normal reference values have accordingly been published for pH and lactate, and they associate with Apgar score slightly better than stationary cutoffs, but they are not widely used in clinical practice. On the basis of good-quality data, it is reasonable to set a cord artery lactate cutoff (mean +2 standard deviations) at 10 mmol/L at 39 to 40 weeks' gestation. For base deficit, it is not possible to establish statistically defined reference values because base deficit is calculated with different equations, and there is no consensus on which to use. Arterial cord blood represents the fetus better than venous blood, and samples from both vessels are needed to validate the arterial origin. A venoarterial pH gradient of <0.02 is commonly used to differentiate arterial from venous samples. Reference values for pH in cord venous blood have been determined, but venous blood comes from the placenta after clearance of a surplus of arterial CO2, and base deficit in venous blood then overestimates the metabolic component of fetal acidosis. The ambition to increase neonatal hemoglobin and iron depots by delaying cord clamping after birth results in falsely acidic blood gas and lactate values if the blood sampling is also delayed. Within seconds after birth, sour metabolites accumulated in peripheral tissues and organs will flood into the central circulation and further to the cord arteries when the newborn starts to breathe, move, and cry. This influence of "hidden acidosis" can be avoided by needle puncture of unclamped cord vessels and blood collection immediately after birth. Because of a continuing anaerobic glycolysis in the collected blood, it should be analyzed within 5 minutes to not result in a falsely high lactate value. If the syringe is placed in ice slurry, the time limit is 20 minutes. For pH, it is reasonable to wait no longer than 15 minutes if not in ice. Routine analyses of cord blood gases enable perinatal audits to gain the wisdom of hindsight, to maintain quality assurance at a maternity unit over years by following the rate of neonatal acidosis, to compare results between hospitals on regional or national bases, and to obtain an objective outcome measure in clinical research. Given that the intrapartum cardiotocogram is an uncertain proxy for fetal hypoxia, and there is no strong correlation between pathologic cardiotocograms and fetal acidosis, a cord artery pH may help rather than hurt a staff person subjected to a malpractice suit based on undesirable cardiotocogram patterns. Contrary to common beliefs and assumptions, up to 90% of cases of cerebral palsy do not originate from intrapartum events. Future research will elucidate whether cell injury markers with point-of-care analysis will become valuable in improving the dating of perinatal injuries and differentiating hypoxic from nonhypoxic injuries.

Prediction of non-reassuring fetal status and umbilical artery acidosis by the maternal characteristic and ultrasound prior to induction of labor

Objective

To investigate the predictive value of pre-induction digital examination, sonographic measurements and parity for the prediction of non-reassuring fetal status and cord arterial pH < 7.2 prior to the induction of labor (IOL).

Method

This was a prospective observational study, including 384 term pregnancies undergoing IOL. Before the IOL, the Bishop score (BS) by digital examination, sonographic Doppler parameters and the estimated fetal weight (EFW) was assessed. The fetal cord arterial was sampled to measure the pH at delivery. Multivariate logistic regression analysis was performed to identify independent predictors of non-reassuring fetal status and low cord arterial pH.

Results

Forty four cases (11.5%) had non-reassuring fetal status, and 76 cases (19.8%) had fetal cord arterial pH < 7.2. In the non-reassuring fetal status group, the incidence of cord arterial pH < 7.2 was significantly higher than that in the normal fetal heart rate group (χ² = 6.401, p = 0.011). Multivariate analysis indicated that significant independent predictors of non-reassuring fetal status were nulliparity (adjusted odds ratio [AOR]: 3.746, p = 0.003), EFW < 10^th percentile (AOR: 3.764, p = 0.003) and cerebroplacental ratio (CPR) < 10^th centile (AOR:4.755, p < 0.001). In the prediction of non-reassuring fetal status, the performance of the combination of nulliparity and EFW < 10th percentile was improved by the addition of CPR < 10th percentile (AUC: 0.681, (95%CI: 0.636 to 0.742) vs 0.756, (95%CI:0.713 to 0.795)), but the difference was not significant (DeLong test: z = 1.039, p = 0.053).. None of the above variables were predictors of cord arterial pH < 7.2.

