Simpson, Kathleen Rice PhD, RNC*; James, Dotti C. PhD, RNC†
Intravenous (IV) fluid boluses, lateral positioning, and oxygen (O2) administration are common intrauterine techniques used during labor when the fetal heart rate (FHR) pattern is nonreassuring. Although these routine interventions are considered to be standard procedures for treating suspected fetal hypoxemia during a nonreassuring FHR pattern,1,2 little data exist about their efficacy in improving fetal oxygen status. No studies of the effect of an IV fluid bolus on fetal oxygen saturation (FSpO2) were found using a MEDLINE and CINAHL search of the terms “fetal oxygen saturation,” “fetal pulse oximetry,” and “intravenous fluid bolus” from 1966 through February 2005. Intravenous fluids can provide rapid volume expansion during periods of maternal hypovolemia and hypotension. However, for women who are not hypovolemic or hypotensive, little is known about the effects of additional IV fluid volume on maternal transfer of O2 to the fetus. It has been suggested that large volumes of IV fluids might negatively affect maternal transfer of O2 to the fetus by causing hemodilution of maternal blood and thus a reduction in its O2 carrying capacity.3 There have been 2 reports comparing the effects of right lateral, left lateral, and supine maternal positions on fetal oxygen status, each suggesting that lateral positioning is more favorable than a supine position.4,5 In a classic study about the effects of maternal O2 administration on the fetus, 100% O2 via face mask corrected nonreassuring FHR patterns by decreasing the baseline rate during fetal tachycardia and reducing or eliminating late decelerations.6 These effects were reversed after the O2 was discontinued. Research about the effects of maternal O2 administration on FSpO2 produced conflicting results. Dildy et al7 concluded that intrapartum O2 administration was of questionable value because of the inability of a simple face mask to achieve a FiO2 greater than 40%. In contrast, McNamara and Lilford8 found a significant increase in FSpO2 after 10 minutes of O2 administration at 15 L/min via a standard face mask (FiO2 27%) as well as 10 minutes via an anesthetic face mask (100% FiO2). According to the most recent review in the Cochrane Database of Systematic Reviews,9 there are not enough data to support the use of prophylactic O2 therapy during labor nor to evaluate its effectiveness for fetal distress.
There has been little recent research attention to intrapartum intrauterine resuscitation techniques. The available evidence about the effects of lateral positioning and O2 administration during labor on FSpO2 is based on small unpowered samples.5,7–8 We therefore conducted this study to evaluate the effects of each of these techniques for their ability to improve fetal oxygen status during labor as measured by FSpO2. We used healthy women with a reassuring FHR pattern as our sample because we wanted to establish a baseline effect for the healthy fetus before studying each of the techniques during a nonreassuring FHR pattern.
MATERIALS AND METHODS
This prospective study evaluated the efficacy of 3 common intrauterine resuscitation techniques used during labor: IV fluid boluses, lateral positioning, and O2 administration at 10 L/min via nonrebreather face mask. Institutional review board approval was obtained from St. John's Mercy Medical Center and Saint Louis University in St. Louis, Missouri. Healthy nulliparous women at term who were admitted for an elective induction of labor at St. John's Mercy Medical Center were evaluated for participation. Addition criteria for study inclusion were IV oxytocin as the induction agent, a singleton fetus in a vertex presentation, plans for epidural anesthesia, and a reassuring FHR pattern at the time of enrollment. Women who were undergoing labor induction and planning epidural anesthesia were chosen because we wanted to study intrauterine resuscitation techniques in a clinical context common in contemporary perinatal practice. Women with medical or obstetrical complications or a maternal condition that could potentially influence maternal SpO2 (eg, history of smoking, asthma, chronic or acute pulmonary or cardiac disease) and those who did not meet criteria for FSpO2 sensor insertion were excluded from participation. The first patients who met eligibility criteria on days selected for research study enrollment were invited to participate. If the first eligible patient declined participation, the next eligible candidate was approached until one woman agreed or there were no further eligible candidates that day. Women were not in labor when they were approached about possible study participation or consented. They were informed that study participation included insertion of an FSpO2 sensor upon rupture of membranes, cervical dilation of at least 2 cm, and fetal station of at least –2. All patients who agreed to participate gave written consent before enrollment.
The efficacy of each intrauterine resuscitation technique was evaluated via continuous FSpO2 monitoring. An electronic fetal monitor (EFM) (model M1350C; Philips Medical Systems, Andover, MA) was used with the OxiFirst FSpO2 sensor (Nellcor Perinatal, Pleasanton, CA). Electronic fetal monitor and FSpO2 data were continuously collected via the OB TraceVue electronic information system (Philips Medical Systems). The FHR tracing and FSpO2 data were printed directly from the EFM for ongoing evaluation during labor.
