Objective
We sought to assess amniotic fluid lactate (AFL) at diagnosis of spontaneous labor at term (≥37 weeks) as a predictor of labor disorders (dystocia) and cesarean delivery (CD).
Study Design
This was a single-institution, prospective cohort study of 905 singleton, cephalic, term (≥37 weeks) nulliparous women in spontaneous labor. A standard management of labor (active management of labor) including a standard oxytocin regimen up to a maximum dose of 30 mU/min was applied. AFL was measured using a point-of-care device (LMU061; ObsteCare, Stockholm, Sweden). Labor arrest in the first stage of labor was defined as the need for oxytocin when cervical dilatation was <1 cm/h over 2 hours and in the second stage of labor by poor descent and rotation over 1 hour. Standard statistical analysis included analysis of variance, Pearson correlations, and binary logistic regression. Unsupervised decision tree analysis with 10-fold cross-validation was used to identify AFL thresholds.
Results
AFL was normally distributed and did not correlate with age, body mass index, or gestation. Unsupervised decision tree analysis demonstrated that AFL could be divided into 3 groups: 0-4.9 mmol/L (n = 118), 5.0-9.9 mmol/L (n = 707), and ≥10.0 mmol/L (n = 80). Increasing AFL was associated with higher total oxytocin dose ( P = .001), labor disorders ( P = .005), and CD ( P ≤ .001). Multivariable regression analysis demonstrated that women with AFL ≥5.0-9.9 mmol/L (odds ratio [OR], 1.6; 95% confidence interval [CI], 1.06–2.39) and AFL ≥10.0 mmol/L (OR, 1.72; 95% CI, 1.01–2.93) were independent predictors of a labor disorder. AFL ≥5.0-9.9 mmol/L did not predict CD but multivariable analysis confirmed that AFL ≥10.0 mmol/L was an independent predictor of CD (OR, 3.35; 95% CI, 1.73–6.46). AFL ≥5.0-9.9 mmol/L had a sensitivity of 89% in predicting a labor disorder and a sensitivity of 93% in predicting CD with a 97% negative predictive value. AFL ≥10.0 mmol/L was highly specific but lacked sensitivity for CD. There was no difference in birthweight of infants according to labor disorder and delivery method.
Conclusion
AFL at diagnosis of labor in spontaneously laboring single cephalic nulliparous term women is an independent predictor of a labor disorder and CD. These data suggest that women with AFL between 5.0-9.9 mmol/L with a labor disorder may be amenable to correction using the active management of labor protocol.
Cesarean delivery (CD) rates have increased significantly over the last 3 decades whereby it is now the most commonly performed surgical procedure in developed countries. There is no consensus on the appropriate rate or indications. Repeat CD remains the largest contributor to the overall CD rate. In nulliparous women, labor disorders (dystocia), defined as abnormal or difficult labor, are the most common indication for the first CD. CD is rare in women who have had a previous vaginal delivery. More focus should be placed on preventing primary CD, particularly on those women in spontaneous labor. The main causes of a labor disorder (dystocia) are inefficient uterine action, malposition, and cephalopelvic disproportion. Inefficient uterine action, also described as dysfunctional uterine action or incoordinate uterine action, refers to the frequency, duration, and force of the uterine contractions and is the most common cause of a labor disorder in nulliparous women. Uterine muscle metabolism and muscle fatigue contribute to labor disorders and subsequent CD.
Myometrial fatigue causes a switch from aerobic to anaerobic metabolism, resulting in an accumulation of intramuscular lactic acid and a subsequent increase in both intracellular and extracellular lactate levels. Increased amniotic fluid lactate (AFL) levels have previously been associated with labor disorders (dystocia), but this study was relatively small and AFL was measured when rupture of membranes occurred during labor and not at the diagnosis of labor.
The purpose of this study was to measure the AFL at the diagnosis and at each assessment of progress in the first stage of labor with last sample measured prior to delivery in an adequately powered cohort of spontaneously laboring single cephalic nulliparous at term cases and to examine labor events and outcomes so as to assess the relationship between AFL and both labor disorders and CD. This prospective cohort study was carried out in an institution where a uniform approach to management of spontaneous nulliparous labor has been used over 4 decades.