Conclusion

The risk of fetal acidosis has increased in cases of non-reassuring fetal status. Nulliparity, small for gestational age and CPR < 10th centile are independent predictors for non-reassuring fetal status in term fetuses, though the addition of CPR < 10th centile could not significantly improve the screening accuracy.

The Present Utility of the Oxytocin Challenge Test-A Single-Center Study

Introduction: The oxytocin challenge test (OCT) used to be one of the most important tools in assessing fetal well-being before ultrasonography became prevalent. We show that, after modifying the classification of the results and the intervention algorithm, OCT can still be a useful tool in present-day obstetrics. Material and methods: The study included 318 OCTs performed in patients admitted to our department from 2010 to 2012. A modified classification of test results was introduced, dividing the results in four groups: I-negative, II-positive, III-non-diagnostic and type IV (fetal tachycardia or increased variability). The purpose of the study was to evaluate the clinical significance of OCT in assessing intrauterinal fetal well-being and predicting the necessity for ending the pregnancy. Results: A significant difference (p < 0.001) in the delivery method and the indications for cesarean sections (CS) was found between negative and positive OCT results. CS indicated by an abnormal fetal heart rate (FHR) pattern had to be performed in 40% of cases with positive OCT results, having constituted 84.6% of all CSs in this group. After negative OCTs, 12.8% pregnancies were ended by CS from FHR indications (62.3% of all the indications). Conclusions: A positive OCT result can be a valuable predictor of an abnormal fetal heart rate pattern after the test and during the delivery, as well as a higher probability of a CS from cardiotocography (CTG) indications, with positive predictive value (PPV) 0.50 and negative predictive value (NPV) 0.85.

Antenatal glucocorticoids, magnesium sulfate, and mode of birth in preterm fetal small for gestational age

A diagnosis of fetal growth restriction and subsequent preterm birth is associated with increased risks of adverse perinatal and neurodevelopmental outcomes and potentially long-lasting effects to adulthood. Most such cases are associated with placental insufficiency and the fetal response to chronic intrauterine hypoxemia and nutrient deprivation leads to substantial physiological and metabolic adaptations. The management of such pregnancies, especially with respect to perinatal interventions and birth mode, remains an unresolved dilemma. The benefits from standard interventions for threatened preterm birth may not be necessarily translated to pregnancies with small-for-gestational-age fetuses. Clinical trials or retrospective studies on outcomes following administration of antenatal glucocorticoids and magnesium sulfate for neuroprotection when preterm birth is imminent either have yielded conflicting results for small-for-gestational-age fetuses, or did not include this subgroup of patients. Experimental models highlight potential harmful effects of administration of antenatal glucocorticoids and magnesium sulfate in the pregnancies with fetal small for gestational age although clinical data do not substantiate these concerns. In addition, heterogeneity in definitions of fetal small for gestational age, variations in the inclusion criteria, and the glucocorticoid regime contribute to inconsistent results. In this review, we discuss the physiologic adaptions of the small-for-gestational-age fetus to its abnormal in utero environment in relation to antenatal glucocorticoids; the impact of antenatal glucocorticoids and intrapartum magnesium sulfate in pregnancies with fetal small for gestational age; the current literature on birth mode for pregnancies with fetal small for gestational age; and the knowledge gaps in the existing literature.

Uterine activity: implications for the condition of the fetus

AIM

The aim of this review is to provide more insight in the fetal mechanisms as a response to uterine contractions and to emphasize the importance of correct assessment of uterine activity (UA) patterns during labor.

STUDY RESULTS

UA causes a decreased flow through the uterine artery. In the healthy uncompromised fetus, this will not cause fetal acidemia. The fetus has developed certain protection mechanisms to survive labor; (1) During a contraction, fetal preload increases and enables the fetus to maintain a constant blood flow through the umbilical artery and (2) UA increases the blood flow in the fetal middle cerebral artery, i.e., a brain sparing effect. The shortcoming of those protection mechanisms in the compromised fetus and in case of excessive UA increases the risk of adverse fetal outcome. The brain sparing effect will become more pronounced to compensate for the decreased umbilical artery blood flow and fetal oxygen saturation. Maintenance of normal UA, especially a sufficiently long relaxation time, is essential so that the supply of well oxygenated maternal blood to the intervillous space will be restored and the fetal cerebral oxygen saturation can remain stable.

