Objective
To examine low maternal admission pulse pressure (PP) as a risk factor for new onset postepidural fetal heart rate (FHR) abnormalities.
Study Design
Retrospective cohort study of nulliparous, singleton, vertex-presenting women admitted to labor and delivery after 37 0/7 weeks that received an epidural during labor. Women with a low admission PP were compared with those with a normal admission PP. The primary outcome was new onset FHR abnormalities defined as recurrent late or prolonged FHR decelerations in the first hour after initial dosing of a labor epidural.
Results
New onset FHR abnormalities, defined as recurrent late decelerations and/or prolonged decelerations, occurred in 6% of subjects in the normal PP cohort compared with 27% in the low PP cohort (odds ratio, 5.6; 95% confidence interval, 2.1–14.3; P < .001). A multivariate logistic regression analysis generated an adjusted odds ratio of 28.9 (95% confidence interval, 3.7–221.4; P < .001).
Conclusion
New onset FHR abnormalities after initial labor epidural dosing occur more frequently in women with a low admission PP than those with a normal admission pulse. Admission PP appears to be a novel predictor of new onset postepidural FHR abnormalities.
Regional anesthesia is the most common labor pain management tool in contemporary obstetric practice in the United States. Labor epidurals have been implicated in some observational studies to increase the risk of certain obstetric interventions and outcomes when compared with either no analgesia or no epidural. These risks include maternal hypotension, operative vaginal delivery, maternal fever, a longer second stage of labor, and cesarean delivery for fetal distress.
In the 60 minutes after initial dosing of regional anesthesia, the frequency of maternal hypotension and fetal heart rate (FHR) changes range from 5–18% and 6–30% respectively, depending on the technique and dose of medication used. Contemporary medications, doses, and techniques for epidural placement are associated with lower rates compared with higher dosage techniques.
Limited information exists on maternal and fetal characteristics that might be risk factors for the changes in maternal vital signs and FHR after dosing of regional anesthesia. The hemodynamic effects of the labor epidural have been described in patients with assumed normal initial intravascular volume. Maternal vital signs have not been examined as indicators for subsequent FHR abnormalities and maternal hypotension after initial dosing of a labor epidural.
Initial dosing of a labor epidural affects the hemodynamic profile by altering the complex mechanisms the body uses to generate and maintain maternal blood pressure, uteroplacental blood flow, and fetal perfusion. Decreases in systemic vascular resistance and venous return, changes in circulating catecholamine levels, and changes in uterine tone are all attributed to the incidence of both FHR abnormalities and maternal hypotension. During these often abrupt and profound alterations in hemodynamics intravascular fluid volume is a relative constant. This physiologic premise is the basis for giving an intravenous (IV) fluid bolus before the initial dosing of a labor epidural as a buffer for these changes. However, a recent Cochrane review of this practice suggests that the incidence of FHR abnormalities and maternal hypotension are not reduced when compared with patients who do not receive this prophylactic IV fluid bolus.
In healthy patients, surrogate clinical indicators of intravascular fluid volume such as mean arterial pressure, heart rate, and urine output are commonly used. Pulse pressure (PP) is a hemodynamic parameter calculated by subtracting the diastolic blood pressure (DBP) from the systolic blood pressure (SBP). The use of PP as perhaps one of the earliest clinical predictors of intravascular volume status is highlighted in both observational and experimental trauma literature. In a series of 28 patients, PP was identified as the only distinguishing prehospital vital sign characteristic associated with a higher likelihood of death follow traumatic injury. This observation was supported by a more robust observational study and in physiologic simulators of central blood volume loss. Convertino and colleagues demonstrate a significant correlation between stroke volume (SV) and PP as well as a linear relationship between PP and central blood volume reductions with no differences in mean arterial pressure (MAP). PP has also been demonstrated as a clinical estimator of the intravascular fluid volume in ambulatory dialysis patients.
Decreased intravascular volume, defined as a PP <45 mm Hg, may be a risk factor for FHR abnormalities and maternal hypotension after the initial dosing of a labor epidural. Our primary objective is to determine whether a low admission PP increases the risk of postepidural FHR abnormalities.
