The incidence of fetal asphyxia in six hundred high-risk monitored pregnancies J.

A.

S. R.

LOW,

M.D.

PANCHAM,

D.

WORTHINGTON,

R.

W.

Kingston,

M.D. M.D.

BOSTON, Ontario,

M.D. Canada

Six hundred high-risk monitored obstetric patients were reviewed for evidence of fetal asphyxia at delivery. The over-all incidence was 20 per cent, i.e., 8 times the incidence in a normal obstetric population. Highly significant indicators of risk for asphyxia were severe toxemia (79 per cent), prematurity with further medical or obstetric complications (36 per cent), and clinical fetal distress, particularly meconium staining with fetal heart rate abnormality (33 per cent). All obstetric, medical, or gestational complications in this review were associated with a normal obstetric population.

an increased

risk

for fetal

Methods Six hundred monitored high-risk pregnancies were analyzed for evidence of fetal asphyxia at delivery.

and of Paediatrics,

Presented at the Thirtieth Annual Meeting of the Society of Obstetricians and Gynaecologists of Canada, Murray Bay, Quebec, Canada, June 20-23, 1974. Reprint requests: Dr. J. A. Low, Department Obstetrics and Gynaecology, Queen’s University, Kingston, Ontario, Canada.

when

compared

to that

in

The definition of fetal asphyxia was an umbilical cord artery buffer base of less than 36.1 mEq. per 1iter.l Uterine contractions were measured by means of an intra-amniotic catheter. Instantaneous fetal rate was obtained from a scalp electrode. Serial acid base assessments (pH, Pcoz, buffer base PoZ) were performed on fetal capillary blood during labor as indicated and on the umbilical cord artery and vein at delivery. The indications for monitoring were the standard obstetric criteria of risk. Some patients had more than one problem; therefore, the number of indications exceeds the number of patients. There were two main categories of indications for monitoring. The first group included those patients in whom a high-risk factor was present, i.e., a maternal obstetric, medical, or fetal complication; these fetuses presumably had an increased potential for morbidity and death. The second group included patients who displayed abnormal uterine activity, had a failed induction of labor, or were trials of labor for previous cesarean section or cephalopelvic disproportion. Abnormal uterine activity included patients with failure to progress in the absence of fetopelvic disproportion or abnormal response to oxytocin stimulation. Failed induction included those patients with spontaneous or artificially ruptured membranes and

THE co N c E PT of the high-risk pregnancy has received extensive endorsement in the recent literature. The clinical implication of risk is an increase in perinatal morbidity and death. Morbidity is difficult to define and has in the past been based on low Apgar scores, early neonatal behavior or longterm central nervous system dysfunction. The increased awareness of this intrapartum risk has led to the widespread use of fetal monitoring units. The purpose of this review of a high-risk monitored obstetric population is to determine the risk of fetal asphyxia at delivery as identified by metabolic acidosis and expressed as an umbilical cord artery buffer base.

From the Department of Obstetrics Gynaecology and the Department Queen’s University.

usphyxia

of

456

Volume Number

121 4

in whom labor was not established in 24 hours despite at least one course of clinical oxytocin stimulation. The fetuses in these patients were presumably normal but at risk because of these abnormal labor and delivery factors. Results The incidence of fetal asphyxia at delivery in 600 high-risk monitored pregnancies was 20 per cent (Table I). Of the 834 indications for monitoring, 22.5 per cent were associated with fetal asphyxia. Six hundred and seventy-four antepartum or intrapartum indices of risk were associated with a 24 per cent incidence of fetal asphyxia; of the 160 in the abnormal labor group, 18 per cent had asphyxia (Table II) . The incidence of fetal asphyxia in patients with high-risk factors is outlined in Table III. Clinical fetal distress was present on 198 occasions, 53 (27 per cent) of which were associated with asphyxia. Table IV shows the incidence of fetal asphyxia, increasing from 10 per cent with clinical fetal heart rate abnormality to 28 per cent with meconium staining of the amniotic fluid and 33 per cent when both criteria were present. The incidence of fetal asphyxia in 64 premature ( < 37 weeks) infants was 31 per cent. Table V shows that there was a higher incidence of fetal asphyxia with prematurity when associated with a further complication, such as toxemia or antepartum hemorrhage, than with prematurity alone. In 98 postterm pregnancies (> 42 weeks), the incidence of fetal asphyxia was 18 per cent. The incidence of asphyxia was no greater in the 27 patients who were 43 weeks’ gestation or greater when compared to the 71 who were 42 completed weeks of gestational age. Table VI shows that the presence of meconium in the amniotic fluid was associated with 27 per cent fetal asphyxia as compared to 12 per cent with clear fluid. The gestational abnormality group was too small to permit detailed analysis (Table III). Of 38 cases of suspected intrauterine growth retardation, the diagnosis was confirmed in 20 cases, of which 7 infants had asphyxia (< tenth percentile on the weight-gestational scale of Gruenwald). Of the remaining 18 without intrauterine growth retardation, only 3 had fetal asphyxia. There were 14 breech presentations, of which 7 infants developed asphyxia. Each breech infant had one or more additional complications so that the development of asphyxia was associated with a combination of