CONCLUSION

Adequate UA monitoring is a prerequisite for proper reading and interpretation of cardiotocograms. It alarms in cases of excessive UA and can help to prevent fetal acidemia. Uterine contraction monitoring deserves full attention in daily obstetric practice.

Fetal ductus venosus, middle cerebral artery and umbilical artery flow responses to uterine contractions in growth-restricted human pregnancies

OBJECTIVE

To explore fetal ductus venosus (DV) flow velocity changes relative to umbilical artery (UA) blood flow and brain-sparing flow (BSF) during uterine contractions.

METHODS

Forty-five term fetuses suspected of having growth restriction were exposed to an oxytocin challenge test (OCT) with simultaneous Doppler velocimetry in the UA, middle cerebral artery (MCA) and DV. Basal BSF was defined as a MCA-to-UA pulsatility index (PI) ratio of < 1.08, and de novo BSF as a decrease in MCA-PI of > or = 1 SD (equivalent to a value of 0.24 units) during the OCT.

RESULTS

Basal DV flow velocities were lower in the BSF group (n = 7) than they were in the non-BSF group (n = 38). During the OCT, DV flow velocity parameters changed in neither group but MCA-PI decreased in the non-BSF group. A crude de novo BSF was not associated with DV flow velocity changes, but when UA-PI changes were considered, a serial relationship was found between decreased UA-PI, increased DV flow velocity, and decreased MCA-PI. When UA-PI increased, the MCA-PI still decreased (though not significantly) but DV flow velocity parameters remained unchanged.

CONCLUSIONS

Established fetal BSF is associated with low DV flow velocities, but in an acute sequence there might be two contrasting courses along which BSF develops: one with an increase and one with a decrease in the UA vascular flow resistance. In the former situation the DV flow velocity increases, while in the latter situation the role of the DV in the acute redistribution of fetal blood flow is unclear.

Intracerebral regional distribution of blood flow in response to uterine contractions in growth-restricted human fetuses

OBJECTIVE

To explore middle cerebral artery (MCA) and anterior cerebral artery (ACA) blood flow responses to superimposed acute hypoxemia in growth-restricted fetuses with and without established brain-sparing flow during basal conditions.

MATERIAL AND METHODS

47 term fetuses suspected of growth restriction were exposed to an oxytocin challenge test with simultaneous cardiotocography and Doppler velocimetry in the umbilical artery, MCA and ACA. The MCA-to-ACA pulsatility index (PI) ratio was calculated during basal conditions, contractions and relaxations. Basal brain-sparing flow was defined as an MCA-to-umbilical artery PI ratio of<1.08, de novo brain-sparing flow in the MCA as an MCA PI decrease with> or =1 standard deviation during uterine contractions or relaxations compared with basal measurements, and de novo brain-sparing flow in the ACA as an ACA PI decrease with > or =1 standard deviation. Non-parametric statistical tests were used with P<0.05 considered significant.

RESULTS

MCA and ACA PI were both significantly lower in the brain-sparing flow group (N=8) during basal conditions (P< or =0.01). During the oxytocin challenge test, MCA and ACA PI both decreased in the non-brain-sparing flow group (N=39) (P< or =0.02) but not in the brain-sparing flow group (P> or =0.4). The MCA-to-ACA PI ratio remained unchanged in both groups. de novo brain-sparing flow calculations revealed no preferential flow to any cerebral artery.

CONCLUSION

Cerebral circulatory responses to acute hypoxemia are synchronized in the middle and anterior cerebral arteries without any preferential regional flow distribution.

