Effects of ephedrine and phenylephrine on uterine and placental circulations and fetal outcome following fetal hypoxaemia and epidural-induced hypotension in a sheep model †

A Randomized Controlled Trial Comparing the Effect of Phenylephrine by Intramuscular Route With Intravenous Infusion in Maintaining Haemodynamic Stability During Elective Lower Segment Caesarean Section Under Spinal Anaesthesia

Background Hypotension is a commonly encountered side effect in patients undergoing spinal anaesthesia, particularly in patients undergoing caesarean section. Phenylephrine is a widely used drug to treat spinal-induced hypotension and to maintain hemodynamic stability. Our aim is to evaluate the effectiveness of phenylephrine given through two different routes prophylactically in prevention of post-spinal hypotension in patients undergoing caesarean section. Methods A total of 150 healthy pregnant women undergoing elective caesarean section were randomly allocated into three groups: Group M (prophylactic intramuscular use of 2 mg phenylephrine), group V (prophylactic intravenous infusion of 30 mcg phenylephrine per minute), and group P (no prophylaxis), rescue phenylephrine 30 mcg IV and atropine 0.6 mg IV were used intraoperatively to treat bradycardia and hypotension in all three groups. The primary outcome was maternal hemodynamic changes. Results There was an insignificant difference in demographic data between the groups. Maternal systolic and diastolic blood pressure were more stable in group M compared to group V and group P. Heart rate was significantly lower only in group V. We did not observe any statistical difference between the groups in the APGAR score or the fetal arterial blood gas values. The incidence of nausea and vomiting was more in group P. Conclusion Preventive intramuscular phenylephrine exhibited a more stable maternal hemodynamics when compared with the prophylactic intravenous infusion of phenylephrine and placebo in elective caesarean under spinal anaesthesia.

Effects of Vasoactive Medications and Maternal Positioning During Cesarean Delivery on Maternal Hemodynamics and Neonatal Acid-Base Status

Maternal hemodynamics, positioning, and anesthesia technique for cesarean delivery influence neonatal acid-base balance; direct effects from drugs that cross the placenta also have an influence. Spinal anesthesia limits fetal exposure to depressant drugs and avoids maternal airway instrumentation, but is associated with hypotension. Hypotension may be prevented/treated with vasopressors and intravenous fluids. Current evidence supports phenylephrine as the first-line vasopressor. Fifteen degrees of lateral tilt during cesarean delivery has been advocated to relieve vena caval obstruction, but routine use may be unnecessary in healthy nonobese women having elective cesarean delivery if maternal blood pressure is maintained near baseline.

Comparison of Phenylephrine and Ephedrine in Treatment of Spinal-Induced Hypotension in High-Risk Pregnancies: A Narrative Review

Purpose

To compare maternal and fetal effects of intravenous phenylephrine and ephedrine administration during spinal anesthesia for cesarean delivery in high-risk pregnancies.

Source

An extensive literature search was conducted using the US National Library of Medicine, MEDLINE search engine, Cochrane review, and Google Scholar using search terms “ephedrine and phenylephrine,” “preterm and term and spinal hypotension,” “preeclampsia and healthy parturients,” or “multiple and singleton gestation and vasopressor.” Society of Obstetric Anesthesia and Perinatology meeting abstracts for the past 4 years were also searched for relevant studies.

Principle findings

Both phenylephrine and ephedrine can be safely used to counteract hypotension after spinal anesthesia in patients with uteroplacental insufficiency, pregnancy-induced hypertension, and in non-elective cesarean deliveries. Vasopressor requirements before delivery in high-risk cesarean sections are reduced compared to healthy parturients. Among the articles reviewed, there were no statistically significant differences in umbilical arterial pH, umbilical venous pH, incidence of fetal acidosis, Apgar scores, or maternal hypotension when comparing maternal phenylephrine and ephedrine use.

