Relationship between fetal intracranial pressure and fetal heart rate during labor

The role of intrapartum fetal head compression in neonatal retinal hemorrhage

PURPOSE

Neonatal retinal hemorrhage is a common finding in newborns, but the underlying mechanisms are not fully understood. A computational simulation was designed to study the events taking place in the eye and orbit when the head is compressed as the neonate passes through the birth canal.

METHODS

A finite element model of the eye, optic nerve sheath, and orbit was simulated and subjected to forces mimicking rises in intracranial pressure (ICP) associated with maternal contractions during normal vaginal delivery. Resulting changes in intraocular pressure (IOP), pressure in the optic nerve sheath, and stress within the sclera and retina were measured.

RESULTS

During contractions, increased ICP was transmitted to the orbit, globe, and optic nerve sheath. IOP rose by 2.71 kPa near the posterior pole. Pressure at the center of the optic nerve sheath rose by 7.31 kPa and up to 9.30 kPa at its interface with the sclera. Stress in the retina was highest near the optic disk and reached 10.93, 10.99, and 13.28 kPa in the preretinal, intraretinal, and subretinal layers, respectively. Stress in the sclera peaked at 12.76 kPa.

CONCLUSIONS

Increasing ICP associated with natural vaginal delivery increases intraorbital pressure, which applies stress to the retina. Associated retinal deformation may cause tearing of the retinal vasculature. Increased pressure within the optic nerve sheath may occlude the central retinal vein, resulting in outflow obstruction and subsequent rupture. Forces accumulated near the optic disk, likely accounting for the tendency of neonatal retinal hemorrhage to occur posteriorly.

Fetal defenses against intrapartum head compression-implications for intrapartum decelerations and hypoxic-ischemic injury

Uterine contractions during labor and engagement of the fetus in the birth canal can compress the fetal head. Its impact on the fetus is unclear and still controversial. In this integrative physiological review, we highlight evidence that decelerations are uncommonly associated with fetal head compression. Next, the fetus has an impressive ability to adapt to increased intracranial pressure through activation of the intracranial baroreflex, such that fetal cerebral perfusion is well-maintained during labor, except in the setting of prolonged systemic hypoxemia leading to secondary cardiovascular compromise. Thus, when it occurs, fetal head compression is not necessarily benign but does not seem to be a common contributor to intrapartum decelerations. Finally, the intracranial baroreflex and the peripheral chemoreflex (the response to acute hypoxemia) have overlapping efferent effects. We propose the hypothesis that these reflexes may work synergistically to promote fetal adaptation to labor.

Approaches to Preventing Intrapartum Fetal Injury

Electronic fetal monitoring (EFM) was introduced into obstetric practice in 1970 as a test to identify early deterioration of fetal acid-base balance in the expectation that prompt intervention ("rescue") would reduce neonatal morbidity and mortality. Clinical trials using a variety of visual or computer-based classifications and algorithms for intervention have failed repeatedly to demonstrate improved immediate or long-term outcomes with this technique, which has, however, contributed to an increased rate of operative deliveries (deemed "unnecessary"). In this review, we discuss the limitations of current classifications of FHR patterns and management guidelines based on them. We argue that these clinical and computer-based formulations pay too much attention to the detection of systemic fetal acidosis/hypoxia and too little attention not only to the pathophysiology of FHR patterns but to the provenance of fetal neurological injury and to the relationship of intrapartum injury to the condition of the newborn. Although they do not reliably predict fetal acidosis, FHR patterns, properly interpreted in the context of the clinical circumstances, do reliably identify fetal neurological integrity (behavior) and are a biomarker of fetal neurological injury (separate from asphyxia). They provide insight into the mechanisms and trajectory (evolution) of any hypoxic or ischemic threat to the fetus and have particular promise in signaling preventive measures (1) to enhance the outcome, (2) to reduce the frequency of "abnormal" FHR patterns that require urgent intervention, and (3) to inform the decision to provide neuroprotection to the newborn.

A Systematic Review of Intrapartum Fetal Head Compression: What Is the Impact on the Fetal Brain?