Materials and Methods
This is a retrospective cohort study, performed with institutional review board approval (IRB # 211070) in a large military teaching medical center. We defined a value of <45 mm Hg as a low PP. It has been observed that in otherwise healthy nonpregnant patients SV is about 1.7 times the PP and in healthy pregnant women an increase in SV after 38 weeks EGA to approximately 100 mL/min has been described. Based on this information we extrapolated that a normal PP would be about 60 mm Hg at term and subsequently selected a value of <45 mm Hg as “low” and a PP of ≥45 mm Hg as “normal.”
For the power analysis we assumed an incidence of 18% (the average of reported rates) for postepidural FHR abnormalities in subjects with a normal PP (control cohort). To detect a 2-fold increase in new onset FHR abnormalities in the low PP cohort and assuming 80% power and an alpha of 0.05 a total sample size of 190 subjects was required.
To identify 95 subjects with a low admission PP who also met inclusion criteria (exposure cohort) we had to review all birth records for a 6 month period (September, 2010–February 2011). There were a total of 1008 deliveries during that 6 month period, with 258 subjects that fulfilled inclusion criteria and had a normal admission PP. We generated a random number table and used permuted blocks of 4 to select 95 of these subjects to use as the control cohort ( Figure 1 ). Inclusion criteria were as follows: age 18 years and older, nulliparity, singleton, vertex-presenting, greater than 37 0/7 weeks, and received a labor epidural.
Demographic, clinical, and laboratory data was extracted from the electronic medical record (EMR). These data included age in years, Estimated gestational age (EGA) calculated in weeks, body mass index (BMI kg/m 2 , admission diagnosis, cervical dilation, Bishop score, maternal comorbidities, and patient reported race, gravidity, and parity.
We also collected data about the placement and initial dosing of the labor epidural. Anesthesia records were reviewed to record the medication concentration and dose used for the initial epidural bolus, timing, and type of IV fluid bolus before placement and labor status at time of epidural request. All the above information was a priori identified as potential confounders to the interpretation of our primary outcome and is part of the routine documentation in our EMR.
The primary outcome, new onset FHR abnormalities, was defined as recurrent late decelerations and/or prolonged decelerations in the first 60 minutes after initial dosing of labor epidural using current National Institute of Child and Human Development (NICHD) electronic fetal monitoring definitions (FHM). The 60 minutes before and after epidural dosing were divided into discrete 20 minute segments. The fetal tracings were reviewed by 2 Association of Women’s Health, Obstetric, and Neonatal Nurses (AWHONN) FHM instructor trainers who were blinded to all subject information, purpose of the study, and admission vital signs. The reviewers used a data collection sheet to describe the FHR baseline, variability, the presence or absence of accelerations, presence or absence of decelerations, the type of decelerations, and whether the decelerations were recurrent. Contraction frequency, calculated Montevideo units and uterine resting tone (if applicable) were also described by these reviewers. If there was a discrepancy between the first 2 reviewers, the tracing was reviewed by a third AWHONN FHM instructor trainer (blinded in the same fashion). The congruent interpretation was used in the assessment of the primary outcome.
We examined several secondary outcomes. New onset maternal hypotension was defined as an absolute SBP <100 mm Hg or a ≥20% decrease in SBP, treatment for hypotension or FHR changes defined as extra measures (multiple position changes, supplemental oxygen, postepidural fluid bolus, vasoconstrictor administration, or tocolytic administration), length of the stages of labor, diagnosis of amnionitis, 1 and 5 minute APGAR scores, birthweight, and mode of delivery. If operative delivery occurred, indication for the mode of delivery was included.
Data were analyzed using PASW statistics for Windows (version 18.0; SPSS Inc, Chicago, IL). The appropriate t test for continuous variables and the appropriate χ 2 test for comparing proportions between the groups were performed. A planned multivariate logistic regression was performed for the primary outcome and for the secondary outcome of new onset maternal hypotension.
