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Clinical significance of fetal heart rate tracings during the second stage of labor
Clinical Significance of Fetal Heart Rate Tracings During the Second Stage of Labor EYAL SHEINER, MD, AMNON HADAR, MD, MORDECHAI HALLAK, MD, MPA, MIRIAM KATZ, MD, MOSHE MAZOR, MD, AND ILANA SHOHAM-VARDI, PhD Objective: To examine the significance of abnormal fetal heart rate (FHR) patterns during the second stage of labor in terms of pregnancy outcome. Methods: A prospective observational study comparing women who had abnormal FHR patterns during the second stage of labor with women who demonstrated normal FHR patterns. Results: Abnormal second-stage FHR patterns were found in 420 tracings (75%), whereas 140 tracings (25%) were normal. In a multivariable analysis, nulliparity (odds ratio [OR] 2.5; 95% confidence interval [CI] 1.5, 4.2), cord problems (OR 1.8; 95% CI 1.03, 3.3), and male sex (OR 1.5; 95% CI 1.01, 2.2) were independent factors affecting the occurrence of abnormal second-stage FHR patterns. Patients with abnormal tracings had significantly higher rates of operative delivery compared with patients with normal tracings. The newborns from the case group had significantly higher percentages of Apgar scores lower than 7 at 1 minute, arterial pH lower than 7.2, and base deficit of 12 mmol/L or higher, and six were admitted to the intensive care unit (ICU). A multiple logistic regression model found second-stage late decelerations, bradycardia less than 70 beats per minute, and the presence of abnormal FHR patterns during the first stage of labor to be independently associated with fetal acidosis (determined by pH less than 7.2 and base deficit greater than 12 mmol/L). Conclusion: Late decelerations, bradycardia less than 70 beats per minute, and abnormal FHR patterns during the first stage of labor might jeopardize fetal well-being, and expedited delivery should be considered. (Obstet Gynecol 2001;97:747–52. © 2001 by The American College of Obstetricians and Gynecologists.)
The importance of fetal heart rate (FHR) monitoring during the first stage of labor has been carefully investigated, and the normal and abnormal patterns have been characterized.1 However, the importance of abnorFrom the Department of Obstetrics and Gynecology and the Epidemiology and Health Services Evaluation Department, Faculty of Health Sciences, Soroka University Medical Center, Ben Gurion University of the Negev, Beer-Sheva, Israel.
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mal FHR patterns during the second stage of labor is unclear. In 1962, Brady et al2 were the first to note extreme changes in FHR patterns during the second stage of labor. Later, Boehm3 described 18 healthy newborns who had demonstrated abnormal FHR patterns during the last 10 minutes before delivery. However, in 1981, Herbert and Boehm4 reported 18 additional newborns with abnormal monitor patterns during the second stage of labor. In this series, there was one intrapartum death and another newborn died after birth. These studies indicate the unpredictable nature of abnormal patterns during the second stage of labor. The second stage of labor is characterized by immense maternal effort from intense pushing, so the illustrative quality of the FHR tracings may be reduced. Even when monitoring is technically satisfactory, the precise meaning of decelerations during the second stage of labor is unclear. Although Katz et al5 found second-stage decelerations in only 3.4% of 1593 deliveries, Krebs et al6 showed abnormal FHR patterns in 91% of 1755 monitor tracings. Moreover, Melchior and Bernard,7 while analyzing 7000 FHR tracings, reported only a small percentage as normal. The high incidence of decelerations on the one hand and the generally normal outcomes (ie, Apgar scores and pH) on the other reduce the ability to correctly predict outcome and hence to manage labor accordingly. The present study attempts to examine the importance of abnormal FHR patterns during the second stage of labor in terms of pregnancy outcome.
Materials and Methods We performed a prospective observational study to compare parturients who showed abnormal FHR patterns during the second stage of labor with parturients who demonstrated normal FHR patterns. The study included 601 consecutive low-risk patients.
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Deliveries occurred at the Department of Obstetrics and Gynecology of the Soroka University Medical Center between January and June 2000. Subjects were eligible if they presented with normal FHR patterns (normal FHR variability, presence of accelerations, and absence of decelerations)8 and normal biophysical profile on admission and if they met the following inclusion criteria: singleton gestation, vertex presentation, and term delivery (greater than 37 completed weeks’ gestation). Monitoring during labor and obtaining umbilical artery pH values after birth are standard practice at our institution, as in some other centers,9 unless there are technical problems. Women were excluded if they had technically uninterpretable tracings; if they required an immediate cesarean because of maternal or fetal indications, such as clinical evidence of cephalopelvic disproportion or placental insufficiency; if they were status post-cesarean (which increases the risk for a subsequent one); if they had preexisting heart or lung disease; and if they were carrying fetuses with known growth restriction or malformations. Data were collected prospectively. Information was obtained upon admission to the labor and delivery suite regarding demographic and obstetric characteristics, including maternal age, gravidity, parity, gestational age, ethnicity (Jewish or Bedouin Arab), and outcome of previous pregnancies. Other variables recorded were the use of prenatal care, pregnancy-induced hypertension, gestational diabetes, hydramnios (amniotic fluid index [AFI] greater than 24 cm), oligohydramnios (AFI less than 5 cm), and premature rupture of membranes. Pregnancy-induced hypertension was defined by the classification of Davey and MacGillivray.10 Patients were defined as having gestational diabetes (class A) or pregestational diabetes (class B-R) according to the modified White classification.11 The mode of labor induction or augmentation, if any, was recorded (ie, Foley catheter, prostaglandins, oxytocin, or amniotomy), and if anesthesia was given, the method was investigated as well (ie, parenteral meperidine hydrochloride or epidural anesthesia). The presence of meconium-stained amniotic fluid (AF) and its characteristics were documented, including thin (greenish or yellowish color) or thick (viscous, tenacious, and opaque). The delivery-room team examined the patients and the monitor patterns hourly and documented their findings. The same obstetrician collected the data after carefully evaluating both the monitor files and the flow charts. Tracings were interpreted using the guidelines of the National Institute of Child Health and Human Development Research Planning Workshop.8 The cumulative depth of decelerations or bradycardia was classified by a nadir of less than 100 but at least 70 beats
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per minute, and decelerations with a nadir less than 70 beats per minute.12 The importance of prospective evaluation of tracings was demonstrated by Zain et al,13 who reported the bias of obstetricians who were aware of adverse neonatal outcomes. On a sample of 52 randomly selected tracings, we performed a reevaluation study of intraobserver and interobserver agreement between two obstetricians to confirm the reliability of interpretation, and the kappa coefficients were calculated. Information was collected about labor duration, performance of an episiotomy, mode of delivery (spontaneous, vacuum, or cesarean), neonatal sex, birth weight, presence of cord problems (nuchal cord or true knot of the cord), Apgar scores, and acid-base status (in particular, metabolic acidosis). The umbilical cord was clamped immediately after delivery. Arterial blood was drawn into a 2-mL plastic syringe that was flushed with heparin, and then transferred to the pH machine located in the delivery ward. The pH was considered abnormal when it was lower than 7.2. Base deficit of 12 mmol/L or greater was considered the threshold of fetal metabolic acidosis at delivery.14 Newborn morbidity included admission to the intensive care unit or delayed discharge from the hospital because of fetal indications. The local ethics institutional review board approved the study. Comparison of group means was performed with the SPSS version 8.0 statistical package (SPSS Inc., Chicago, IL). Chi-square or Fisher exact tests were used for comparison of proportions. Student t tests were applied for comparison of means. P ⬍ .05 was considered statistically significant. Multiple logistic regression models were used to investigate the independent contributions of obstetric factors to abnormal FHR patterns during the second stage of labor and to investigate the contributions of those patterns to selected fetal outcomes. Odds ratios (ORs) and their 95% confidence intervals (CIs) were calculated from the regression coefficients.