Objective During labor the fetal head is subjected to pressure related to uterine contractions and maternal pushing. Here we systematically review what is known about fetal head compression and its effects on fetal intracranial pressure, oxygenation, blood flow and cerebral function, and the plausibility that it might cause isolated fetal brain injury. Study Design Systematic review of intrapartum fetal head compression and fetal brain injury in accordance with the MOOSE methodology. The PubMed database was searched using a combination of the terms "fetal," "head," "cranial," "extracranial," "pressure," and "compression." Additional references were obtained using multiple strategies. Results were evaluated, and relevant studies encompassing animal and human data using several approaches are summarized in this review. Results Studies support a significant increase in fetal extracranial pressure with contractions and pushing. However, available data do not support a concomitant significant relative increase in intracranial pressure, a reduction in cerebral circulation or oxygenation, or an impact on cerebral function. Conclusion A review of the literature indicates that fetal intracranial pressure is well protected from extracranial forces. Available data do not support intrapartum fetal extracranial pressure as a cause of fetal brain injury. Precis The fetal brain is relatively unaffected by intrapartum fetal head compression.

Categorization of Fetal Heart Rate Decelerations in American and European Practice: Importance and Imperative of Avoiding Framing and Confirmation Biases

Interpretation of electronic fetal monitoring (EFM) remains controversial and unsatisfactory. Fetal heart rate (FHR) decelerations are the commonest aberrant feature on cardiotocographs and considered “center-stage” in the interpretation of EFM. A recent American study suggested that the lack of correlation of American three-tier system to neonatal acidemia may be due to the current peculiar nomenclature of FHR decelerations leading to loss of meaning. The pioneers like Hon and Caldeyro-Barcia classified decelerations based primarily on time relationship to contractions and not on etiology per se. This critical analysis debates pros and cons of significant anchoring/framing and confirmation biases in defining different types of decelerations based primarily on the shape (slope) or time of descent. It would be important to identify benign early decelerations correctly to avoid unnecessary intervention as well as to improve the positive predictive value of the other types of decelerations. Currently the vast majority of decelerations are classed as “variable”. This review shows that the most common rapid decelerations during contractions with trough corresponding to peak of contraction cannot be explained by “cord-compression” hypothesis but by direct/pure (defined here as not mediated through baro-/chemoreceptors) or non-hypoxic vagal reflex. These decelerations are benign, most likely and mainly a result of head-compression and hence should be called “early” rather than “variable”. Standardization is important but should be appropriate and withstand scientific scrutiny. Significant framing and confirmation biases are necessarily unscientific and the succeeding three-tier interpretation systems and structures embodying these biases would be dysfunctional and clinically unhelpful. Clinical/pathophysiological analysis and avoidance of flaws/biases suggest that a more physiological and scientific categorization of decelerations should be based on time relationship to contractions alone irrespective of shape or descent time as indeed proposed by pioneers like Hon and Caldeyro-Barcia. Such meaningful categorization, apart from being a scientific necessity, could improve the practical performance of three-tier FHR interpretation systems and possibly application of dependent complementary techniques like fetal ECG/pulse oximetry/computer-aided analysis, thus facilitating future progress in the field of intrapartum fetal monitoring.

Pathophysiology of intracranial epidural haematoma following birth

Epidural haematoma in newborn infants is rare, and few specific obstetrical data related to its formation are available in the literature. The aim of this study is to discuss the pathophysiology of this condition. EDH is always a post traumatic lesion and it is only possible if the insult has produced a cleavage of the dura mater from bone. Therefore, EDH results from the mechanical forces exerted on the foetal head during birth, with or with no instrumental interference. Although it is still unclear whether the injury (and dura mater cleavage) was directly caused by the forceps or had already been inflicted by natural forces, or a combination of both however, in some patients (with neither dystocia nor skull fracture), there is no basis for explaining EDH formation, apart from propulsion of the fore coming head through the birth canal. Excessive moulding, whether or not associated with iatrogenic trauma, has been incriminated in most cases of EDH. As dystocia cannot always be anticipated, EDH will remain an ever-present cause of morbidity in the neonatal population, albeit a rare occurrence.

The Role of Uterine Fundal Pressure in the Management of the Second Stage of Labor: A Reappraisal

Among the maneuvers that are used in the second stage of labor, uterine fundal pressure is one of the most controversial. The prevalence of its use is unknown. We reviewed the existing literature to assess whether there is justification for the use of fundal pressure in the contemporary management of the second stage of labor. Only one randomized, controlled study and a few prospective studies, review articles, and case reports have been published. No confirmed benefit of the procedure has been documented and a few adverse events have been reported in association with its use. Alternative management strategies in the second stage of labor exist and should be considered whenever possible. In conclusion, the role of fundal pressure is understudied and remains controversial in the management of the second stage of labor. We believe that caution should be exercised using this maneuver until it is proven to be safe and effective.

TARGET AUDIENCE

Obstetricians & Gynecologists, Family Physicians.