Conclusion

From the limited existing data, phenylephrine and ephedrine are both appropriate selections for treating or preventing hypotension induced by neuraxial blockade in high-risk pregnancies. There is no clear evidence that either medication is more effective at maintaining maternal blood pressure or has a superior safety profile in this setting. Further investigations are required to determine the efficacy, ideal dosing regimens, and overall safety of phenylephrine and ephedrine administration in high-risk obstetric patients, especially in the presence uteroplacental insufficiency.

Effect of Hypoxemia with or without Increased Placental Vascular Resistance on Fetal Left and Right Ventricular Myocardial Performance Index in Chronically Instrumented Sheep

Myocardial performance index (MPI) is increased in growth-restricted fetuses with placental insufficiency, but it is unknown if this is due to fetal hypoxemia or increased placental vascular resistance (R_plac). We used chronically instrumented sheep fetuses (n = 24). In 12 fetuses, placental embolization was performed 24 h before experiments. On the day of the experiment, left (LV) and right (RV) ventricular MPIs were obtained by pulsed Doppler at baseline and in the hypoxemia and recovery phases. At baseline, R_plac was greater and fetal pO₂ lower in the placental embolization group, but RV and LV MPIs were comparable to those of the control group. During hypoxemia, mean LV MPI increased significantly only in fetuses with an intact placenta (0.34 vs. 0.46), returning to baseline during the recovery phase. Right ventricular MPI was unaffected. We conclude that fetal LV function is sensitive to acute hypoxemia. Exposure to chronic hypoxemia could pre-condition the fetal heart and protect its function with worsening hypoxemia.

Hemodynamic effects of ephedrine and phenylephrine bolus injection in patients in the prone position under general anesthesia for lumbar spinal surgery

Ephedrine and phenylephrine (PE) are vasoconstrictors commonly used to restore the blood pressure (BP) to normal values. The aim of the present study was to investigate the effects of ephedrine and PE bolus administration on intra-arterial systolic BP (ISBP), intra-arterial diastolic BP (IDBP) and cardiac output (CO) in patients undergoing lumbar spine surgery in the prone position under general anesthesia (GA). In this prospective, randomized, and double-blind study, a total of 60 patients aged 20-60 years and undergoing elective lumbar spine surgery were administered either a single dose of ephedrine (0.1 mg/kg) or PE (1 µg/kg) through a central venous catheter as a bolus injection following the achievement of a stable hemodynamic status for ≥10 min. Following bolus injection of ephedrine or PE, a significant increase in ISBP was observed in the two experimental groups compared with pre-ephedrine and pre-PE values. The duration of the increment in ISBP however was significantly longer in the ephedrine group compared with the PE group. A similar response was observed in IDBP. A significant increase in CO began 1 min following ephedrine injection and lasted for the entire observation period, whereas the increase was only sustained for 3 min following bolus injection in the PE group. The results of the present study demonstrated that bolus ephedrine produces a more persistent pressor response and durable increase in CO and CI compared with PE when patients are in the prone position with GA for spine surgery.

Can we measure the spiral and uterine artery blood flow by real-time sonography and Doppler indices to predict spontaneous miscarriage in a normal-risk population?