After giving written informed consent, women who agreed to participate were observed until they met criteria for FSpO2 insertion, after which they were enrolled in the study. The first phase of the study was an evaluation of the effects of an IV fluid bolus on FSpO2. Women were randomized via computer-generated allocation to receive either a 500-mL or 1,000-mL IV fluid bolus of lactated Ringers solution over a 20-minute period for pre-epidural anesthesia hydration. The standard unit protocol for IV fluids during labor induction of lactated Ringers solution at 125 mL/h was used before and after the IV fluid bolus. To control for potential differences in FSpO2 based on maternal position, all women were assisted to the left lateral position 15 minutes before the IV fluid bolus and remained in this position during the bolus and for 15 minutes after the bolus was completed. Approximately 15 minutes after completion of the IV bolus, epidural anesthesia was administered based on the standard unit protocol of an initial dose of 100 μg of fentanyl and 8–10 mL of 0.25% bupivacaine with epinephrine, followed by 0.0725% bupivacaine at a rate of 10–15 mL/h via a continuous infusion pump.
The second phase of the study was an evaluation of the effects of maternal positions on FSpO2. Approximately 1 hour after receiving epidural anesthesia, women were randomized via computer-generated allocation to 1 of 6 possible position sequences that included all of the following positions for 15 minutes each in succession based on the randomized sequence: supine with the head of the bed elevated 30°, left lateral, and right lateral.
The final phase of the study was an evaluation of the effects of maternal O2 administration on FSpO2. Approximately 1 hour after the position sequences, women were assisted to the left lateral position for 15 minutes to control for potential differences in FSpO2 based on maternal position. After 15 minutes, O2 was administered with an Airlife (Allegiance Healthcare Corporation, McGaw Park, IL) nonrebreather face mask at 10 L/min. With the nonrebreather mask, 100% O2 enters the mask from the reservoir bag when the patient inhales. The nonrebreather mask delivers approximately 80–100% FiO2 at 10 L/min with all flaps closed and a tight seal.10 After 15 minutes, the O2 was discontinued. The women remained in the left lateral position for the next 30 minutes, during which FSpO2 data were continuously collected.
Oxytocin was administered intravenously using the standard unit protocol, starting at 1 mU/min and increasing by 1–2 mU/min every 30 minutes until an adequate contraction pattern was achieved. The oxytocin infusion was titrated, based on labor progress and maternal-fetal response, to promote uterine contractions no closer than every 2 1/2 to 3 minutes. Oxytocin infusion rates were not increased during the period of evaluation for each study intervention.
For 15 minutes before and during all study interventions, the FHR was continuously monitored. Maternal blood pressure (BP) was assessed every 5 minutes with a noninvasive automatic BP device; maternal pulse and SpO2 were continuously monitored using a pulse oximeter. The noninvasive BP device and pulse oximeter were components of the EFM. Maternal temperature was assessed before study interventions. Study interventions were not initiated unless the FHR was reassuring, FSpO2 was between 30% and 70%, and maternal vital signs were normal; normotensive BP (systolic < 140 mm Hg, diastolic > 60 mm Hg), pulse within normal limits (60–100 bpm), SpO2 at 96% or greater, and temperature (97.6–99.6°F). One of the investigators and a research nurse were in continuous bedside attendance during each of the study interventions.
A power analysis was conducted to determine the approximate sample size necessary to achieve a power of .80 with a 2 × 3 analysis of variance (ANOVA) design to detect a difference of at least 5% in FSpO2. This reflects the mean FSpO2 during labor (47%), a standard deviation of 8.4, and a nondirectional test at the .05 level of significance. Based on a medium effect size of .03, a sample size of at least 42 was determined to be necessary. Data analysis was performed with SPSS 12.0 for Windows (SPSS Inc, Chicago, IL). The OB TraceVue system did not allow electronic averaging of FSpO2 data. The FSpO2 data set entered into SPSS consisted of FSpO2 values using the first reading recorded by the OB TraceVue system of each minute during the periods under evaluation before, during, and after each study intervention. Although data were recorded prospectively, data entry into the SPSS program occurred retrospectively, with investigators blinded to subject allocation.