Results Six hundred one consecutive FHR tracings qualified for inclusion and represented the study population. Fortyone women were operated on because of nonreassuring FHR patterns during the first stage of labor and did not reach the second stage of labor. A total of 420 patients (75%) had shown one or more abnormal patterns, whereas 140 patients (25%) had normal patterns. Interobserver and intraobserver agreements of the categoric data were good, as indicated by a kappa of 0.71 and 0.86, respectively. The demographic and clinical characteristics of pa-
Obstetrics & Gynecology
Table 1. Demographic and Clinical Characteristics of Patients With Normal and Abnormal Fetal Heart Rate Patterns During the Second Stage of Labor
Characteristic Ethnic origin Jewish Bedouin Maternal age (y) ⬍20 20 –29 30 –39 ⱖ40 Gravidity 1 2–5 ⱖ6 Parity 1 2–5 ⱖ6 Gestational age (wk) Birth weight (g) Fetal sex Male Female
Abnormal FHR patterns (n ⫽ 420)
Normal FHR patterns (n ⫽ 140)
P
236 (56.2) 184 (43.6)
76 (54.3) 64 (45.7)
11 (2.6) 247 (58.8) 144 (34.3) 18 (4.3)
3 (2.1) 68 (48.6) 65 (46.4) 4 (2.9)
.081
105 (25.0) 228 (54.3) 87 (20.7)
17 (12.1) 92 (65.7) 31 (22.1)
.005
126 (30.1) 231 (55.1) 62 (14.8) 39.39 ⫾ 1.37 3217.92 ⫾ 466.58
20 (14.3) 95 (67.9) 25 (17.9) 39.34 ⫾ 1.33 3298.57 ⫾ 430.74
228 (54.3) 192 (45.7)
60 (42.9) 80 (57.1)
.694
.001 .681 .072
.019
FHR ⫽ fetal heart rate. Data are presented as n (%) or mean ⫾ standard deviation.
tients with abnormal FHR tracings (case group) and of women with normal FHR tracings (comparison group) are presented in Table 1. Women with abnormal FHR patterns were of significantly lower birth order and were more often carrying male fetuses compared with women with normal FHR patterns. Table 2 shows obstetric risk factors of the two groups. The patients with abnormal FHR tracings during the Table 2. Obstetric Factors of Patients With Normal and Abnormal Fetal Heart Rate Patterns During the Second Stage of Labor
Characteristic Lack of prenatal care Pregnancy-induced hypertension Gestational diabetes mellitus Oligohydramnios Hydramnios Foley catheter induction Prostaglandin induction Oxytocin augmentation Amniotomy Premature rupture of membranes
Abnormal FHR Normal FHR patterns patterns (n ⫽ 140) (n ⫽ 420) 35 (8.3) 9 (2.1) 19 (4.5) 19 (4.5) 15 (3.6) 16 (3.8) 2 (0.5) 46 (11.0) 118 (28.3) 97 (23.1)
Abbreviation as in Table 1. Data are presented as n (%).
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11 (7.9) 4 (2.9) 8 (5.7) 1 (0.7) 1 (0.7) 4 (2.9) 2 (2.1) 12 (8.6) 48 (34.3) 28 (20.0)
P .858 .713 .695 .031 .054 .575 .086 .432 .477 .446
Table 3. Labor Characteristics and Fetal Outcomes of Patients With Normal and Abnormal Fetal Heart Rate Patterns During the Second Stage of Labor
Characteristic Abnormal tracing during first stage Anesthesia None Epidural Parenteral Amniotic fluid Clear Bloody Thin meconium Thick meconium First-stage duration (h) Second-stage duration (min) Episiotomy Mode of delivery Spontaneous Cesarean Vacuum Cord problems None Nuchal cord True knot of cord Apgar score 1 min ⬍7 5 min ⬍7 Arterial pH ⬍7.2 ⬍7.1 Base deficit ⱖ12 (mmol/L) Birth weight (g) Fetal complications
Abnormal FHR patterns (n ⫽ 420)
Normal FHR patterns (n ⫽ 140)
P
254 (62.4)
9 (6.4)
⬍.001
172 (41.0) 69 (16.4) 179 (42.6)
75 (53.6) 18 (12.9) 47 (33.6)
.034
330 (78.6) 9 (2.1) 58 (13.8) 23 (5.5) 3.46 ⫾ 2.51 29.41 ⫾ 37.84
118 (84.3) 4 (2.9) 17 (12.1) 1 (0.7) 3.09 ⫾ 2.32 20.21 ⫾ 33.49
.048 .124 .011
142 (33.8)
25 (17.9)
.001
359 (85.5) 22 (5.2) 39 (9.3)
131 (93.6) 4 (2.9) 5 (3.6)
.040
340 (80.9) 70 (16.7) 10 (2.4)
124 (88.6) 16 (11.4)
22 (5.2) 3 (0.7)
2 (1.4)
.032 .188
54 (12.9) 11 (2.6) 26 (6.2)
3 (2.1)
⬍.001 .040 .011
.050
2 (1.4)
3217.92 ⫾ 466.58 3298.57 ⫾ 430.74 6 (1.4)
.072 .155
Abbreviation as in Table 1. Data are presented as n (%) or mean ⫾ standard deviation.