Materials and Methods
Population
The study was conducted over 9 months in 2011 through 2012. Approval from the National Maternity Hospital Ethics Committee and human subject exemption from University College Dublin was obtained. Inclusion criteria were single cephalic nulliparous term (≥37 weeks’ gestation) women in spontaneous labor (group 1, Ten-Group Classification System). All participants signed an informed consent form. Women were recruited to the study immediately following the vaginal examination confirming the diagnosis of labor.
Management of labor
Labor was diagnosed when the cervix was fully effaced and at least 1 cm dilated. Amniotomy was performed at the diagnosis of labor if spontaneous rupture of the membranes had not already occurred. In single cephalic nulliparous at term women, vaginal examinations were carried out every 2 hours to assess progress in labor. Criteria for diagnosis of labor arrest were dilation of the cervix at <1 cm/h over 2 hours in the first stage of labor and in the second stage of labor was assessed by descent and rotation of the presenting part in the pelvis. Early diagnosis of labor arrest and inadequate descent were treated with oxytocin according to hospital protocol.
Oxytocin was administered with a dose of 5 mU/min increasing to a maximum dose of 30 mU/min. The oxytocin was increased or decreased at 15-minute intervals and by 5 mU/min according to uterine activity. A maximum of 7 contractions per 15-minute interval was permitted and midwifery staff palpated and recorded the number of contractions. Increased duration of first stage of labor is associated with increased maternal and neonatal morbidity. Duration of the first stage of labor was recorded for AFL groups. Prolonged labor was defined as duration of labor lasting >720 minutes.
Data collection
Demographic and clinical data were collected from the clinical labor record and neonatal records of all women recruited to the study. All indications for CD were recorded according to a standard defined classification for intrapartum CD ( Figure 1 ).
This classification differentiates between suspected fetal distress without oxytocin as opposed to suspected fetal distress after oxytocin was started, but when the primary problem was a labor disorder. It also attempts to differentiate between different subtypes of labor disorders related to the rate of progress in labor, irrespective of the definitions and guidelines for managing labor used in different institutions. These are listed under variables, which means that it can be used universally.
Quantification of AFL levels
Amniotic fluid samples were collected at each vaginal examination to assess labor progress, including the last sample before delivery, in a clean clear tube. The samples were collected by catching the amniotic fluid sample flowing onto a sterile glove. We needed 1 mL of amniotic fluid to perform the test. All samples were immediately labeled with patient name, identification number, and the date and time of collection. Samples were stored and tested according to guidelines from the manufacture of the point-of-care device (LMU061, ObsteCare; Stockholm, Sweden). This instrument is built on patented technology and is currently the only system on the market for point-of-care use that is tailored for the measurement of lactate in amniotic fluid during labor (US patent no. US 7,318,809).
Statistical analysis
Statistical analysis was performed using software (SPSS 20; IBM Corp, Armonk, NY). Unsupervised decision tree analysis with a 10-fold cross-validation was used to identify AFL thresholds. A series of cross-tabulations and analyses of variance were performed to explore the relationships and differences between clinical characteristic variables and lactate levels. Univariate and multivariable binary logistic regression analysis were used to identify predictors of labor disorders and CD. Variables found to be significant in univariate analysis were included in the multivariable analysis. An alpha level of P < .05 was set for all statistical tests.
Power calculations
Power calculation was based on the incidence of labor disorders from the National Maternity Hospital clinical report. The assumptions (based on analysis of outcomes obtained from the clinical report) were a level of significance of .05 that 20% of the patients would present with elevated AFL and the rate of operative delivery would be 3 times higher in the AFL ≥10.0 mmol/L group. A sample of 575 patients in the study could detect a difference in CD rates between 14.2% in the elevated group and 4.7% in the control group with a power of 90% at a level of significance of .05. A sample size of 500 patients was required for the initial analysis. In all, 905 patients were recruited to the study.
