Relationship of fetal asphyxia to brain damage

During the last 50 years, asphyxia has been examined in the research laboratory using a number of different animal models. These studies have confirmed that fetal asphyxia of a particular degree and duration may cause brain damage. However, the striking feature of all these studies is that in spite of a uniform single exposure to asphyxia many fetuses have no brain damage, some will have brain damage and a few fetal deaths will occur. The outcome is influenced by the fetal response to asphyxia. Fetal cardiovascular compensation with an increase of arterial pressure, centralization of cardiac output and increased cerebral blood flow will maintain cerebral oxygen consumption in spite of the hypoxemia. However if the asphyxia continues, a point will be reached when fetal cardiovascular decompensation reverses this process leading to cerebral hypoxia and, if sustained, brain damage. Variability of this fetal cardiovascular response is an important

Fig. 8.1 The outcome, i.e. no brain damage/brain damage is a result of the characteristics of the fetal asphyxia and the quality of the fetal cardiovascular compensation.

factor in the differing outcome in these laboratory studies.

Our understanding of the relationship between fetal asphyxia and brain damage has been based on these studies in the research laboratory. It has become evident that the relationship between fetal asphyxia and brain damage is complex and may be influenced by a number of factors including: maturity of the fetus; degree, duration and nature of the asphyxia and the quality of the fetal cardiovascular response (Fig. 8.1).

The clinical introduction of microelectrode blood gas systems provided the opportunity to examine these measures in all pregnancies at delivery without risk to the fetus and newborn. Umbilical vein and artery blood gas and acid-base measures at delivery represent valuable reference points of asphyxia during labour. The umbilical vein reflects the effectiveness of maternal fetal blood gas exchange while the umbilical artery reflects the acid-base status of the fetus. Reference data for these measures in the umbilical vein and artery for 21,744 deliveries in our centre are presented in Table 8.1.

Recognizing the importance of these measures, recent studies have emphasized the need for quality data to provide the basis for interpretation [2]. Several procedural and technical errors may occur during umbilical cord blood sampling and subsequent blood gas analysis. Optimal

Table 8.1 Mean blood gas and acid-base measures for 21,744 deliveries

Umbilical vein

Umbilical artery

Mean

SD

Mean

SD

pH

7.340

0.07

7.248

0.069

pco2

40.4

7.7

54.5

9.7

P02

27.2

6.1

15.1

5.1

BD

3.0

2.7

6.8

BD, base deficit.

interpretation requires a paired sample from both umbilical vein and artery. A singlesamplefrom theumbilical vein will define venous metabolic state but cannot rule out an arterial metabolic acidosis. During sampling, two aliquots may be drawn from the umbilical vein. Such a procedural error is implied when the vein-artery pH difference is less than 0.02. The accuracy of the calculated measures of metabolic acidosis is dependent upon the quality of the pH and pC02 estimations. The accuracy of the pC02 estimation should be questioned when the pC02 value is outside the physiological range or the pC02 artery-vein difference is a negative value or less than 4 mgHg. In these circumstances, the interpretation should be limited to pH alone.

An umbilical artery blood measure of metabolic acido-sis is the best indicator of tissue oxygen debt experienced by the fetus. Recent studies have suggested an increased risk to the fetus begins when the umbilical artery base deficit exceeds the mean [3]. In a study to determine the threshold for significant morbidity, moderate and severe newborn complications occurred only in the fetuses with an umbilical artery base deficit >12mmol/l. Theincidence of moderate and severe complications with an umbilical artery base deficit 12-16 mmol/l was 10% [4]. There is a progression of frequency of such complications with increasing metabolic acidosis with a 40% incidence of moderate and severe complications when the umbilical artery base deficit was >16 mmol/l.

The objective of clinical studies since the introduction of microelectrode blood gas systems has been to determine if the concepts emerging from the research laboratory are relevant to the human fetus. The findings are consistent with the contention that a fetus may experience asphyxia without morbidity; however, the occurrence of fetal asphyxia of a particular degree and duration may cause cerebral dysfunction in the newborn [5,6] and in some cases brain damage accounting for handicap in surviving children [7,8].

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Getting Back Into Shape After The Pregnancy

Getting Back Into Shape After The Pregnancy

Once your pregnancy is over and done with, your baby is happily in your arms, and youre headed back home from the hospital, youll begin to realize that things have only just begun. Over the next few days, weeks, and months, youre going to increasingly notice that your entire life has changed in more ways than you could ever imagine.

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