second stage of labor had a significantly higher rate of oligohydramnios and a nonsignificantly higher rate of hydramnios. No other significant differences were seen between the groups for the risk factors listed in the table. Table 3 compares labor complications and perinatal outcomes in both groups. Parturients with abnormal FHR patterns had significantly higher rates of abnormal tracings during the first stage of labor, longer second stage of labor, meconium-stained AF, episiotomies, cord problems, and either cesarean or operative vaginal deliveries compared with those with normal fetal monitor tracings. Significantly more women with abnormal FHR patterns received epidural and parenteral anesthesia compared with women with normal FHR tracings. The newborns from the case group had significantly higher percentages of Apgar scores lower than 7 at 1
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Table 4. Factors Associated With Abnormal Fetal Heart Rate Patterns During the Second Stage of Labor Characteristic
OR
95% CI
P
Nulliparity Cord problems Male fetus
2.5 1.8 1.5
1.5, 4.2 1.03, 3.3 1.01, 2.2
⬍.001 .042 .044
OR ⫽ odds ratio; CI ⫽ confidence interval. Results are from a multiple logistic regression model. Variables were entered by forward selection into the model. The criterion for selection was P ⬍ .05.
minute, arterial pH lower than 7.2, and a base deficit of 12 mmol/L or higher. Six neonates were admitted to the ICU and had a delayed discharge from the hospital. All belonged to the case group: Two had meconium aspiration syndrome, three had sepsis, and one had a previously undetected meningomyelocele. To assess independent factors associated with abnormal FHR patterns during the second stage of labor, we constructed a multiple logistic regression model (Table 4). The model included all relevant variables that were shown to be significantly associated with abnormal FHR patterns in the univariate analysis: duration of the second stage (minutes), fetal sex (male or female), epidural anesthesia, meperidine hydrochloride use, cord problems (nuchal cord or true knot of the cord), nulliparity, meconium-stained AF, and oligohydramnios. Variables were entered by forward selection into the model. The criterion for selection was P ⬍ .05. Nulliparity, cord problems, and male sex were found to be statistically significant factors associated with the occurrence of abnormal FHR tracings during the second stage of labor. Table 5 presents the frequencies of abnormal FHR patterns during the second stage of labor. A separate analysis was done to investigate monitor patterns associated with fetal acidosis (Table 6). Only 11 cases of arterial pH lower than 7.1 were found in the study, so the outcome variables selected were arterial pH lower than 7.2 and a base deficit of 12 mmol/L or greater. Late decelerations and deep variable decelerations (less than 70 beats per minute) were found to be significant
Table 5. Frequencies of Pathologic Fetal Heart Rate Patterns During the Second Stage of Labor Characteristic
n
%
Variable decelerations ⱖ70 bpm Variable decelerations ⬍70 bpm Late decelerations Bradycardia ⱖ70 bpm Bradycardia ⬍70 bpm Reduced variability
264 36 20 55 21 43
62.8 8.6 4.8 13.1 5.0 10.2
bpm ⫽ beats per minute.
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Table 6. Pathologic Fetal Heart Rate Patterns During the Second Stage of Labor (Compared With Normal Tracings) Associated With pH Less Than 7.2 and With Base Deficit of 12 mmol/L or More: Univariate Analysis pH ⬍7.2 (n ⫽ 57) Characteristic
OR
95% CI
P
Base deficit ⱖ12 mmol/L (n ⫽ 28) OR
95% CI
Variable decelerations 5.1 1.4, 21.4 .008 3.5 0.8, 15.8 ⱖ70 bpm Variable decelerations 16.3 3.8, 80.5 ⬍.001 10.5 1.9, 56.4 ⬍70 bpm Late decelerations 15.2 2.8, 91.4 ⬍.001 17.3 2.9, 101.9 Bradycardia ⱖ70 bpm 2.3 0.3, 17.1 .390 3.8 0.3, 44.2 Bradycardia ⬍70 bpm 26.6 5.2, 150.3 ⬍.001 5.2 0.8, 31.9 Reduced variability 2.2 0.3, 17.1 .728 5.1 0.6, 46.1
P .101 .006 .002 .282 .007 .098
Abbreviations as in Tables 4 and 5.
contributors to both outcome variables investigated. Other patterns significantly associated with pH lower than 7.2 were variable decelerations of 70 beats per minute or more and bradycardia at a rate lower than 70 beats per minute. Because several tracings demonstrated abnormal patterns during the first stage of labor and some tracings showed more than one abnormality, a multiple logistic regression model was used to analyze independent contributions of specific abnormal patterns, found to be significant in the univariate analysis, to the occurrence of fetal acidosis (determined by both arterial pH less than 7.2 and a base deficit of 12 mmol/L or more) (Table 7). The model included the variables that were statistically significant in the univariate analysis: deep variable decelerations (less than 70 beats per minute), late decelerations, and bradycardia (less than 70 beats per minute), plus abnormal patterns during the first stage of labor. Variables were entered by backward stepwise selection into the model. The criterion for selection was P ⬍ .05. Second-stage late decelerations, bradycardia of less than 70 beats per minute, and abnormal FHR
Table 7. Pathologic Fetal Heart Rate Patterns During the Second Stage of Labor (Compared With Normal Tracings) Associated With Fetal Acidosis*: Multivariable Analysis Characteristic
OR
95% CI
P
Late decelerations Abnormal tracing during first stage Bradycardia ⬍70 bpm
3.9 3.4 3.0
1.1, 13.1 1.3, 8.7 1.02, 8.6
.029 .011 .045
Abbreviations as in Tables 4 and 5. Variables were entered by backward stepwise selection into the model. The criterion for selection was P ⬍ .05. * Fetal acidosis was determined by both pH less than 7.2 and base deficit of 12 mmol/L or more (n ⫽ 28).