Results
From the 1008 birth records reviewed, we identified 95 who met inclusion criteria and had a low admission PP. Over the same time period, 258 patients were admitted with a normal PP and met inclusion criteria. We selected 95 control subjects as previously described ( Figure 1 ). Admission characteristics between the cohorts were similar ( Table 1 ). Subjects with a normal admission PP had a higher average admission BMI than those admitted with a low PP (32.2 ± 5.2 vs 29.3 ± 4.9, P < .01).
| Characteristic | PP ≥45 mm Hg (n = 95) | PP <45 mm Hg (n = 95) | P value |
|---|---|---|---|
| Age, y | 24.4 ± 4.8 | 24.6 ± 4.3 | .70 |
| EGA, wks | 39.3 ± 1.1 | 39.5 ±1.1 | .32 |
| Gravidity | .94 | ||
| G1 | 63 (66) | 65 (68) | |
| G2 | 24 (25) | 23 (24) | |
| ≥ G3 | 8 (8) | 7 (7) | |
| Race | .26 | ||
| White | 64 (67) | 68 (71) | |
| African American | 9 (10) | 8 (8) | |
| Hispanic | 1 (1) | 0 (0) | |
| Asian | 8 (8) | 2 (2) | |
| Other | 13 (14) | 17 (18) | |
| Admission labor diagnosis | .67 | ||
| Spontaneous | 57 (60) | 54 (57) | |
| Induction | 38 (40) | 31 (43) | |
| Maternal comorbidities | .26 | ||
| Pregestational diabetes | 0 (0) | 2 (2) | |
| Gestational diabetes | 10 (11) | 10 (11) | |
| Chronic hypertension | 7 (7) | 3 (3) | |
| Proteinuria | 3 (3) | 1 (1) | |
| Gestational hypertension | 14 (15) | 9 (9) | |
| Preeclampsia | 7 (7) | 1 (1) | |
| BMI, kg/m 2 | 32.2 ± 5.2 | 29.3 ± 4.9 | < .001 |
| Tobacco use | 8 (8) | 10 (11) | .28 |
| Cervical dilation, cm | 3 ± 1.5 | 3 ± 1.7 | .56 |
| Bishop score | 8 ± 3 | 8 ± 3 | .76 |
| Admission hematocrit, % | 36.9 ± 3.4 | 36.4 ± 3.5 | .40 |
The total volume of IV fluid given from admission and the amount given as a preload IV bolus was similar between groups. The number of subjects on oxytocin at the time of epidural dosing, frequency of cervical examinations, and indwelling bladder catheter placement within the first hour postepidural were all similar. Total time to place and dose the epidural was 14 ± 6.4 minutes in the low PP vs 17 ± 12.5 minutes for the normal PP cohort ( P = .02). Extra measures or additional interventions were provided more frequently to those subjects with a low PP than those with a normal PP. The type and dose of epidural medication bolused were similar between groups ( Table 2 ).
| Characteristic | PP ≥45 mm Hg (n = 95) | PP <45 mm Hg (n = 95) | P value |
|---|---|---|---|
| Admission to request, h | 5.9 ± 5.8 | 7.4 ± 5.2 | .054 |
| Fluid from admission to request, mL | 995 ± 886 | 925 ± 766 | .56 |
| Cervix at request | 4.1 ± 1.4 | 4.3 ± 1.3 | .26 |
| Request to placement, min | 40 ± 22 | 43 ± 22 | .37 |
| Narcotics before epidural placement | 30 (32) | 32 (34) | .78 |
| Epidural medication (concentration) | .54 | ||
| Ropivacaine (0.2%); fentanyl (2 mcg/cc) | 79 (83) | 78 (82) | |
| Initial bolus, mL | 6.4 ± 1.7 | 6.3 ± 1.6 | |
| Lidocaine (1.5%); fentanyl (2 mcg/cc) | 6 (6) | 11 (12) | |
| Initial bolus, mL | 5 ± 0 | 4.6 ± 0.5 | |
| Lidocaine (1.5%) | 4 (4) | 3 (3) | |
| Initial bolus, mL | 5 ± 0 | 5 ± 0 | |
| Fentanyl (2 mcg/cc) | 5 (5) | 3 (3) | |
| Initial bolus, mcg | 90 + 22 | 83 + 28 | |
