Limitations in the clinical prediction of intrapartum fetal asphyxia

J.A. Low, MD, L.L. Simpson, MD, G. Tonni, MD, and S. Chamberlain, MD

Kingston, Ontario, Canada

OBJECTIVE: Our purpose was to demonstrate the predictive value of clinical risk scoring and fetal assessment for intrapartum fetal asphyxia. STUDY DESIGN: lntrapartum fetal asphyxia was defined by an umbilical artery buffer base ~34 mmol/L. The predictive value of 20 antepartum and intrapat-tum risk factors was examined in 1909 consecutive pregnancies. The predictive value of clinical risk factors with periodic fetal assessment was examined in a second population of 100 consecutive pregnancies with biochemically determined intrapat-tum fetal asphyxia. RESULTS: The incidence of intrapartum was 2.3%. Two problems were apparent in these studies. A significant proportion of intrapartum fetal asphyxia occurred in pregnancies with no risk factors. The positive predictive value of clinical risk factors was low, 3%, resulting in a large number of false positives requiring clarification. CONCLUSION: Screening and fetal assessment methods must be improved to ensure the early recognition of intrapartum fetal asphyxia that may require intervention during labor to avoid morbidity and mortality. (AM J OBSTET GYNECOL 1995;i 72:801-4.)

Key words: Clinical risk scoring, fetal assessment, fetal asphyxia

Intrapartum fetal asphyxia may cause newborn com- plications in the central nervous sytem, cardiovascular system, respiratory system, and kidney’” and, in a few of these children, brain damage with long-term neu- rodevelopmental sequelae.4, 5 Prevention of these com- plications is dependent on the early identification of asphyxia during labor, with appropriate intervention in selected cases.

The diagnosis of intrapartum fetal asphyxia can be made with a blood gas and acid-base assessment of fetal blood during labor or at delivery.” The clinical problem is the identification of those pregnancies at risk for this complication. Current clinical practice is the definition of risk pregnancies on the basis of the presence of certain complications followed by an assessment of fetal behavior.

The limited sensitivity of such assessment programs was evident in a review of perinatal mortality in which neuropathologic features resulting from fetal asphyxia was identified during postmortem examination.’ The results of this study were (1) 50% of antepartum as- phyxia occurred in pregnancies with no risk factors; (2)

From the Department of Obstetrics and Gynecology, Queen’s Uni- versity. Received for publication May 11, 1994; revised July 8, 1994; nccepted September 9, 1994. Reprint requests: J.A. Low, MD, Department of Obstetrics and Gynecology, QueenS University, Kingston, Ontario, Canada K7L 2V7. Cofiyright 0 1995 by Mosby-Year Book, Inc. 0002-9378/95 $3.00 + 0 6/l/60511

50% of antepartum asphyxia occurred in risk preg- nancies; however, periodic fetal assessment failed to identify the asphyxial episode responsible for the neu- ropathologic feature; and (3) the indicators of intrapar- turn asphyxia were observed after the central nervous system injury occurred.

This report presents the results of two studies that examine the limitations of risk scoring and periodic fetal assessment in the prediction of the pregnancy at risk of intrapartum fetal asphyxia determined bio- chemically at delivery.

Methods and results Predictive value of clinical risk scoring. The pre-

dictive value of clinical risk factors for intrapartum fetal asphyxia was examined in 1909 consecutive pregnancies in which an umbilical vein and artery blood gas and acid-base analysis was obtained at delivery. The criterion of intrapartum fetal asphyxia was an umbilical artery buffer base <34 mmol/L (equivalent to a base deficit > 12 mmol/L). This occurred in 44 newborns (2.3%).

This was a retrospective review conducted without awareness of the outcome at delivery. The antepartum and intrapartum risk factors from the medical record were documented, and incomplete data were obtained by interview with the responsible physician. Current risk scoring systems were reviewed. The 20 risk factors that were considered most relevant for fetal asphyxia were examined.