Fetal

asphyxia

Table I. Incidence obstetric

in monitored

pregnancies

of fetal asphyxia

in a high-risk

population Fetal

Patients Indications

for

monitoring

No.

600 834

119 188

of fetal asphyxia for monitoring

indication

Indication

for

High-risk Abnormal

monitoring

1

No.

Total

20 22.5

per

I-

674 160

pregnancies labor group

asphyxia

7%

Total

Table II. Incidence

159 29

24 18

834

Table III. Incidence patients

with

of fetal asphyxia indices

high-risk

in obstetric

1 Fetal High-risk

indices

1 No.

Clinical fetal distress Abnormal maturity Prematurity Postmaturity Gestational abnormality Suspect intrauterine growth retardation Breech Twins Rh isoimmunization Maternal medical abnormality Diabetes Hypertension Obstetric complications Toxemia Antepartum hemorrhage Other Total

Table IV. Incidence clinical

457

asphyxia

IA;o.

198

53

27

64 98

20 17

31 18

38 14 12 21

10 7 3 2

26 50 14 10

32 30

6 7

19 23

100 46 14 674

25 10 1

25 22 7

of fetal asphyxia

in

fetal distress 1 “eta: Clinical

fetal

Clinical fetal heart abnormality Meconium staining fluid Meconium staining

distress

asphz

No.

rate 30

3

10

113

32

28

55

18

33

of amniotic plus

heart rate abnormality

fetal

458

Low et al.

Table V. Incidence of the premature

February Am. .I. Obstet.

of fetal asphyxia group

in 64 patients

Fetal

Prematurity Prematurity

I NO.

I No.

14 50

2 18

alone with complications

Table VII. Incidence 100 cases of toxemia

asphyxia 1

NO.

14 36

Mild toxemia Severe toxemia Totai

Table VIII. of fetal asphyxia pregnancies

98 postterm

No.

No.

Clear Meconium

57 41

7 11

patients

in

asphyxia

No.

71 19 100

Incidence abnormal

with

I

1

%

10 15

asphyxia I

Abnormal

of fetal asphyxia labor

labor

Failed induction Abnormal uterine activity Cephalopelvic disproportion Trial of labor-previous cesarean section Total

% 12 17

98

--

14 79

in

Fetal

Fetal

Amniotic fluid

in

Fetal

%

64

Table VI. Incidence

of fetal asphyxia

15. 1975 Cynrcol.

asphyxia

No.

No.

3P a7 30

8 15 4

% 25 17 13

11 160

?

18

I

factors and not the abnormal presentation alone. There were 62 diabetic or hypertensive complications of pregnancy in the study. In 32 diabetic patients, the incidence of fetal asphyxia was 19 per cent. In 18 Class A pregnant diabetic patients, 3 had asphyxia; in 14 cases of Classes B, C, D diabetes combined, 3 had asphyxia. In 30 pregnancies complicated by hypertension (blood pressure 140/90) without toxemia, 7 infants (23 per cent) had fetal asphyxia. The incidence of fetal asphyxia in 100 patients with toxemia was 25 per cent. Table VII shows a marked increase in fetal asphyxia from 14 per cent in mild toxemia to 79 per cent in severe toxemia (blood pressure 160, systolic; 110 or greater, diastolic, proteinuria, 5 Gm. per 24 hours or more). In the 15 cases of severe toxemia with asphyxia, the fetus was further complicated in 9 cases by intrauterine growth retardation and in 6 by prematurity. Antepartum hemorrhage was a primary or associated complication in 46 cases, 10 (22 per cent) of which had fetal asphyxia. In 14 cases where definite pathology was identified (abruptio placentae, major infarcts, circumvallate placenta, vasa previa), 6 had fetal asphyxia. In the remaining 32 cases where no pathologic explanation of bleeding was established, only 4 had asphyxia. The abnormal labor group is outlined in Table VIII. There were 32 cases of failed induction of

labor was present in 23 cases; 6 of these had asphyxia at delivery. There were 9 patients who had labor induced without any major complications; 2 developed asphyxia. In 87 cases of abnormal uterine activity, there were 15 ( 17 per cent) who had fetal asphyxia. In 30 patients who had trials of labor for fetopelvic disproportion, 4 (13 per cent) had asphyxia.