Obstetrics & Gynecology
patterns during the first stage of labor were found to be independently associated with fetal acidosis.
Discussion The present prospective study, performed in a cohort of low-risk parturients, found that abnormal FHR patterns during the second stage of labor were significantly associated with nulliparity, cord problems, and male sex. Interestingly, the effect of nulliparity remained significant even when duration of the second stage of labor was adjusted for in the multivariable analysis and not the length of labor per se, as was shown in other studies.15,16 Dawes et al17 analyzed the effect of fetal sex on FHR during labor and found that the response to normal labor differed between the sexes. Female fetuses had significantly faster heart rates than male fetuses. In addition, Blickstein et al18 found a statistically significant correlation between male fetuses and true knot of the umbilical cord, two factors found to be independently associated with abnormal FHR patterns during the second stage of labor in our study. These findings further support the conclusion of Lau et al19 that male fetuses are at higher risk for developing distress during labor. Indeed, Lieberman et al20 found a 70% increase in cesarean deliveries for fetal distress in women carrying male fetuses. In that study, male infants delivered by cesarean for fetal distress also had lower Apgar scores than female infants delivered for the same indication. The authors suggested that developmental differences between male and female fetuses could be responsible for these findings. Controversy exists in the medical literature regarding the ability of abnormal FHR patterns during the second stage of labor to predict fetal well-being. Whereas several authors found abnormal FHR patterns during the second stage to be significantly associated with low Apgar scores and a pH less than 7.2,6,21–23 others did not.24,25 However, the abnormal patterns had led to higher rates of operative vaginal and cesarean deliveries, even in the latter studies.24,25 In this observational study, interventions were used in response to the abnormalities of the FHR tracings, thereby confounding analysis of the correlation between the outcome and the observed abnormality. However, patients with abnormal tracings during the second stage of labor had significantly higher rates of operative vaginal and cesarean deliveries. Moreover, these newborns had higher rates of pH lower than 7.2 and a base deficit of 12 mmol/L or higher. Given that most fetuses were shown to tolerate acidemia during labor with a pH cutoff of 7.0 without incurring neurologic impairment,26,27 the clinical significance of these findings is questionable. In-
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deed, in this low-risk population, we had small numbers of complications. Only six of 11 neonates with a cord pH less than 7.10 were admitted to the ICU and had complications necessitating prolonged hospitalization. However, although the positive predictive value of fetal monitoring is low, patients showing normal FHR patterns during the second stage of labor have a negative predictive value of 98.6% for a base deficit lower than 12 mmol/L. Monitor patterns found to be independently associated with fetal acidosis (determined by both pH less than 7.2 and a base deficit of 12 mmol/L or more) in our study were late decelerations, bradycardia less than 70 beats per minute in the second stage of labor, and the presence of abnormal FHR patterns during the first stage. These patterns might jeopardize fetal well-being, and thus an expedited delivery should be considered. Katz et al5 found that the end-stage stress is particularly dangerous to fetuses that showed pathologic FHR patterns earlier in labor. Late decelerations are considered to be the consequence of uteroplacental insufficiency resulting in fetal acidemia.28 Last, in agreement with our results, a significant association was shown by several studies between bradycardia and a pH lower than 7.2.6,21,22 Several studies have attempted to find a direct cause for abnormal FHR patterns during the second stage of labor. Boehm3 observed cord entanglement in most cases of abnormal tracings and concluded that cord compression was the cause of the decelerations. Other authors concluded that second-stage decelerations are probably from dural pressure or fetal cerebral hypoxia, causing a vagal response mediated through chemoreceptors and baroreceptors.29 In our low-risk study population, only 19% of the patients with abnormal monitor tracings had cord problems, suggesting cord pathology as an independent contributor, but not as a major cause of abnormal FHR tracings.
References 1. Goodlin RC. History of fetal monitoring. Am J Obstet Gynecol 1979;133:323–52. 2. Brady JP, James LS, Backer MA. Heart rate changes in the fetus and newborn infant during labor. Am J Obstet Gynecol 1962;84:1–11. 3. Boehm FH. Prolonged end stage fetal heart rate decelerations. Obstet Gynecol 1975;45:579 – 82. 4. Herbert CM, Boehm FH. Prolonged end stage fetal heart rate deceleration: A reanalysis. Obstet Gynecol 1981;57:589 –93. 5. Katz M, Shani N, Meizner I, Insler V. Is end-stage deceleration of the fetal heart ominous? Br J Obstet Gynaecol 1982;89:186 –9. 6. Krebs HB, Petres RE, Dunn LJ. Intrapartum fetal heart rate monitoring. V. Fetal heart rate patterns in the second stage of labor. Am J Obstet Gynecol 1981;140:435–9. 7. Melchior J, Bernard N. Incidence and pattern of fetal heart rate