The antepartum risk factors were (1) prior stillbirth

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802 Low et al March 1993 Am J Obstet Gym01

Table I. Predictive value of antepartum risk factors (group 1) and antepartum plus intrapartum risk factors (group 2) for intrapartum fetal asphyxia

Group 1 Group 2

Asphyxia present* Asphyxia absent Asphyxia present” Asphyxia absent

Risk present 22 663 34 1081 Risk absent 22 1202 10 784 Sensitivity (%) 50 77 Specificity (%) 64 42 Positive predictive value (%) 3 3 iSegative predictive value (%) 98 99

*Umbilical artery buffer base < 34 mmol/L.

Table II. Incidence of intrapartum fetal asphyxia in relation to antepartum and intrapartum risk factors

Asphyxia present”

Risk factors Total (No.) No. 70

Antepartum Prior perinatal death Medical complications PIH or preeclampsia Antepartum hemorrhage Growth retardation Fetal, other

Intrapartum Preterm Postterm Meconium Abnormal labor

67 134 133

84 126

93

198 8 134 2 354 16 400 12

PIH, Pregnancy-induced hypertension. *Umbilical artery buffer base < 34 mmol/L.

or neonatal death; (2) maternal medical complications, including hypertension, renal disease, diabetes, col- lagen disease, respiratory disease, endocrinopathies, and severe acute infections during pregnancy; (3) ob- stetric complications, including pregnancy-induced hy- pertension or preeclampsia and antepartum hemor- rhage; and (4) fetal complications, including major congenital anomalies, fetal growth retardation, and oli- gohydramnios or polyhydramnios. One or more an- tepartum risk factors were present in 685 pregnancies, or 36% of the total population.

The intrapartum risk factors were (1) preterm or postterm labor, (2) meconium in the amniotic fluid, and (3) abnormal labor, including prolonged labor, unfavor- able progress in labor, or a major malpresentation. One or more antepartum or intrapartum risk factor were present in 1115 pregnancies, or 58% of the study population.

The predictive value of antepartum risk factors and antepartum plus intrapartum risk factors is presented in Table I. Fifty percent of the pregnancies with intra- partum fetal asphyxia with metabolic acidosis at deliv- ery had a risk factor before the onset of labor, increas-

ing to 77% at delivery. Risk was determined by a wide range of clinical complications with no single risk factor demonstrating a strong association with intrapartum fetal asphyxia. However, one in four cases of intrapar- turn fetal asphyxia occurred in a pregnancy with no risk factors.

The positive predictive value of both antepartum and intrapartum risk factors for intrapartum fetal asphyxia was 3%. Table II demonstrates that this low predictive value was true for all risk factors. Thus clinical risk scoring has a major problem with regard to false- positive results in the prediction of intrapartum fetal asphyxia.

Predictive value of clinical risk scoring and fetal

assessment. The predictive value of clinical risk factors with periodic fetal assessment was examined in a second population of 100 consecutive pregnancies with bio- chemically determined intrapartum fetal asphyxia (i.e., an umbilical artery buffer base < 34 mmol/L [equivalent to a base deficit greater than 12 mmol/L]). These oc- curred in a series of 4368 pregnancies with umbilical cord blood gas and acid base assessment at delivery, an incidence of 2.3%.

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Low et al. 803

Table III. Clinical complications and fetal assessment in antepartum and intrapartum periods in obstetric patients in antepartum risk group, intrapartum risk group, and no antepartum or intrapartum risk group

Antepartum risk Intrapartum risk No antepartum or (n = 40) (n = 25) intrapartum risk (n = 35)

Antepartum Clinical

Medical Obstetric Fetal

BPP 26 16

NST Reactive Nonreactive

Intrapartum Clinical

Pretermiposterm Meconium

NST Reactive Nonreactive

22 16

8

9 2

12 12

12 13

1 0 4 4 6 0

9 17

0 0

BPP, Biophysical profile.