LEARNING OBJECTIVES

After completion of this article, the reader should be able to recall that there is a scarcity of literature related to the efficacy and safety of using fundal pressure during the second stage of labor, state that there is no confirmed benefit of the procedure and there may be some adverse maternal/fetal effects, and explain that there are alternative strategies for management of the second stage of labor.

Efficacy of the cushing response in maintaining cerebral blood flow in premature and near-term fetal sheep

Fetal head compression during labor may increase intracranial pressure (ICP) and decrease cerebral perfusion pressure (CPP). An increase in mean arterial pressure (MAP) associated with the Cushing response normally acts to mitigate an ischemic insult when the increase in ICP approaches MAP. However, the premature fetus may be limited in its ability to increase MAP. We compared the efficacy of the pressor response in sustaining CPP, cerebral blood flow (CBF), and cerebral O2 consumption (CMRO2) in chronically catheterized fetal sheep at 0.6 gestation (92 d; n = 7) and 0.9 gestation (133 d; n = 7). When fetal ICP was increased to baseline MAP (41 +/- 3 mm Hg; +/-SEM) in 92-d fetuses, MAP increased by 7 +/- 2 mm Hg and remained stable during 30 min of constant ICP elevation; CBF decreased by 72% and CMRO2 decreased by 46%. In 133-d fetuses, MAP increased from 53 +/- 2 to 65 +/- 4 mm Hg at 3 min of elevated ICP; CBF decreased by 62% and CMRO2 decreased 30%. However, MAP continued to increase after 3 min and reached a stable level of 75 +/- 3 mmHg at 30 min in 133-d fetuses. The additional increase in MAP restored CBF and CMRO2 to baseline values. Plasma epinephrine and vasopressin concentrations increased between 6 and 33 min of elevated ICP to levels, exceeding those in 92-d fetuses. We conclude that the arterial pressure response to intracranial hypertension is present at 0.6 gestation but is less well developed than at 0.9 gestation in fetal sheep, possibly due to immaturity of the sympathoadrenal and vasopressin systems. Consequently, CBF and CMRO2 are not as well defended at mid-gestation against elevated ICP as might occur during difficult labor.

Acid accumulation during end-stage bradycardia in term fetuses: how long is too long?

OBJECTIVE

To estimate the existence and degree of fetal accumulation of acid during end-stage bradycardia as reflected by the base deficit. This may set a criterion for proper intervention during labour.

SETTING

Maternity unit of the Tel Aviv Medical Centre.

SUBJECTS

Forty-three consecutively born term infants whose mothers were delivered by vacuum extraction were analysed: 27 because of end-stage bradycardia and 16 controls whose mothers were delivered electively because of maternal indications.

MAIN OUTCOME MEASURES

Analysis of umbilical arterial cord blood for pH, PCO2 and base deficit. The base deficit was compared between the groups using the two-tailed Student's t test, and was correlated with variables of fetal heart rate monitoring using the Pearson correlation coefficient.

RESULTS

The base deficit was greater in newborns who had end-stage bradycardia than in controls (11.02 vs 5.01, P < 0.0001). The duration of loss of short term variability in fetal heart rate during end-stage bradycardia correlated positively with the base deficit (r = 0.8, P < 0.0005). Conversely, the time until the loss of short term variability during end-stage bradycardia correlated negatively with the base deficit. The length and the depth of the bradycardia and their product, had a weaker correlation with the base deficit.

CONCLUSIONS

End-stage bradycardia, which presumably reflects fetal hypoxia, is associated with acidaemia in the umbilical artery at birth in some fetuses. The fetuses who are predisposed to acidaemia, as reflected by an increased base deficit, are those who lost their fetal heart rate variability during end-stage bradycardia for more than 4 min or started to lose this in less than 3 min from the beginning of the end-stage bradycardia. Operative vaginal delivery should be reserved for these indications.

Intracranial pressure and fetal heart rate in a hydrocephalic fetus during labor

Intra-uterine pressure (IUP), intracranial pressure (ICP) and fetal heart rate (FHR) were simultaneously recorded during labor in a severely hydrocephalic fetus. After cephalocentesis, 600 ml of liquor cerebrospinalis was drained. ICP exceeded IUP, but the increase in ICP was less than the increase in IUP during most of the contractions. The FHR showed marked decelerations during uterine contractions and changed gradually into a persistent bradycardia. The pathophysiology of fetal heart rate patterns during labor is discussed and the literature has been reviewed.