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alterations during labor. In: Kunzel W, ed. Fetal heart rate monitoring. Heidelberg: Springer-Verlag, 1985:73– 81. National Institute of Child Health and Human Development Research Planning Workshop. Electronic fetal heart rate monitoring: Research guidelines for interpretation. Am J Obstet Gynecol 1997;177:1385–90. Johnson JW, Richards DS, Wagaman RA. The case for routine umbilical blood acid-base studies at delivery. Am J Obstet Gynecol 1990;162:621–5. Davey DA, MacGillivray I. The classification and definition of hypertensive disorders of pregnancy. Am J Obstet Gynecol 1988; 158:892– 8. White P. Classification of obstetric diabetes. Am J Obstet Gynecol 1978;130:228 –30. Spong CY, Rasul C, Collea JV, Eglinton GS, Ghidini A. Characterization and prognostic significance of variable decelerations in the second stage of labor. Am J Perinatol 1998;15:369 –74. Zain HA, Wright JW, Parrish GE, Diehl SJ. Interpreting the fetal heart rate tracing: Effect of knowledge of neonatal outcome. J Reprod Med 1998;43:367–70. Low JA, Lindsay BG, Derrick EJ. Threshold of metabolic acidosis associated with newborn complications. Am J Obstet Gynecol 1997;177:1391– 4. Katz M, Lunenfeld E, Meizner I, Bashan N, Gross J. The effect of the duration of the second stage of labor on acid-base state of the fetus. Br J Obstet Gynaecol 1987;94:425–30. Nordstrom L, Malcus P, Chua S, Shimojo N, Arulkumaran S. Lactate and acid base balance at delivery in relation to cardiotocography and T/QRS ratios in the second stage of labour. Eur J Obstet Gynecol Reprod Biol 1998;76:157– 60. Dawes NW, Dawes GS, Moulden M, Redman CWG. Fetal heart rate patterns in term labor vary with sex, gestational age, epidural analgesia, and fetal weight. Am J Obstet Gynecol 1999;180:181–7. Blickstein I, Shoham-Schwartz Z, Lancet M. Predisposing factors in the formation of true knots of the umbilical cord—analysis of morphometric and perinatal data. Int J Gynaecol Obstet 1987;25: 395– 8. Lau TK, Chung KH, Haines CJ, Chang AM. Fetal sex as a risk factor for fetal distress leading to abdominal delivery. Aust N Z J Obstet Gynaecol 1996;36:146 –9. Lieberman E, Lang JM, Cohen AP, Frigoletto FD Jr, Acker D, Rao R. The association of fetal sex with the rate of cesarean section. Am J Obstet Gynecol 1997;176:667–71. Gilstrap LC, Hauth JC, Hankins GD, Beck AW. Second stage fetal
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heart rate abnormalities and type of neonatal acidemia. Obstet Gynecol 1987;70:191–5. Piquard F, Hsiung R, Mettauer M, Schaefer A, Haberey P, Dellenbach P. The validity of fetal heart rate monitoring during the second stage of labor. Obstet Gynecol 1988;72:746 –51. Gull I, Jaffa AJ, Oren M, Grisaru D, Peyser MR, Lessing JB. Acid accumulation during end-stage bradycardia in term fetuses: How long is too long? Br J Obstet Gynaecol 1996;103:1096 –101. Loghis C, Salamalekis E, Panayotopoulos N, Vitoratos N, Zourlas PA. The effect of early second stage bradycardia on newborn status. Eur J Obstet Gynecol Reprod Biol 1997;72:149 –52. Wu RW, Chen CP, Wang KG. Implications of prolonged fetal heart rate deceleration during the second stage of labor. J Formos Med Assoc 1996;95:231–5. Freeman JM, Nelson KB. Intrapartum asphyxia and cerebral palsy. Pediatrics 1988;82:240 –9. Gilstrap LC, Leveno KJ, Burris J, Williams ML, Little BB. Diagnosis of birth asphyxia based on fetal pH, Apgar score, and newborn cerebral dysfunction. Am J Obstet Gynecol 1989;161:825–30. Murata Y, Martin CB, Ikenoue T, Hashimoto T, Taira S, Sagawa T, et al. Fetal heart rate accelerations and late decelerations during the course of intrauterine death in chronically catheterized rhesus monkeys. Am J Obstet Gynecol 1982;144:218 –23. Ball RH, Parer JT. The physiologic mechanisms of variable decelerations. Am J Obstet Gynecol 1992;166:1683– 8.
Address reprint requests to:
Eyal Sheiner, MD Department of Obstetrics and Gynecology Soroka University Medical Center Ben Gurion University of the Negev Faculty of Health Sciences P.O. Box 151 Beer-Sheva, 84101 Israel E-mail: [email protected]
Received September 11, 2000. Received in revised form December 4, 2000. Accepted December 15, 2000. Copyright © 2001 by The American College of Obstetricians and Gynecologists. Published by Elsevier Science Inc.
Obstetrics & Gynecology
The importance of fetal heart rate (FHR) monitoring during the first stage of labor has been carefully investigated, and the normal and abnormal patterns have been characterized.1 However, the importance of abnorFrom the Department of Obstetrics and Gynecology and the Epidemiology and Health Services Evaluation Department, Faculty of Health Sciences, Soroka University Medical Center, Ben Gurion University of the Negev, Beer-Sheva, Israel.
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mal FHR patterns during the second stage of labor is unclear. In 1962, Brady et al2 were the first to note extreme changes in FHR patterns during the second stage of labor. Later, Boehm3 described 18 healthy newborns who had demonstrated abnormal FHR patterns during the last 10 minutes before delivery. However, in 1981, Herbert and Boehm4 reported 18 additional newborns with abnormal monitor patterns during the second stage of labor. In this series, there was one intrapartum death and another newborn died after birth. These studies indicate the unpredictable nature of abnormal patterns during the second stage of labor. The second stage of labor is characterized by immense maternal effort from intense pushing, so the illustrative quality of the FHR tracings may be reduced. Even when monitoring is technically satisfactory, the precise meaning of decelerations during the second stage of labor is unclear. Although Katz et al5 found second-stage decelerations in only 3.4% of 1593 deliveries, Krebs et al6 showed abnormal FHR patterns in 91% of 1755 monitor tracings. Moreover, Melchior and Bernard,7 while analyzing 7000 FHR tracings, reported only a small percentage as normal. The high incidence of decelerations on the one hand and the generally normal outcomes (ie, Apgar scores and pH) on the other reduce the ability to correctly predict outcome and hence to manage labor accordingly. The present study attempts to examine the importance of abnormal FHR patterns during the second stage of labor in terms of pregnancy outcome.
Materials and Methods We performed a prospective observational study to compare parturients who showed abnormal FHR patterns during the second stage of labor with parturients who demonstrated normal FHR patterns. The study included 601 consecutive low-risk patients.