Results
Recruitment process to this study is presented in Figure 2 . The study cohort (n = 905) comprised 25 nationalities with native Irish women representing 66.3% (n = 600) of the population and Eastern European women contributing 20.7% (n = 188); other European women, 4.0% (n = 37); Asian women, 3.0% (n = 27); Chinese women, 2.7% (n = 25); US women, 2.0% (n = 19); and other nationalities, 1% (n = 9). Clinical characteristics are described in Table 1 .
| Characteristic | Value |
|---|---|
| Age, y | |
| Mean (minimum-maximum) | 30 (16–44) |
| BMI, kg/m 2 | |
| Mean (minimum-maximum) | 24.2 (16.9–50.6) |
| Gestation at delivery, wk | |
| Mean completed (minimum-maximum) | 40 (37–42) |
| Amniotic fluid lactate at diagnosis of labor, mmol/L | |
| Mean (minimum-maximum) | 7.25 (1.0–20.9) |
| Amniotomy | |
| Spontaneous | 278 (31) |
| Artificial | 627 (69) |
| Cervical dilatation at diagnosis, cm | |
| Median (range) | 1 (1–10) |
| Epidural anesthesia | |
| No | 161 (18) |
| Yes | 744 (82) |
| Electronic fetal monitoring | |
| No | 168 (19) |
| Yes | 737 (81) |
| Labor disorder (dystocia) | |
| None | 368 (40) |
| First stage | 441 (49) |
| Second stage | 96 (11) |
| Oxytocin dose, mU | |
| Median (range) | 650 (0–18,7500) |
| Duration of oxytocin infusion, min | |
| Median (range) | 75 (0–737) |
| Delivery | |
| Spontaneous vaginal | 609 (67) |
| Instrumental vaginal | 235 (26) |
| Cesarean | 61 (7) |
| Duration of first stage of labor, min | |
| Mean (SD) | 300 (213.6) |
| Duration of second stage of labor, min | |
| Mean (range) | 51 (0–231) |
| Infant birthweight, g | |
| Median (range) | 3550 (2470–5220) |
To examine any relationship between AFL levels and delivery outcomes, unsupervised decision tree analysis was used to divide AFL into 3 groups: 0-4.9 mmol/L (n = 118), 5.0-9.9 mmol/L (n = 707), and ≥10.0 mmol/L (n = 80). Delivery outcomes for the 3 groups are presented in Table 2 . The rates of labor disorders (dystocia), spontaneous vaginal delivery, and CD were significantly different among the 3 groups with levels of AFL 0-4.9 mmol/L associated with a higher spontaneous delivery rate, less labor disorder, and a lower CD rate. The CD rate in the AFL ≥10.0 mmol/L group was 17.5%, almost 3 times that of the 0-4.9 mmol/L group ( Table 2 ). The mean duration of the first stage of labor was 276 minutes in the AFL 0-4.9 mmol/L group and was 77 minutes longer in the AFL ≥10.0 mmol/L group.
| Variable | AFL 0-4.9 mmol/L (n = 118) | AFL ≥5.0-9.9 mmol/L (n = 707) | AFL ≥10.0 mmol/L (n = 80) | P value |
|---|---|---|---|---|
| Mean maternal age, y (SD) | 30.1 (4.8) | 29.6 (5.1) | 29.9 (5.1) | .546 |
| Mean gestation at delivery, wk (SD) | 39.4 (0.9) | 39.7 (1.0) | 40.0 (1.0) | .046 |
| Mean cervical dilation at diagnosis, cm (SD) | 2 (1.6) | 1.8 (1.63) | 1.63 (1.06) | < .001 |
| Mean BMI, kg/m 2 (SD) | 23.7 (3.6) | 24.1 (4.0) | 25.5 (4.4) | .001 |
| SROM | ||||
| Yes | 27.0% | 32.4% | 27.5% | |
| No | 63.0% | 37.6% | 73.5% | |
| Labor disorder (dystocia) | ||||
| None | 61 (52) | 286 (41) | 21 (26) | .005 a |
| Dystocia diagnosed first stage | 46 (39) | 344 (49) | 51 (64) | |
| Dystocia diagnosed second stage | 11 (11) | 77 (11) | 8 (10) | |
| Delivery method | ||||
| Spontaneous vaginal | 87 (74) | 474 (67) | 48 (60) | .052 a |