| Bupivicaine (0.25%) | 1 (1) | 0 (0) | |
| Initial bolus, mL | 1 | — | |
| Received preepidural bolus | 92 (97) | 94 (99) | .31 |
| Volume of bolus | 918 ± 231 | 968 ± 160 | .083 |
| Type of fluid | .23 | ||
| Lactated ringers | 88 (93) | 93 (98) | |
| Hespan | 4 (4) | 1 (1) | |
| None | 3 (3) | 1 (1) | |
| Time for placement, min | 17 ± 12.5 | 14 ± 6.4 | .02 |
| Oxytocin during epidural | 38 (40) | 52 (55) | .059 |
| Foley placed in 1st hour | 63 (66) | 69 (73) | .35 |
| Cervical exam in 1st hour | 8 (8) | 10 (11) | .62 |
| Extra measures initiated | 27 (28) | 61 (64) | < .001 |
| Multiple position changes | 20 (21) | 59 (62) | < .001 |
| Supplemental oxygen | 12 (12) | 40 (42) | < .001 |
| Intravenous fluid bolus | 10 (11) | 28 (30) | < .001 |
| Vasoconstrictor or tocolytic | 6 (6) | 16 (17) | .02 |
Admission PP, SBP, and DBP were different between the 2 cohorts ( Table 3 ). At the time of epidural request and at the time of epidural timeout there were no differences in any of the vital sign parameters. However, after initial dosing of the epidural bolus, differences in vital signs were observed. The low PP group had significantly lower SBP and PP for the 60 minutes after initial epidural dosing. Figure 2 describes the SBP from admission until 60 minutes after the initial dosing of the labor epidural. This shows that despite similar mean SBP at time of epidural request and timeout there was a significant difference in the postepidural SBP values. The low PP cohort had significantly lower average MAP at 20-40 minutes. DBP and heart rate did not differ during the 60 minutes after initial epidural dosing.
| Variable | PP ≥45 mm Hg | PP <45 mm Hg | P value |
|---|---|---|---|
| Admission | n = 95 | n = 95 | |
| PP | 55 ± 10.8 | 40 ± 3.6 | < .001 |
| SBP | 132 ± 15 | 119 ± 10.0 | < .001 |
| DBP | 76 ± 10.2 | 79 ± 9.7 | .037 |
| MAP | 95 ± 10.9 | 93 ± 9.7 | .15 |
| HR | 89 ± 15.1 | 91 ± 15.5 | .24 |
| Epidural request | n = 72 | n = 78 | |
| PP | 57 ± 13.2 | 55 ± 11.3 | .3 |
| SBP | 130 ± 18.2 | 128 ± 14.6 | .58 |
| DBP | 73 ± 11.5 | 74 ± 11.5 | .53 |
| MAP | 92 ± 12.8 | 92 ± 11.3 | .97 |
| HR | 83 ± 14.6 | 84 ± 15.3 | .67 |
| Epidural timeout | n = 70 | n = 88 | |
| PP | 54 ± 11.3 | 54 ± 11.6 | .99 |
| SBP | 133 ± 13.3 | 130 ± 11.0 | .23 |
| DBP | 79 ± 11.7 | 77 ± 11.0 | .20 |
| MAP | 97 ± 11.7 | 95 ± 10.3 | .28 |
| HR | 91 ± 14.6 | 90 ± 15.4 | .66 |
| 0-20 min after epidural | n = 95 | n = 95 | |
| PP | 51 ± 9.3 | 48 ± 9.2 | .041 |
| SBP | 120 ± 15.9 | 115 ± 13.0 | .027 |
| DBP | 69 ± 11.5 | 67 ± 9.9 | .27 |
| MAP | 86 ± 12.3 | 83 ± 10.5 | .74 |
| HR | 89 ± 15.1 | 90 ± 17.0 | .53 |
| 20-40 min after epidural | n = 87 | n = 92 | |
| PP | 52 ± 9.8 | 48 ± 8.9 | < .001 |
| SBP | 119 ± 15.3 | 113 ± 10.8 | < .001 |
| DBP | 67 ± 11.7 | 65 ± 8.4 | .18 |
| MAP | 84 ± 12.1 | 81 ± 8.3 | .035 |
| HR | 85 ± 14.9 | 87 ± 16.8 | .37 |
| 40-60 min after epidural | n = 87 | n = 92 | |
| PP | 54 ± 11.1 | 49 ± 9.1 | < .001 |
| SBP | 120 ± 16.0 | 115 ± 12.9 | .03 |
| DBP | 66 ± 9.4 | 65 ± 9.1 | .99 |
| MAP | 84 ± 10.1 | 82 ± 9.6 | .27 |
| HR | 85 ± 16.1 | 88 ± 17.0 | .28 |
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