Types of intrapartum hypoxia in the newborn at term with metabolic acidemia: A retrospective study

Hypoxic-ischemic encephalopathy following intrapartum asphyxia: is it avoidable?

BACKGROUND

The proportion of term hypoxic-ischemic encephalopathy arising during intrapartum fetal surveillance remains unclear. Moreover, recent Cochrane review and other studies emphasized the need for research on the impact of admission cardiotocography and highlighted the necessity for a definition of "avoidable perinatal brain injury".

OBJECTIVE

To assess the impact of intrapartum asphyxia on neonatal hypoxic-ischemic encephalopathy occurrence and identify the proportion of cases that benefit from preventive measures.

STUDY DESIGN

This retrospective 20-year birth cohort study included admission and intrapartum cardiotocography recordings from spontaneous term (≥37 weeks of gestation) singleton deliveries at 7 maternity hospitals within the Helsinki University Hospital area, Finland, between 2005 and 2024. In newborns diagnosed with hypoxic-ischemic encephalopathy, cases following intrapartum asphyxia were identified by a normal cardiotocogram at admission, whereas antepartum exposure was indicated by an abnormal admission cardiotocogram. Cord blood gases, erythropoietin, and serum S100β concentrations were analyzed, and placentas underwent histopathological examination. Primary outcome was hypoxic-ischemic encephalopathy. Secondary outcome was fetal asphyxia, defined as the presence of severe or moderate acidemia.

RESULTS

Among 317,126 term newborns, 314 cases of hypoxic-ischemic encephalopathy were identified. Admission cardiotocogram was normal in 141 (44.9%) and abnormal in 173 (55.1%). Of those with a normal admission cardiotocogram, severe acidemia (umbilical artery pH <7.00 and/or base excess ≤-12.0 mmol/L) evolved in 127/141 (90.1%) and moderate acidemia (umbilical artery pH 7.09-7.00 and base excess -10.0 to -11.9 mmol/L) in 11/141 (7.8%). Excluding cases with a perinatal sentinel event and timely deliveries, 70 cases (49.6%) remained in which hypoxic-ischemic encephalopathy presumably developed during labor and was considered potentially avoidable. These findings suggest that in 22.3% (70/314), preventive measures should have been implemented. Newborns with abnormal cardiotocograms had higher median umbilical blood erythropoietin concentrations than those with normal admission cardiotocograms (112 U/L, interquartile range 22-1130 vs 29 U/L, interquartile range 7-680, P<.001), indicating more chronic hypoxia.

CONCLUSION

Of term newborns with hypoxic-ischemic encephalopathy and normal admission cardiotocogram, 98% were attributable to intrapartum asphyxia. Our findings indicate that half of the cases of intrapartum hypoxic-ischemic encephalopathy with a normal admission cardiotocogram were potentially avoidable, suggesting that one-fifth of all cases could have benefited from preventive measures. The findings underscore the role of optimal intrapartum care in preventing hypoxic-ischemic encephalopathy.

The peripheral chemoreflex and fetal defenses against intrapartum hypoxic-ischemic brain injury at term gestation

Fetal hypoxemia is ubiquitous during labor and, when severe, is associated with perinatal death and long-term neurodevelopmental disability. Adverse outcomes are highly associated with barriers to care, such that developing countries have a disproportionate burden of perinatal injury. The prevalence of hypoxemia and its link to injury can be obscure, simply because the healthy fetus has robust coordinated defense mechanisms, spearheaded by the peripheral chemoreflex, such that hypoxemia only becomes apparent in the minority of cases associated with stillbirth, severe metabolic acidemia or adverse neurodevelopmental outcomes. This represents only the extreme end of the spectrum, when defense mechanisms have failed due to severe/prolonged hypoxemia, or the fetal defenses are compromised by additional risk factors. Understanding the fetal defenses to hypoxemia and when the fetus begins to decompensate is crucial to understanding perinatal health and disease, by linking antenatal health, intrapartum events, the neonatal trajectory and ultimately life-long neurodevelopmental health.