| Forceps | 8 (7) | 77 (11) | 6 (8) | |
| Ventouse | 19 (16) | 113 (16) | 12 (15) | |
| Cesarean delivery | ||||
| Yes | 4 (3) | 43 (6) | 14 (18) | < .001 a |
| No | 114 (97) | 664 (94) | 66 (82) | |
| Reason for operative intervention | ||||
| No intervention | 87 (73) | 476 (67) | 49 (61) | .027 a |
| Fetal reason | 10 (9) | 50 (7) | 5 (6) | |
| Dystocia | 21 (18) | 181 (26) | 26 (33) |
Mean birthweight in the AFL 0-4.9 mmol/L group was 3564 g (SD 422), in AFL 5.0-9.9 mmol/L group was 3597 g (SD 468), and in the AFL ≥10.0 mmol/L group was 3585 g (SD 431). A series of Pearson correlations were performed to explore if a relationship exists among birthweight, labor disorder, and AFL at diagnosis of labor within each AFL group. There were no significant relationships found in AFL 0-4.9 mmol/L group or the AFL ≥5.0-9.9 mmol/L group. A significant relationship between birthweight and labor disorder (r = 0.263, n = 80) was found. There was also a significant relationship between birthweight and CD (r = 0.227, n = 80) found in the AFL ≥10.0 mmol/L.
Having demonstrated an association among increased AFL at diagnosis of labor, labor disorders (dystocia), and subsequent CD, we sought to further investigate the relationship between AFL and labor disorders. Univariate predictors of labor disorders included body mass index, gestational age, and cervical dilatation at presentation ( Table 3 ). AFL measured as a continuous variable was associated with a labor disorder (odds ratio [OR], 1.09; 95% confidence interval [CI], 1.02–1.16). To ascertain if AFL was an independent predictor of a labor disorder (dystocia), a multivariable binary logistic regression model using body mass index >25 or <25, gestational age, and cervical dilatation at diagnosis was employed. Multivariable analysis demonstrated that AFL, when measured as a continuous variable, was an independent predictor of a labor disorder (OR, 1.08; 95% CI, 1.01–1.16). Using the same multivariable model demonstrated that with AFL >5.0-9.9 mmol/L (OR, 1.6; 95% CI, 1.06–2.39) and AFL ≥10.0 mmol/L (OR, 1.72; 95% CI, 1.01–2.93) were independent predictors of a labor disorder (dystocia).
| Variable | Univariate | Multivariate a | Multivariate a | Multivariate a | |||||
|---|---|---|---|---|---|---|---|---|---|
| OR (95% CI) | P value | OR (95% CI) | P value | OR (95% CI) | P value | OR (95% CI) | P value | ||
| Age (continuous) | 905 | 1.02 (0.99–1.04) | .257 | ||||||
| BMI (continuous) | 862 | 1.07 (1.03–1.11) | < .001 | 1.06 (1.02–1.10) | .001 | 1.06 (1.03–1.10) | .001 | 1.06 (1.03–1.10) | .002 |
| Gestation (continuous) | 905 | 1.17 (1.03 –1.34) | .017 | 1.17 (1.02–1.34) | .028 | 1.17 (1.02–1.34) | .03 | 1.15 (1.01–1.32) | .046 |
| Cervical dilatation at diagnosis (continuous) | 905 | 0.25 (0.16–0.40) | < .001 | 0.26 (0.16–0.42) | < .001 | 0.26 (0.16–0.42) | < .001 | 0.26 (0.16–0.42) | < .001 |
| Lactate (continuous) | 905 | 1.09 (1.02–1.16) | .01 | 1.08 (1.01–1.16) | .024 | ||||
| Lactate | |||||||||
| 0-4.9 mmol/L | 118 | 1 | .31 | 1 | .23 | ||||
| ≥5 mmol/L | 787 | 1.67 (1.14–2.47) | .009 | 1.60 (1.06–2.39) | .024 | ||||
| 0-9.9 mmol/L | 825 | 1 | .14 | 1 | .15 | ||||
| ≥10 mmol/L | 80 | 2.04 (1.22–3.42) | .005 | 1.72 (1.01–2.93) | .046 | ||||
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