The intrapartum deceleration in center stage: a physiologic approach to the interpretation of fetal heart rate changes in labor

One of the most distinctive features of fetal heart rate recordings in labor is the deceleration. In clinical practice, there has been much confusion about the types of decelerations and their significance. In the present review, we examined uteroplacental perfusion in labor, describe the pathophysiologic condition of decelerations, and explain some of the reasons behind the confusion about the terminology. We summarize recent studies that systematically have dissected the features of variable decelerations that may help to identify developing fetal compromise, such as the slope of the deceleration, overshoot, and variability changes. Although no pattern of repeated deep decelerations is necessarily benign, fetuses with normal placental reserve can compensate fully, even for frequent deep but brief decelerations, for surprisingly prolonged intervals before the development of profound acidosis and hypotension. This tolerance reflects the remarkable ability of the fetus to adapt to repeated hypoxia. We propose that, rather than focus on descriptive labels, clinicians should be trained to understand the physiologic mechanisms of fetal heart rate decelerations and the patterns of fetal heart rate change that indicate progressive loss of fetal compensation.

Acute centralization of blood flow in compromised human fetuses evoked by uterine contractions

BACKGROUND

During fetal hypoxia blood is redistributed to the brain ('brain-sparing'). Sequential changes of the cerebral and placental circulation in parallel in comparisons between basal conditions and acute hypoxic stress have not yet been thoroughly studied in human fetuses.

AIM

To explore acute fetal middle cerebral artery (MCA) circulatory changes relative to umbilical artery (UA) blood flow in a clinical experimental model with hypoxic stress provoked by uterine contractions during an oxytocin challenge test (OCT).

STUDY DESIGN

Prospective comparative between imminently compromised (OCT positive) and un-compromised (OCT negative) fetuses.

SUBJECTS AND METHODS

82 term pregnancies suspected of intrauterine growth restriction were exposed to simultaneous electronic fetal heart rate monitoring and Doppler recordings of pulsatility index (PI) in the UA and MCA during basal conditions and during uterine contractions and relaxations at an OCT.

OUTCOME MEASURES

Sequential changes of UA and MCA PI, OCT positive vs. negative cases. Nonparametric statistics with a P < 0.05 considered significant.

RESULTS

The UA PI was significantly higher in OCT positive cases (N = 10) compared with OCT negative cases (N = 72) during uterine contractions and relaxations, but not during basal measurements. During contractions and relaxations the MCA PI decreased significantly in both groups (brain-sparing), but significantly more in OCT positive cases.

CONCLUSIONS

During acute hypoxic stress, changes towards a centralization of blood flow to the brain develop in imminently compromised (OCT positive) fetuses at an expense of the umbilicoplacental blood flow, and the brain-sparing flow is more pronounced than in un-compromised (OCT negative) fetuses.

Uterine activity and ductus venosus flow velocity patterns during the first stage of labor

OBJECTIVE

To analyze the effects of uterine contractions on ductus venosus (DV) pulsatility during the first stage of labor.

METHODS

Twenty healthy women were examined. Measurements were taken at three stages of cervical dilatation (<4 cm, 4-7 cm and >or=8 cm) during and between contractions. Peak velocity during ventricular systole (S) and atrial contraction (A), pulsatility index for veins (DV PIV), ductus venosus index (DVI) and the S/A ratio were measured.

RESULTS

The DV was observed successfully in 16 cases. The mean S velocity did not change significantly (64 cm/s during and 65 cm/s between contractions). The mean A velocity decreased significantly from 35 cm/s measured between contractions to 29 cm/s during contractions (P<0.0001). The mean DV PIV and DVI were significantly higher during contractions (0.72 and 0.55) than between contractions (0.57 and 0.45) (P<0.0001). There were no significant differences in means between stages of cervical dilatation.

CONCLUSION

Significant differences during and between uterine contractions can be observed in DV pulsatility during normal labor.

Restrained cerebral hyperperfusion in response to superimposed acute hypoxemia in growth-restricted human fetuses with established brain-sparing blood flow

OBJECTIVE

To investigate the cerebral circulatory response to superimposed acute hypoxemia in growth-restricted fetuses with established brain-sparing flow (BSF) during basal conditions.