International expert consensus statement on physiological interpretation of cardiotocograph (CTG): First revision (2024)

The first international consensus guideline on physiological interpretation of cardiotocograph (CTG) produced by 44 CTG experts from 14 countries was published in 2018. This guideline ensured a paradigm shift from classifying CTG by arbitrarily grouping certain features of the fetal heart rate into different "categories", and then, randomly combining them to arrive at an overall classification of CTG traces into "Normal, Suspicious and Pathological" (or Category I, II and III) to a classification which is based on the understanding of fetal pathophysiology. The guideline recommended the recognition of different types of fetal hypoxia, and the determination of features of fetal compensatory responses as well as decompensation to ongoing hypoxic stress on the CTG trace. Since its first publication in 2018, there have been several scientific publications relating physiological interpretation of CTG, especially relating to features indicative of autonomic instability due to hypoxic stress (i.e., the ZigZag pattern), and of fetal inflammation. Moreover, emerging evidence has suggested improvement in maternal and perinatal outcomes in maternity units which had implemented physiological interpretation of CTG. Therefore, the guideline on Physiological Interpretation of CTG has been revised to incorporate new scientific evidence, and the interpretation table has been expanded to include features of chorioamnionitis and relative utero-placental insufficiency of labour (RUPI-L).

Relative uteroplacental insufficiency of labor

Relative uteroplacental insufficiency of labor (RUPI-L) is a clinical condition that refers to alterations in the fetal oxygen "demand-supply" equation caused by the onset of regular uterine activity. The term RUPI-L indicates a condition of "relative" uteroplacental insufficiency which is relative to a specific stressful circumstance, such as the onset of regular uterine activity. RUPI-L may be more prevalent in fetuses in which the ratio between the fetal oxygen supply and demand is already slightly reduced, such as in cases of subclinical placental insufficiency, post-term pregnancies, gestational diabetes, and other similar conditions. Prior to the onset of regular uterine activity, fetuses with a RUPI-L may present with normal features on the cardiotocography. However, with the onset of uterine contractions, these fetuses start to manifest abnormal fetal heart rate patterns which reflect the attempt to maintain adequate perfusion to essential central organs during episodes of transient reduction in oxygenation. If labor is allowed to continue without an appropriate intervention, progressively more frequent, and stronger uterine contractions may result in a rapid deterioration of the fetal oxygenation leading to hypoxia and acidosis. In this Commentary, we introduce the term relative uteroplacental insufficiency of labor and highlight the pathophysiology, as well as the common features observed in the fetal heart rate tracing and clinical implications.

[Fetal monitoring: Current limitations and new approaches based on analysis of the fetal autonomic nervous system]

OBJECTIVE

Currently, fetal monitoring during labor is based on visual analysis of the fetal heart rate (FHR). This test is imperfect, with high intra- and inter-observer variability and a moderate to poor prediction of the occurrence of neonatal acidosis or anoxic-ischaemic encephalopathy. In situations where there is an intermediate risk of acidosis, it is possible to use second-line tests such as blood scalp sampling (with pH or lactate measurement) or ST segment analysis of the fetal ECG. However, these invasive tests have many limitations and their place is debated. Some authors suggest a more physiological approach to FHR assessment. The main actor in maintaining fetal homeostasis is the autonomic nervous system (ANS). Its activity can be assessed by analysing heart rate variability (HRV). The aim is to assess whether HRV can be used to identify situations at risk of acidosis.

MATERIALS AND METHODS

Our team has developed an index, the Fetal Stress Index, to measure HRV. To test it in a situation of acidosis, we used a pregnant ewe model. We also developed in parallel a human fetal ECG recording system.

RESULTS

In our experimental model, we have shown that this index reflects variations in the parasympathetic system and correlates with the onset of acidosis. As its use in clinical practice requires the acquisition of a beat-to-beat FHR signal, we have also developed an abdominal patch that allows highly accurate analysis of the fetal ECG.

CONCLUSION

The future is therefore to validate the FSI as a marker of acidosis in a prospective cohort using the signal obtained from our patch. This could be a new tool for fetal monitoring during labor.