Pulse Pressure and Post-epidural Fetal Heart Rate Changes

Objective:

To assess the efficacy of increased IV fluid preload for the prevention of post-epidural FHR changes in women with narrow pulse pressure

Hypothesis:

In individual with narrow pulse pressure, an increased IV fluid bolus will reduce the rate of post-epidural FHR changes.

Background:

In contemporary obstetric practice, regional anesthesia is the most commonly utilized method for pain management in labor. Compared with no anesthesia, neuraxial techniques have been associated with an increased risk of maternal hypotension, operative vaginal delivery, maternal fever, and cesarean delivery for fetal distress. Following the initial dosing of regional anesthesia, maternal hypotension and FHR abnormalities occur with reported frequencies of 5-18% and 6-30% respectively.

Neuraxial anesthesia can induce a sympathetic blockade, which results in decreased maternal systemic vascular resistance and venous return. These alterations in the maternal hemodynamic profile impact maternal blood pressure, uteroplacental blood flow and fetal perfusion, and may contribute to maternal hypotension and changes in the FHR after initiation of regional anesthesia. Importantly, these hemodynamic changes generally occur in the setting of a constant maternal intravascular volume. Given that the uteroplacental circulation has limited capacity for autoregulation, uterine perfusion is sensitive to changes in both maternal blood pressure and venous return. This physiology represents the basis for the administration of an intravenous (IV) bolus prior to or during the placement of regional anesthesia .

While maternal hypotension and FHR changes after neuraxial anesthesia are common occurrences, limited information on maternal and fetal characteristics or risk factors has been published. A recent retrospective cohort study by Miller et al. reported that a narrow maternal pulse pressure was a risk factor for post-epidural FHR changes. In this cohort study, women with an admission pulse pressure < 45, compared with pulse pressure > 45, had a marked increase in the risk of post-epidural FHR changes (27% vs 6%, p < 0.001, OR 5.6 [2.1-14.3], aOR 28.9 [3.8-221.4]). Interestingly, the incidence of maternal hypotension was not different between the two pulse pressure groups (21% vs 25%, p=0.49), suggesting that fetal perfusion is sensitive to the reduced venous return associated with increased venous capacitance related to reduced sympathetic tone. Further supporting this premise, a study by Vricella et al. demonstrated an increased rate of post-epidural maternal hypotension, vasopressor support and FHR changes in women with severe preeclampsia, a clinical state of volume contraction, compared with normotensive controls. The authors concluded that the vasoconstriction and intravascular depletion central to the syndrome of severe preeclampsia make this subset of women vulnerable to post-epidural hemodynamic changes and that administration of a standard IV fluid bolus may not adequately compensate for increased venous capacitance.

Data from the critical care and trauma literature further support the use of pulse pressure as a marker of hypovolemia. Conventional static indicators of cardiac preload, such as central venous pressure (CVP) or pulmonary capillary wedge pressure (PCWP), are poor predictors of "volume responsiveness", which is the cardiovascular response (i.e. changes in preload) to intravascular volume expansion with IV fluid. However, dynamic parameters such as variation in pulse pressure or stroke volume, are highly predictive of volume responsiveness. While data on pulse pressure variation are primarily derived from non-pregnant, mechanically ventilated individuals, a small case series demonstrated that pulse pressure variation accurately reflects volume responsiveness in pregnant women undergoing cesarean delivery under regional anesthesia. Additionally, data from the trauma literature have also demonstrated pulse pressure to represent an accurate surrogate of stroke volume and marker of central hypovolemia. Covertino et al. reported a direct relationship between pulse pressure and central volume reductions without any difference in mean arterial pressure. As such, pulse pressure may serve as a better maker of central hypovolemia than blood pressure alone. Therefore, pulse pressure may better predict low maternal intravascular volume and serve as a marker for post-epidural FHR changes.

Pregnancy itself is associated with alterations in blood pressure with decreased vascular tone and widening of the pulse pressure. Normal pulse pressure and variations in pregnancy are poorly described. However, in otherwise healthy non-pregnant adults, stroke volume is approximately 1.7 times the pulse pressure. Given that the stroke volume in a term pregnancy is approximated to be 100 mL/min, a normal pulse pressure in pregnancy should be approximately 60 mmHg. Therefore, a level of 45 mmHg has been proposed to define a low pulse pressure in pregnancy.