Repeated measures ANOVA and paired samples t tests with Bonferroni post hoc analysis were used to evaluate the differences in the mean FSpO2 for the 15 minutes before the IV fluid bolus, during the 20-minute bolus, and for the 15 minutes after the bolus was completed. Repeated measures ANOVA and paired samples t tests with Bonferroni post hoc analysis were used to evaluate the differences in the mean FSpO2 for 15 minutes during each maternal position. A linear regression repeated measures model was used to test the effects of the position sequence on the differences between means during each of the 3 positions. Repeated measures ANOVA and paired samples t tests with Bonferroni post hoc analysis were used to evaluate the differences in the mean FSpO2 15 minutes before and during O2 administration and the means during the first, second, and third 10-minute periods after the O2 was discontinued. Differences were measured for all patients who received O2 and for the following 2 subgroups: those with fetuses with a mean FSpO2 less than 40% in the 15 minutes before O2 administration (n = 15) and those with fetuses with a mean FSpO2 of 40% or greater during the same period (n = 34).
Sixty healthy nulliparous women at term undergoing elective induction of labor were approached for participation. Fifty-six women agreed to participate, and 4 women declined participation. Figure 1 illustrates participant flow through the study. Patient demographics and clinical characteristics are outlined in Table 1. Eleven of the 56 women eventually had a cesarean birth—7 for failure to progress in labor and 4 for nonreassuring fetal status—representing a 19.6% cesarean birth rate, which is not unusual for nulliparous women undergoing elective labor induction.11
Fourteen of the 56 women who initially agreed to participate declined FSpO2 sensor insertion until after they received epidural anesthesia, citing concerns about possible discomfort during FSpO2 sensor insertion without anesthesia. Thus, they were not included in the evaluation of the IV fluid bolus. Forty-two women were randomized to receive 1 of 2 amounts of an IV fluid bolus of lactated Ringers solution over a 20-minute period for pre-epidural hydration. Twenty-one women received 500 mL, and 21 women received 1,000 mL. The mean FSpO2 values for the 15 minutes before the IV fluid bolus, the 20 minutes during the bolus, and the 15 minutes after the bolus were compared. The epidural catheter was dosed at least 15 minutes after the IV bolus was completed. The mean FSpO2 values with standard deviations (SDs) before, during, and after the IV boluses are presented in Table 2. Only the 1,000-mL fluid bolus resulted in a significant increase in FSpO2 when comparing the means before and during the bolus (500 mL: mean increase 3.7, P = 17; 1,000 mL: mean increase 5.2, P = .03). The increase in mean FSpO2 as a result of the 1,000-mL bolus remained significantly different during the 15 minutes after the bolus FSpO2 (mean increase 6.3, P = .05) when compared with the mean FSpO2 15 minutes before the bolus (Fig. 2). There were no significant differences in oxytocin dosage rates between groups (500 mL: mean 6.2 mU/min, SD 4.9; 1,000 mL: mean 6.6 mU/min, SD 3.8, P = .78).
After the epidural catheter was placed and dosed and the women reported adequate pain relief, the FSpO2 sensor was inserted for the 14 patients who declined participation until after epidural anesthesia. During the interim, 5 of the 42 women who participated in the IV fluid bolus phase of the study progressed rapidly to the second stage of labor and did not participate further.
Fifty-one women were randomized to 1 of 6 possible position sequences that included supine with the head elevated 30°, left lateral, and right lateral. Fetal oxygen saturation was higher in a lateral position (left mean 48.3%, SD 7.8; right mean 47.7%, SD 9.4) than in a supine position (supine mean 37.5%, SD 9.3, P < .03). There was no significant difference in mean FSpO2 between left lateral and right lateral positions (P = .90). The sequence of the positions had no effect on the mean FSpO2 of each position (P = .34). The mean dose of oxytocin during the position change sequences was 7.5 mU/min (SD 4.9). Two women progressed to the second stage of labor after the position changes and did not participate further.
The remaining 49 women received O2 at 10 L/min per nonrebreather face mask. The mean FSpO2 values with standard deviations before, during, and after O2 administration are presented in Table 3. Oxygen administration resulted in an increase in FSpO2 (mean increase 8.7, P = .03). The effect persisted for more than 30 minutes after the O2 was discontinued (P = .03). The steepest rise in FSpO2 (from a mean of 43.5% for the 15 minutes before O2 administration to 52.0%) occurred within 4 minutes after initiating O2, reaching its peak (55.3%) at 15 minutes during O2 administration. The O2 was discontinued after 15 minutes so it is not known if the increase in FSpO2 would have continued or for how long, if the O2 had been administered for a longer period. During the next 30 minutes after O2 removal, FSpO2 gradually decreased, but the mean FSpO2 during the last 10 minutes of the 30-minute period after O2 removal remained higher than the mean of the 15 minutes before O2 administration (pre-O2 mean 43.5%, SD 7.8; third 10-minute period post-O2 mean 47.6%, SD 8.1, P = .03). The mean dose of oxytocin during O2 administration was 8.9 mU/min (SD 4.9).