The documented clinical risk factors included the obstetric history and maternal, obstetric, fetal, and

labor complications recorded in the first study. An- tepartum biophysical profiles and nonstress tests (NSTs) were performed as requested by the attending physician. Biophysical profiles were scored as follows: fetal breathing, 2; fetal movement, 2; fetal tone, 2; and amniotic fluid volume, 2; for a total score of 8. NSTs included 20 or 40 minutes of fetal heart rate recording. A NST was classified as reactive on the basis of at least two accelerations of 15 beats/min in a 20-minute pe- riod. Some patients had an admissions test early in labor of 1 hour of fetal heart rate recording. This was scored as an NST. The association between clinical risk factors and the periodic fetal assessments and intrapar- turn fetal asphyxia is presented in Table III. There were 40 pregnancies in the antepartum risk group. This classification was determined on the basis of a wide range of clinical risk factors and abnormal fetal assess- ments. The designation was based on abnormal fetal assessment alone in five cases. There were 25 pregnan- cies in the intrapartum risk group. This designation was based on an abnormal admissions test alone in four cases. There were 35 pregnancies with no clinical risk factors or abnormal fetal assessments. The degree of metabolic acidosis at delivery was of the same order in the three groups (Table IV).

Table IV; Degree of metabolic acidosis in three risk groups

,-_J

Antepartum risk 40 31.2 3.7 Intrapartum risk 25 30.8 3.5 No antepartum or 35 31.6 2.4

intrapartum risk

significant proportion of intrapartum fetal asphyxia occurs in pregnancies with no risk factors. This group is equally important, with metabolic acidosis at delivery of the same order as in the pregnancies with risk factors. This is in keeping with the earlier observation of preg- nancies with no risk factors in the perinatal mortality study with neuropathologic features related to as- phyxial insults.

Comment

Risk factors were present in approximately half the pregnancies at the onset of labor and in two thirds of the pregnancies before delivery. There were a wide range of risk factors. No individual r&k factor demon- strated a strong association with intrapartum fetal as- phyxia. Biophysical profiles and NSTs were periodic assessments that reflect the fetal state at the time of the examination. These tests, as presently used, may add modestly to the prediction of intrapartum fetal as- phyxia.

The limitations of clinical risk scoring and periodic Risk scoring systems have been used for many pur- fetal assessment in the prediction of intrapartum fetal poses. One of the earliest was the prediction of perina- asphyxia is apparent. Both studies demonstrate that a tal mortality. A review of these studies indicates that to

804 Low et al. March 1995 Am J Obstet Gynecol

achieve a reasonable sensitivity in the prediction of perinatal mortality it was necessary for the risk scoring systems to identify a large proportion (i.e., >50%) of the obstetric population to be at risk.H-” On the basis of the selected risk factors used in our studies, the pro- portion of pregnancies with risk factors before the onset of labor was 36% and before delivery 58%. The inci- dence of intrapartum fetal asphyxia is approximately 2%. The result is a low positive predictive value of 3% and a large number of false positives that require clarification.

These findings highlight some of the problems facing the clinician in the diagnosis of fetal asphyxia during labor and delivery. Our objective is to assure the early recognition of intrapartum asphyxia during labor to avoid mortality or morbidity. To achieve this goal, screening methods must be developed to identify those asphyxial events occurring in pregnancies with no clini- cal risk factors, and diagnostic methods such as elec- tronic fetal heart rate monitoring, fetal electrocardio- graphic monitoring, or continuous recording of fetal blood gases must be improved to increase the sensitivity and decrease the false positives in risk pregnancies.

Until this goal is achieved the only definitive method to assure the diagnosis of intrapartum fetal asphyxia is routine umbilical cord blood gas analysis at delivery. This permits the nursery to be forewarned of a possible problemwhen a significant metabolic acidosis is present. It also serves to confirm that fetal asphyxia during labor did not occur in 98% of the babies delivered.

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