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Deliveries occurred at the Department of Obstetrics and Gynecology of the Soroka University Medical Center between January and June 2000. Subjects were eligible if they presented with normal FHR patterns (normal FHR variability, presence of accelerations, and absence of decelerations)8 and normal biophysical profile on admission and if they met the following inclusion criteria: singleton gestation, vertex presentation, and term delivery (greater than 37 completed weeks’ gestation). Monitoring during labor and obtaining umbilical artery pH values after birth are standard practice at our institution, as in some other centers,9 unless there are technical problems. Women were excluded if they had technically uninterpretable tracings; if they required an immediate cesarean because of maternal or fetal indications, such as clinical evidence of cephalopelvic disproportion or placental insufficiency; if they were status post-cesarean (which increases the risk for a subsequent one); if they had preexisting heart or lung disease; and if they were carrying fetuses with known growth restriction or malformations. Data were collected prospectively. Information was obtained upon admission to the labor and delivery suite regarding demographic and obstetric characteristics, including maternal age, gravidity, parity, gestational age, ethnicity (Jewish or Bedouin Arab), and outcome of previous pregnancies. Other variables recorded were the use of prenatal care, pregnancy-induced hypertension, gestational diabetes, hydramnios (amniotic fluid index [AFI] greater than 24 cm), oligohydramnios (AFI less than 5 cm), and premature rupture of membranes. Pregnancy-induced hypertension was defined by the classification of Davey and MacGillivray.10 Patients were defined as having gestational diabetes (class A) or pregestational diabetes (class B-R) according to the modified White classification.11 The mode of labor induction or augmentation, if any, was recorded (ie, Foley catheter, prostaglandins, oxytocin, or amniotomy), and if anesthesia was given, the method was investigated as well (ie, parenteral meperidine hydrochloride or epidural anesthesia). The presence of meconium-stained amniotic fluid (AF) and its characteristics were documented, including thin (greenish or yellowish color) or thick (viscous, tenacious, and opaque). The delivery-room team examined the patients and the monitor patterns hourly and documented their findings. The same obstetrician collected the data after carefully evaluating both the monitor files and the flow charts. Tracings were interpreted using the guidelines of the National Institute of Child Health and Human Development Research Planning Workshop.8 The cumulative depth of decelerations or bradycardia was classified by a nadir of less than 100 but at least 70 beats
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per minute, and decelerations with a nadir less than 70 beats per minute.12 The importance of prospective evaluation of tracings was demonstrated by Zain et al,13 who reported the bias of obstetricians who were aware of adverse neonatal outcomes. On a sample of 52 randomly selected tracings, we performed a reevaluation study of intraobserver and interobserver agreement between two obstetricians to confirm the reliability of interpretation, and the kappa coefficients were calculated. Information was collected about labor duration, performance of an episiotomy, mode of delivery (spontaneous, vacuum, or cesarean), neonatal sex, birth weight, presence of cord problems (nuchal cord or true knot of the cord), Apgar scores, and acid-base status (in particular, metabolic acidosis). The umbilical cord was clamped immediately after delivery. Arterial blood was drawn into a 2-mL plastic syringe that was flushed with heparin, and then transferred to the pH machine located in the delivery ward. The pH was considered abnormal when it was lower than 7.2. Base deficit of 12 mmol/L or greater was considered the threshold of fetal metabolic acidosis at delivery.14 Newborn morbidity included admission to the intensive care unit or delayed discharge from the hospital because of fetal indications. The local ethics institutional review board approved the study. Comparison of group means was performed with the SPSS version 8.0 statistical package (SPSS Inc., Chicago, IL). Chi-square or Fisher exact tests were used for comparison of proportions. Student t tests were applied for comparison of means. P ⬍ .05 was considered statistically significant. Multiple logistic regression models were used to investigate the independent contributions of obstetric factors to abnormal FHR patterns during the second stage of labor and to investigate the contributions of those patterns to selected fetal outcomes. Odds ratios (ORs) and their 95% confidence intervals (CIs) were calculated from the regression coefficients.
Results Six hundred one consecutive FHR tracings qualified for inclusion and represented the study population. Fortyone women were operated on because of nonreassuring FHR patterns during the first stage of labor and did not reach the second stage of labor. A total of 420 patients (75%) had shown one or more abnormal patterns, whereas 140 patients (25%) had normal patterns. Interobserver and intraobserver agreements of the categoric data were good, as indicated by a kappa of 0.71 and 0.86, respectively. The demographic and clinical characteristics of pa-
Obstetrics & Gynecology
Table 1. Demographic and Clinical Characteristics of Patients With Normal and Abnormal Fetal Heart Rate Patterns During the Second Stage of Labor
Characteristic Ethnic origin Jewish Bedouin Maternal age (y) ⬍20 20 –29 30 –39 ⱖ40 Gravidity 1 2–5 ⱖ6 Parity 1 2–5 ⱖ6 Gestational age (wk) Birth weight (g) Fetal sex Male Female
Abnormal FHR patterns (n ⫽ 420)
Normal FHR patterns (n ⫽ 140)
P
236 (56.2) 184 (43.6)
76 (54.3) 64 (45.7)
11 (2.6) 247 (58.8) 144 (34.3) 18 (4.3)
3 (2.1) 68 (48.6) 65 (46.4) 4 (2.9)
.081
105 (25.0) 228 (54.3) 87 (20.7)
17 (12.1) 92 (65.7) 31 (22.1)
.005
126 (30.1) 231 (55.1) 62 (14.8) 39.39 ⫾ 1.37 3217.92 ⫾ 466.58
20 (14.3) 95 (67.9) 25 (17.9) 39.34 ⫾ 1.33 3298.57 ⫾ 430.74
228 (54.3) 192 (45.7)
60 (42.9) 80 (57.1)
.694
.001 .681 .072
.019
FHR ⫽ fetal heart rate. Data are presented as n (%) or mean ⫾ standard deviation.
tients with abnormal FHR tracings (case group) and of women with normal FHR tracings (comparison group) are presented in Table 1. Women with abnormal FHR patterns were of significantly lower birth order and were more often carrying male fetuses compared with women with normal FHR patterns. Table 2 shows obstetric risk factors of the two groups. The patients with abnormal FHR tracings during the Table 2. Obstetric Factors of Patients With Normal and Abnormal Fetal Heart Rate Patterns During the Second Stage of Labor
Characteristic Lack of prenatal care Pregnancy-induced hypertension Gestational diabetes mellitus Oligohydramnios Hydramnios Foley catheter induction Prostaglandin induction Oxytocin augmentation Amniotomy Premature rupture of membranes
Abnormal FHR Normal FHR patterns patterns (n ⫽ 140) (n ⫽ 420) 35 (8.3) 9 (2.1) 19 (4.5) 19 (4.5) 15 (3.6) 16 (3.8) 2 (0.5) 46 (11.0) 118 (28.3) 97 (23.1)
Abbreviation as in Table 1. Data are presented as n (%).