MATERIAL AND METHODS

76 term fetuses suspected of growth restriction were exposed to Doppler velocimetry in the umbilical artery (UA) and middle cerebral artery (MCA), and in 38-39 cases also in Galen's vein (GV), straight sinus (SS), and transverse sinus (TS), before and during an oxytocin challenge test (OCT), and simultaneous to electronic fetal heart rate monitoring. Nonparametric statistical analyses compared presence/absence of established BSF (MCA-to-UA pulsatility index [PI] ratio <1.08) with a two-tailed P<0.05 considered significant.

RESULTS

The OCT (positive/negative) was not different in the BSF group (BSFG, N=16) and the normal flow group (NFG, N=60) (P=0.2). During uterine contractions, the MCA PI decreased in the NFG, but not in the BSFG. De novo GV pulsations and increase of GV maximum flow velocity occurred during contractions in the NFG, but not in the BSFG. Significant SS flow velocity waveform changes were found in neither group and TS flow changes in the BSFG only.

CONCLUSIONS

Fetuses without established brain-sparing flow during basal conditions responded with both arterial and venous brain-sparing flow during acute hypoxemia, whereas in fetuses with established brain-sparing flow the cerebral circulatory responses were absent or equivocal. Fetuses with established brain-sparing flow may have a limited capacity of further cerebral hyperperfusion during superimposed acute hypoxic stress.

Foetal heart rate power spectrum response to uterine contraction

Cardiotocography is the most diffused prenatal diagnostic technique in clinical routine. The simultaneous recording of foetal heart rate (FHR) and uterine contractions (UC) provides useful information about foetal well-being during pregnancy and labour. However, foetal electronic monitoring interpretation still lacks reproducibility and objectivity. New methods of interpretation and new parameters can further support physicians' decisions. Besides common time-domain analysis, study of the variability of FHR can potentially reveal autonomic nervous system activity of the foetus. In particular, it is clinically relevant to investigate foetal reactions to UC to diagnose foetal distress early. Uterine contraction being a strong stimulus for the foetus and its autonomic nervous system, it is worth exploring the FHR variability response. This study aims to analyse modifications of the power spectrum of FHR variability corresponding to UC. Cardiotocographic signal tracts corresponding to 127 UC relative to 30 healthy foetuses were analysed. Results mainly show a general, statistically significant (t test, p<0.01) power increase of the FHR variability in the LF 0.03-0.2 Hz and HF 0.2-1 in correspondence of the contraction with respect to a reference tract set before contraction onset. Time evolution of the power within these bands was computed by means of time-varying spectral estimation to concisely show the FHR response along a uterine contraction. A synchronised grand average of these responses was also computed to verify repeatability, using the contraction apex as time reference. Such modifications of the foetal HRV that follow a contraction can be a sign of ANS reaction and, therefore, additional, objective information about foetal reactivity during labour.

Acute changes of cerebral venous blood flow in growth-restricted human fetuses in response to uterine contractions

OBJECTIVE

The fetal cerebral venous circulation during acute hypoxic stress provoked by uterine contractions has not been studied previously. The aim of this study was to explore the cerebral venous circulation during an oxytocin challenge test (OCT) in intrauterine growth-restricted (IUGR) fetuses.

METHODS

Doppler recordings of blood flow in the vein of Galen (GV), straight sinus (SS) and transverse sinus (TS) before and during uterine contractions and relaxation were obtained at the same time as electronic fetal heart rate (FHR) monitoring in 44 term IUGR fetuses. The OCT was classified as negative (normal FHR) or positive (late FHR decelerations). Non-parametric statistics were used to test differences between OCT groups.

RESULTS

In OCT-negative cases (n = 39), de novo pulsations occurred in the GV, and SS flow velocities increased during contractions compared with basal measurements. There were no significant differences in TS flow. Flow recordings were less often obtained from OCT-positive cases (n = 5), making comparisons with OCT-negative cases uncertain.

CONCLUSIONS

In uncompromised IUGR fetuses an acute cerebral venous hyperperfusion develops in response to uterine contractions. Pulsations in the GV are detected but are unlikely to be an ominous sign in this situation. The more centrally located TS was less discriminating for acute cerebral venous blood flow changes.