labor,

delivery.

of

which

An

8

(25

obstetric

per

cent)

indication

Comment Standard obstetric criteria of risk have been based on the clinical observation of the association between antepartum and intrapartum complications and subsequent outcome of the fetus. This review is an attempt to assess these factors on the basis of biochemical evidence of asphyxia at delivery. Because there are no widely accepted laboratory criteria of fetal asphyxia, the definition used in this review was the identification of metabolic acidosis based on an umbilical cord artery buffer base of less than 36.1 mEq. per liter. This figure represents 2 standard deviations below the mean in a previously reported normal obstetric popu1ation.l Every complication of pregnancy examined in this review was associated with an increased risk of fetal asphyxia at delivery. The over-all incidence of asphyxia in this high-risk group was 8 times greater than in the normal obstetric population. Clinical

had

asphyxia

at

with

for

induction

of

Meconium

criteria

a high

of

fetal

incidence

staining

of the

distress

of

were

asphyxia

amniotic

fluid

associated

at

delivery.

showed

an

Volume Number

121 4

elevenfold increase in asphyxia when compared to the incidence in the normal population with clear fluid. The low incidence of asphyxia when only clinical fetal heart rate abnormalities were present re-emphasizes the difficulty of interpretation or fetal heart rate patterns by auscultation as an indicator of asphyxia. There was an increased incidence of fetal asphyxia in premature infants. However, this was probably principally a reflection of other obstetric, medical, and gestational complications. Postterm pregnancies had a sevenfold increase in asphyxia when compared to the incidence in the normal term pregnancies. The increased incidence of fetal asphyxia with meconium staining of the amniotic fluid points to the value of this observation as an additional indicator of risk in this group. There is evidence in the literature to suggest an increased perinatal mortality rate after 43 weeks’ gestation”; however, in this review, there was no further increase in the incidence of asphyxia past 43 weeks’ gestation. The difficulty in the diagnosis of intrauterine growth retardation is emphasized by about a 50 per cent predictive accuracy. However, when the diagnosis is correct, there is a marked increase in

Fetal

asphyxia

in monitored

pregnancies

459

the incidence of asphyxia as previously reported and again evidenced in this review.’ Severe toxemia was associated with the highest incidence of fetal asphyxia in this review. Associated factors were intrauterine growth retardation and prematurity. The incidence of fetal asphyxia was higher in cases of antepartum hemorrhage associated with definite pathology when compared to those cases without an identified cause. The significance of maternal medical disorders is shown by the increased risk of fetal asphyxia in obstetric patients whose pregnancies were complicated by diabetes or hypertension. The high risk of the Class A diabetic pregnancy is demonstrated by the presence of asphyxia in 3 fetuses. Abnormal labor was associated with a sevenfold increase in asphyxia. In the failed induction group, the increased incidence of fetal asphyxia might, in part, be related to associated complications. This review has defined and analyzed fetal asphyxia on the basis of an umbilical cord artery buffer base of less than 36.1 mEq. per liter. Every complication studied was associated with an increased incidence of asphyxia. Therefore, highrisk indices have been reasonable predictors of intrapartum asphyxia in this review.

REFERENCES

1.

2.

Low, J. A., Pancham, S. R., Worthington, D., and Boston, R. W.: AM. J. OBSTET. GYNECOL. 120: 862, 1974. Butler, N. R., and Bonham, D. G.: The First Report

of the British Perinatal Mortality Survey, Edinburgh, 1963, E. & S. Livingston, Ltd., Section F, p. 118. 3. Low, J. A., Boston, R. W., and Pancham, S. R.: AM. J. OBSTET. GYNECOL. 113: 351 1972.

The incidence of fetal asphyxia in six hundred high-risk monitored pregnancies.

Six hundred high-risk monitored obstetric patients were reviewed for evidence of fetal asphyxia at delivery. The over-all incidence was 20 per cent, i...
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