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11 (7.9) 4 (2.9) 8 (5.7) 1 (0.7) 1 (0.7) 4 (2.9) 2 (2.1) 12 (8.6) 48 (34.3) 28 (20.0)
P .858 .713 .695 .031 .054 .575 .086 .432 .477 .446
Table 3. Labor Characteristics and Fetal Outcomes of Patients With Normal and Abnormal Fetal Heart Rate Patterns During the Second Stage of Labor
Characteristic Abnormal tracing during first stage Anesthesia None Epidural Parenteral Amniotic fluid Clear Bloody Thin meconium Thick meconium First-stage duration (h) Second-stage duration (min) Episiotomy Mode of delivery Spontaneous Cesarean Vacuum Cord problems None Nuchal cord True knot of cord Apgar score 1 min ⬍7 5 min ⬍7 Arterial pH ⬍7.2 ⬍7.1 Base deficit ⱖ12 (mmol/L) Birth weight (g) Fetal complications
Abnormal FHR patterns (n ⫽ 420)
Normal FHR patterns (n ⫽ 140)
P
254 (62.4)
9 (6.4)
⬍.001
172 (41.0) 69 (16.4) 179 (42.6)
75 (53.6) 18 (12.9) 47 (33.6)
.034
330 (78.6) 9 (2.1) 58 (13.8) 23 (5.5) 3.46 ⫾ 2.51 29.41 ⫾ 37.84
118 (84.3) 4 (2.9) 17 (12.1) 1 (0.7) 3.09 ⫾ 2.32 20.21 ⫾ 33.49
.048 .124 .011
142 (33.8)
25 (17.9)
.001
359 (85.5) 22 (5.2) 39 (9.3)
131 (93.6) 4 (2.9) 5 (3.6)
.040
340 (80.9) 70 (16.7) 10 (2.4)
124 (88.6) 16 (11.4)
22 (5.2) 3 (0.7)
2 (1.4)
.032 .188
54 (12.9) 11 (2.6) 26 (6.2)
3 (2.1)
⬍.001 .040 .011
.050
2 (1.4)
3217.92 ⫾ 466.58 3298.57 ⫾ 430.74 6 (1.4)
.072 .155
Abbreviation as in Table 1. Data are presented as n (%) or mean ⫾ standard deviation.
second stage of labor had a significantly higher rate of oligohydramnios and a nonsignificantly higher rate of hydramnios. No other significant differences were seen between the groups for the risk factors listed in the table. Table 3 compares labor complications and perinatal outcomes in both groups. Parturients with abnormal FHR patterns had significantly higher rates of abnormal tracings during the first stage of labor, longer second stage of labor, meconium-stained AF, episiotomies, cord problems, and either cesarean or operative vaginal deliveries compared with those with normal fetal monitor tracings. Significantly more women with abnormal FHR patterns received epidural and parenteral anesthesia compared with women with normal FHR tracings. The newborns from the case group had significantly higher percentages of Apgar scores lower than 7 at 1
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Table 4. Factors Associated With Abnormal Fetal Heart Rate Patterns During the Second Stage of Labor Characteristic
OR
95% CI
P
Nulliparity Cord problems Male fetus
2.5 1.8 1.5
1.5, 4.2 1.03, 3.3 1.01, 2.2
⬍.001 .042 .044
OR ⫽ odds ratio; CI ⫽ confidence interval. Results are from a multiple logistic regression model. Variables were entered by forward selection into the model. The criterion for selection was P ⬍ .05.
minute, arterial pH lower than 7.2, and a base deficit of 12 mmol/L or higher. Six neonates were admitted to the ICU and had a delayed discharge from the hospital. All belonged to the case group: Two had meconium aspiration syndrome, three had sepsis, and one had a previously undetected meningomyelocele. To assess independent factors associated with abnormal FHR patterns during the second stage of labor, we constructed a multiple logistic regression model (Table 4). The model included all relevant variables that were shown to be significantly associated with abnormal FHR patterns in the univariate analysis: duration of the second stage (minutes), fetal sex (male or female), epidural anesthesia, meperidine hydrochloride use, cord problems (nuchal cord or true knot of the cord), nulliparity, meconium-stained AF, and oligohydramnios. Variables were entered by forward selection into the model. The criterion for selection was P ⬍ .05. Nulliparity, cord problems, and male sex were found to be statistically significant factors associated with the occurrence of abnormal FHR tracings during the second stage of labor. Table 5 presents the frequencies of abnormal FHR patterns during the second stage of labor. A separate analysis was done to investigate monitor patterns associated with fetal acidosis (Table 6). Only 11 cases of arterial pH lower than 7.1 were found in the study, so the outcome variables selected were arterial pH lower than 7.2 and a base deficit of 12 mmol/L or greater. Late decelerations and deep variable decelerations (less than 70 beats per minute) were found to be significant
Table 5. Frequencies of Pathologic Fetal Heart Rate Patterns During the Second Stage of Labor Characteristic
n
%
Variable decelerations ⱖ70 bpm Variable decelerations ⬍70 bpm Late decelerations Bradycardia ⱖ70 bpm Bradycardia ⬍70 bpm Reduced variability
264 36 20 55 21 43
62.8 8.6 4.8 13.1 5.0 10.2
bpm ⫽ beats per minute.