Cerebral handicap in full-term neonates related to the mechanical forces of labour

Sometimes the relationship between peripartum events and neonatal CNS injury is obvious: for example, following complete abruptio placentae or umbilical cord prolapse and occlusion with a delay of many minutes before delivery of the baby. These circumstances are, of course, rare in modern obstetrics. Usually, when a neonate develops neurological injury, a host of various potentially adverse peripartum factors are assumed to be the aetiology, but without definitive evidence. Among these latter factors are those we have focused on in this paper: the mechanical forces exerted on the fetal head during labour when the full-term fetus is in cephalic presentation. The mechanical events during the first stage of labour are reviewed, showing how uterine contractions result in cervical dilatation and descent and rotation of the fetal head. The consequences of these forces on the fetal intracranial pressure and blood flow are discussed: FHR remains normal up to a certain pressure threshold, above which decelerations occur. In other words, excessive pressures applied to the fetal head, either spontaneously (e.g. uterine tetany) or iatrogenically (e.g. traumatic forceps delivery or excessive fundal pressure) can increase fetal intracranial pressure to such a degree as to result in significant decreases in cerebral blood flow that are associated with fetal heart rate decelerations. Even when decelerations are simultaneous to contractions, decelerations cannot be considered as reflex and innocuous, as they are indeed associated with a decreasing cerebral blood flow. They must therefore be considered and evaluated in the management of labour. Cord compression and functional modifications of intervillous space by mechanical forces may further compromise the biological status of the fetus, leading to severe asphyxia. Neurological evaluation of the neonate within the first few days after delivery is currently the only way to provide the obstetricians with information on the possible consequences of an abnormal labour. The assessment of normality of the CNS in the neonate born at term, and its value in predicting late outcome are discussed. When abnormalities are detected after one or repeated assessments, abnormal neurological signs and symptoms are classified into three grades at the end of the first week. According to our data, a good correlation exists between this neonatal grading of cerebral dysfunction and late outcome. A careful evaluation of fetal head deformation, extensive caput succedaneum, and extensive retinal haemorrhages can help to interpret an abnormal labour retrospectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Is end-stage deceleration of the fetal heart ominous?

End-stage deceleration (ESD) defined as deep and sustained fetal bradycardia during the second stage of labour was observed in 55 patients. Fetal well-being and neonatal outcome were assessed with fetal-scalp pH, umbilical-vein pH and 1-min Apgar score values. Only six infants with pathological fetal heart-rate patterns before ESD had a 1-min Apgar score of less than 7 and an acidotic umbilical-vein pH. The remaining 49 babies were born in excellent condition, although when ESD persisted for more than 15 min umbilical-vein pH decreased.

Clinical significance of fetal heart rate patterns during labor. VI. Early decelerations

In a population of high-risk patients who had continuous "direct" monitoring during labor, 598 (46%) had no decelerations during the first stage, while 247 (19%) had presented early decelerations before completion of dilatation. The clinical characteristics, the fetal heart rate (FHR) baseline alterations, and neonatal outcome were compared between these two groups: there were no differences in any of the aspects evaluated, except that there was transient tachycardia more often among the early-decelerations group. The patients who had no decelerations were subdivided into vaginal deliveries and cesarean deliveries, and the same parameters were compared again: the cesarean section group had longer recordings and more contractions, lower Apgar 1 and 5 minute scores, and higher incidences of Apgar scores less than or equal to 6 at 1 minute, while all other aspects were similar. A possible explanation was that 82% of cesarean sections were done under general anesthesia and only 3% of vaginal deliveries. The implication of ruptured membranes in the etiology of early decelerations was extensively reviewed and discussed in view of these findings. It is concluded that amniotomy does not seem a maneuver deleterious to the fetal well-being.

Pressure in the hydrocephalic fetal head during the first stage of labor

Intraamniotic pressure was studied in the 30th week of amenorrhea in relationship with fetal intracranial pressure with open-tip catheters. The fetus had a severe hydrocephalus (echoscopy 16 cm) due to a teratologic malformation of the cerebrum. Clinically nonoperative treatment was indicated. Intracranial pressure (X) was invariably higher than intraamniotic pressure (Y) between contractions: Y = 2.04 + 0.54 X, and during contractions: Y = 5.30 + 0.55 X. There was no definite relationship between intrauterine and intracranial pressure, and the fetal tachogram. A definite relationship was established with the supine position of the patient and decelerations in the fetal tachogram. It is suggested that when fetal cardiac decelerations are seen during the first stage of labor it seems advisable to look for factors such as umbilical cord compression and decrease of materno-placental perfusion rather than fetal head compression.