For fetuses with a mean FSpO2 less than 40% during the 15 minutes before maternal O2 administration, the increase was greater and longer lasting than for those with a mean FSpO2 of 40% or greater during the same period (FSpO2 < 40% mean increase 11.4; FSpO2 ≥ 40% mean increase 7.6, P = .03; Fig. 3). There were no significant differences in oxytocin dosage rates between groups (FSpO2 < 40% mean 9.4 mU/min, SD 4.9; FSpO2 ≥ 40% mean 7.7 mU/min, SD 4.8, P = .25). Continuous attendance at the bedside by 2 members of the research team during all interventions under investigation and sensor adjustments as necessary to maintain an uninterrupted FSpO2 tracing resulted in a greater than 90% rate of data recorded.
An IV fluid bolus of 1,000 mL, lateral positioning, and O2 administration at 10 L/min via nonrebreather face mask resulted in significant increases in FSpO2 when used for healthy women in labor with a reassuring FHR. It is possible that each of these techniques would be at least as beneficial for a fetus with a nonreassuring FHR, but we only evaluated the effects of intrauterine resuscitation techniques for healthy fetuses in this study. Lateral positioning resulted in an increase in FSpO2 (right lateral mean 10.2; left lateral mean 10.8) when compared with the supine position, consistent with an earlier study that reported higher FSpO2 in a lateral position than a supine position.5 Other researchers have noted that the supine position is associated with late decelerations, a decrease in fetal scalp pH, and a decrease in maternal cardiac output when compared with a lateral position.12,13 Although some physicians and nurses believe that the maternal positioning on the left side is better for the fetus, in our study there was no significant difference in FSpO2 between left and right lateral positions.
Even though healthy women in labor have nearly 100% SpO2 (usually between 96% and 99%), it has been suggested that increasing inspired O2 increases blood O2 tension and results in more O2 delivered to the fetus.8 In our study, as in that of McNamara et al,8 there was a more rapid increase in FSpO2 when O2 was given, compared with the decrease in FSpO2 when it was discontinued, suggesting that the fetus responds to the new placental O2 gradient by accepting O2 more rapidly than it gives it up. Fetal hemoglobin has a higher affinity for O2 than adult hemoglobin, and fetal hematocrit is higher than adults. These physiologic factors allow for a steeper increase in fetal oxygen concentration and FSpO2 during maternal O2 therapy. The FSpO2 levels higher than those pre-O2 administration persisted for 30 minutes after the O2 was discontinued. Thus, the effects were not limited to the period of O2 therapy. The fetuses with a baseline FSpO2 less than 40% before the mother was given O2 had a greater increase in FSpO2 than the fetuses with a baseline FSpO2 of 40% or greater, suggesting that fetuses with low FSpO2 may benefit more from O2 therapy than fetuses with normal FSpO2.
Although our results suggest that short-term (15 minutes) O2 therapy may be beneficial for the fetus during labor, these results cannot be extrapolated to suggest that long-term O2 therapy during labor is of similar benefit. The effects of long-term O2 therapy during labor have not been studied extensively. Thorp and colleagues14 found that more than 10 minutes of O2 administration during the second stage of labor resulted in lower umbilical artery cord blood gas values compared with less than 10 minutes of O2 administration and no O2 administration (control group). In the Thorp14 study, the 10-minute period was arbitrarily determined after evaluating results. It was not intended initially to give 10 minutes of O2 to one group and more than 10 minutes of O2 to another group.
Our results suggest that a 1,000-mL IV fluid bolus may be beneficial to the fetus. These fetal benefits were noted for women who were not hypotensive or hypovolemic before the IV fluid bolus. Other researchers have found that increasing IV fluid hydration during labor above that which is commonly used (250 mL/h versus 125 mL/h) has a positive effect on labor progress.15
We conclude that a 1,000-mL IV fluid bolus, lateral positioning, and maternal O2 administration at 10 L/min via nonrebreather face mask are effective in improving FSpO2 during labor. More data are needed about the effects of each of these commonly used intrauterine resuscitation techniques when the FHR is nonreassuring and the effects of long-term (> 15 minutes) maternal O2 therapy during labor on fetal and newborn outcomes.
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