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Table 6. Pathologic Fetal Heart Rate Patterns During the Second Stage of Labor (Compared With Normal Tracings) Associated With pH Less Than 7.2 and With Base Deficit of 12 mmol/L or More: Univariate Analysis pH ⬍7.2 (n ⫽ 57) Characteristic
OR
95% CI
P
Base deficit ⱖ12 mmol/L (n ⫽ 28) OR
95% CI
Variable decelerations 5.1 1.4, 21.4 .008 3.5 0.8, 15.8 ⱖ70 bpm Variable decelerations 16.3 3.8, 80.5 ⬍.001 10.5 1.9, 56.4 ⬍70 bpm Late decelerations 15.2 2.8, 91.4 ⬍.001 17.3 2.9, 101.9 Bradycardia ⱖ70 bpm 2.3 0.3, 17.1 .390 3.8 0.3, 44.2 Bradycardia ⬍70 bpm 26.6 5.2, 150.3 ⬍.001 5.2 0.8, 31.9 Reduced variability 2.2 0.3, 17.1 .728 5.1 0.6, 46.1
P .101 .006 .002 .282 .007 .098
Abbreviations as in Tables 4 and 5.
contributors to both outcome variables investigated. Other patterns significantly associated with pH lower than 7.2 were variable decelerations of 70 beats per minute or more and bradycardia at a rate lower than 70 beats per minute. Because several tracings demonstrated abnormal patterns during the first stage of labor and some tracings showed more than one abnormality, a multiple logistic regression model was used to analyze independent contributions of specific abnormal patterns, found to be significant in the univariate analysis, to the occurrence of fetal acidosis (determined by both arterial pH less than 7.2 and a base deficit of 12 mmol/L or more) (Table 7). The model included the variables that were statistically significant in the univariate analysis: deep variable decelerations (less than 70 beats per minute), late decelerations, and bradycardia (less than 70 beats per minute), plus abnormal patterns during the first stage of labor. Variables were entered by backward stepwise selection into the model. The criterion for selection was P ⬍ .05. Second-stage late decelerations, bradycardia of less than 70 beats per minute, and abnormal FHR
Table 7. Pathologic Fetal Heart Rate Patterns During the Second Stage of Labor (Compared With Normal Tracings) Associated With Fetal Acidosis*: Multivariable Analysis Characteristic
OR
95% CI
P
Late decelerations Abnormal tracing during first stage Bradycardia ⬍70 bpm
3.9 3.4 3.0
1.1, 13.1 1.3, 8.7 1.02, 8.6
.029 .011 .045
Abbreviations as in Tables 4 and 5. Variables were entered by backward stepwise selection into the model. The criterion for selection was P ⬍ .05. * Fetal acidosis was determined by both pH less than 7.2 and base deficit of 12 mmol/L or more (n ⫽ 28).
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patterns during the first stage of labor were found to be independently associated with fetal acidosis.
Discussion The present prospective study, performed in a cohort of low-risk parturients, found that abnormal FHR patterns during the second stage of labor were significantly associated with nulliparity, cord problems, and male sex. Interestingly, the effect of nulliparity remained significant even when duration of the second stage of labor was adjusted for in the multivariable analysis and not the length of labor per se, as was shown in other studies.15,16 Dawes et al17 analyzed the effect of fetal sex on FHR during labor and found that the response to normal labor differed between the sexes. Female fetuses had significantly faster heart rates than male fetuses. In addition, Blickstein et al18 found a statistically significant correlation between male fetuses and true knot of the umbilical cord, two factors found to be independently associated with abnormal FHR patterns during the second stage of labor in our study. These findings further support the conclusion of Lau et al19 that male fetuses are at higher risk for developing distress during labor. Indeed, Lieberman et al20 found a 70% increase in cesarean deliveries for fetal distress in women carrying male fetuses. In that study, male infants delivered by cesarean for fetal distress also had lower Apgar scores than female infants delivered for the same indication. The authors suggested that developmental differences between male and female fetuses could be responsible for these findings. Controversy exists in the medical literature regarding the ability of abnormal FHR patterns during the second stage of labor to predict fetal well-being. Whereas several authors found abnormal FHR patterns during the second stage to be significantly associated with low Apgar scores and a pH less than 7.2,6,21–23 others did not.24,25 However, the abnormal patterns had led to higher rates of operative vaginal and cesarean deliveries, even in the latter studies.24,25 In this observational study, interventions were used in response to the abnormalities of the FHR tracings, thereby confounding analysis of the correlation between the outcome and the observed abnormality. However, patients with abnormal tracings during the second stage of labor had significantly higher rates of operative vaginal and cesarean deliveries. Moreover, these newborns had higher rates of pH lower than 7.2 and a base deficit of 12 mmol/L or higher. Given that most fetuses were shown to tolerate acidemia during labor with a pH cutoff of 7.0 without incurring neurologic impairment,26,27 the clinical significance of these findings is questionable. In-
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deed, in this low-risk population, we had small numbers of complications. Only six of 11 neonates with a cord pH less than 7.10 were admitted to the ICU and had complications necessitating prolonged hospitalization. However, although the positive predictive value of fetal monitoring is low, patients showing normal FHR patterns during the second stage of labor have a negative predictive value of 98.6% for a base deficit lower than 12 mmol/L. Monitor patterns found to be independently associated with fetal acidosis (determined by both pH less than 7.2 and a base deficit of 12 mmol/L or more) in our study were late decelerations, bradycardia less than 70 beats per minute in the second stage of labor, and the presence of abnormal FHR patterns during the first stage. These patterns might jeopardize fetal well-being, and thus an expedited delivery should be considered. Katz et al5 found that the end-stage stress is particularly dangerous to fetuses that showed pathologic FHR patterns earlier in labor. Late decelerations are considered to be the consequence of uteroplacental insufficiency resulting in fetal acidemia.28 Last, in agreement with our results, a significant association was shown by several studies between bradycardia and a pH lower than 7.2.6,21,22 Several studies have attempted to find a direct cause for abnormal FHR patterns during the second stage of labor. Boehm3 observed cord entanglement in most cases of abnormal tracings and concluded that cord compression was the cause of the decelerations. Other authors concluded that second-stage decelerations are probably from dural pressure or fetal cerebral hypoxia, causing a vagal response mediated through chemoreceptors and baroreceptors.29 In our low-risk study population, only 19% of the patients with abnormal monitor tracings had cord problems, suggesting cord pathology as an independent contributor, but not as a major cause of abnormal FHR tracings.
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Address reprint requests to:
Eyal Sheiner, MD Department of Obstetrics and Gynecology Soroka University Medical Center Ben Gurion University of the Negev Faculty of Health Sciences P.O. Box 151 Beer-Sheva, 84101 Israel E-mail: [email protected]
Received September 11, 2000. Received in revised form December 4, 2000. Accepted December 15, 2000. Copyright © 2001 by The American College of Obstetricians and Gynecologists. Published by Elsevier Science Inc.
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