Notably, no prospective studies have evaluated a risk-factor based approach, namely the use of maternal volume status, to guide the volume of the IV fluid bolus administered prior to neuraxial anesthesia. The investigators hypothesize that in individuals with narrow pulse pressure, an increased IV fluid bolus will reduce the rate of post-epidural FHR changes. While pulse pressure variation has been best characterized as a marker of fluid responsiveness in previously healthy critically ill patients, this parameter is only readily obtained through use of an automated device (FloTrac/Vigileo system, Edwards Lifesciences, Irvine, CA) that requires an arterial line. Furthermore, pulse pressure variation has primarily been assessed in mechanically ventilated patients, which is rare in the obstetric setting. Therefore, based on the preliminary data by Miller et al., the investigators plan to use pulse pressure (and not pulse pressure variation) as a marker of intravascular volume this study with a cut point of 45 mmHg to define low pulse pressure. Given that previous studies have used IV fluid preload doses ranging from 500-1500 mL, the IV fluid dosing regimens that will be used in this study represent variations in the standard of care. A practical study design is proposed using current practice patterns for a low risk intervention in a low risk population.

Study Design:

Prospective Randomized Controlled Trial

Study Protocol:

Women meeting eligibility criteria will be recruited and randomized in a 1:1 ratio into two study groups: standard IV fluid bolus prior to epidural placement (500 mL Lactated Ringers) or "volume replacement" IV fluid bolus of 1500mL Lactated Ringers. IV fluid boluses will be infused over 30 minutes and administered within 1 hour of epidural placement. A third group will also be recruited as a control, which will include women with a pulse pressure > 45 on admission (and otherwise meet the aforementioned inclusion/exclusion criteria). The control group will receive the standard preload bolus of 500 mL prior to epidural placement.

This is a practical study using standard clinical practice methods to assess vital signs. Maternal vital signs (blood pressure, heart rate) will be collected in the left lateral recumbent position after an initial 5 minute period without activity. Automated devices will be used to assess blood pressure and pulse pressure at the following times: on admission, immediately prior to infusion of IV fluid bolus in preparation for epidural placement, immediately following IV fluid bolus infusion, at epidural test dose (T=0), every 5 minutes for 30 minutes (T=0 to T=30), and then every 10 minutes for the next 30 minutes (T=30 to T=60).

The chart will be abstracted to identify changes in the FHR and interventions. Continuous external (or internal, if placed for typical obstetric indications), fetal monitoring will be performed before and after epidural placement as is standard on our L\&D unit. Treatment of hypotension with administration of vasopressors or additional IV fluid boluses along with the treatment of FHR abnormalities (maternal position changes, supplemental oxygen administration, tocolytic use, cessation of oxytocin) will be recorded. Operative delivery for non-reassuring fetal status in the 60 minutes after epidural placement will be recorded. Additionally, maternal demographic and medical/obstetric data, maternal and neonatal delivery and outcome data will be recorded.

The technique for epidural placement and dosing will be standardized as follows. Epidural dosing will consist of 5 mL of 0.125% bupivicane with 10 mcg/mL of fentanyl. This standard bolus dose will be administered and levels of sensory block and numeric patient pain scores will be recorded. If patients have not achieved a pain score of 4 or less at 15 minutes after the epidural bolus, an additional bolus will be administered to achieve appropriate pain control. The volume of additional epidural bolus will be recorded.

Two independent obstetricians blinded to the randomization group and delivery outcome will interpret the FHR tracing for one hour before and after epidural placement. The FHR tracings will be divided into 15 minute epochs and categorized using the standard ACOG categories: I, II and III. Additional information including FHR baseline, FHR variability, and the presence or absence of accelerations or decelerations will be recorded for each 15min epoch. Discrepancies between the tracing interpretations will be reviewed by a third obstetrician for development of a consensus interpretation.