Introduction: The predictive value of spiral artery flow Doppler measurements of a subsequent early miscarriage in first trimester pregnancy is explored here. Objective: The aim of this study is to determine uterine and spiral artery blood flow changes in first trimester subsequent miscarriages and correlate within the mechanisms of the Doppler indicies. Study design: The uterine artery and spiral artery pulsatility and resistance indexes, systolic and diastolic ratios, acceleration times, and blood flow of both the right and left uterine arteries were obtained by trans vaginal color Doppler ultrasonography in consecutive viable pregnancies between 5 and 12 gestational week. Women were subsequently classified as having continuing pregnancies or pregnancy loss before 20 weeks gestation. To predict subsequent pregnancy loss, Doppler findings were adjusted for maternal age, history of previous abortion, presence of subchorionic hematoma, embryonic bradycardia, and gestational age by means of multivariate logistic regression analysis. The cut-off values are used for the ROC curve. Results: Twenty-five pregnancies (11.7%) were spontaneously aborted before 20 weeks of gestational age. In 29 (13.6%) cases there were previously abortion history, 30 (14%) had bradycardia, and 37 (17.3%) had subchoronic hematoma. Regarding the parameters of uterine and spiral artery pulsatility and resistive index, acceleration time, systolic/diastolic ratios and blood flows, only uterine artery S/D low values were significantly associated with pregnancy loss in the multivariate logistic regression analysis (P = 0.0001,95% CI: 4.968-55.675). Conclusion: The uterine artery systolic/diastolic ratios have a predictive value for early pregnancy loss and seem to be useful as a marker. On the other hand, spiral artery changes could be so local that they cannot be determined by the parameters of spectral Doppler techniques. This suggests that uterine vascular bed alterations should be measured to understand the prognosis of early pregnancy loss during the first trimester.

Anesthetic Techniques for Fetal Surgery

Background:

Use of high-dose inhalational anesthesia during open fetal surgery may induce maternal–fetal hemodynamic instability and fetal myocardial depression. The authors’ preliminary human retrospective study demonstrated less fetal bradycardia and left ventricular systolic dysfunction with lower dose desflurane supplemented with propofol and remifentanil IV anesthesia (SIVA). In this animal study, the authors compare maternal–fetal effects of high-dose desflurane anesthesia (HD-DES) and SIVA.

Methods:

Of 26 instrumented midgestational ewes, data from 11 animals exposed to both SIVA and HD-DES in random sequences and six animals exposed to HD-DES while maternal normotension was maintained were analyzed. Maternal electroencephalography was used to guide comparable depths of anesthesia in both techniques. Hemodynamic parameters, blood gas, and fetal cardiac function from echocardiography were recorded.

Results:

Compared with SIVA, HD-DES resulted in significant maternal hypotension (mean arterial pressure difference, 19.53 mmHg; 95% CI, 17.6–21.4; P < 0.0001), fetal acidosis (pH 7.11 vs. 7.24 at 150 min, P < 0.001), and decreased uterine blood flow. In the HD-DES group with maternal normotension, uterine blood flow still declined and fetal acidosis persisted, with no statistically significant difference from the group exposed to HD-DES that had maternal hypotension. There was no statistically significant difference in fetal cardiac function.

Conclusion:

In sheep, SIVA affects maternal hemodynamics less and provides better fetal acid/base status than high-dose desflurane. Fetal echocardiography did not reflect myocardial dysfunction in this model.

[Prophylaxis and treatment of arterial hypotension during caesarean with spinal anaesthesia]

Caesarean section is one of the most common surgical procedures worldwide. Arterial hypotension is the most prevalent adverse effect after spinal anaesthesia. Various methods have been used to prevent or treat hypotension. Since there is no treatment 100% effective by itself, a multimodal management is required to achieve an optimum balance and avoidance of hemodynamic imbalance. Strategies to avoid this side effect are analyzed on the basis of the best evidence available so far, summarized as mechanical factors, anesthetics, fluids and vasopressors. After spinal anaesthesia for caesarean section, the best strategy available for prevention of hypotension appears to be the combination of crystalloids along with an alpha 1 agonist vasopressor.

Update in fetal anesthesia for the ex utero intrapartum treatment (EXIT) procedure

The ex-utero intrapartum treatment (EXIT) is one type of fetal surgery, performed before delivery while the fetus remains attached to the uteroplacental circulation. This intervention improves neonatal morbidity and mortality of certain congenital diseases. For instance, securing the airway of a fetus with congenital airway obstruction while on uteroplacental circulation prevents the hypoxemia during the establishment of an airway post-delivery. Anesthesia for fetal surgery now incorporates new knowledge of the maternal/fetal response to anesthetic agents. This chapter reviews for the EXIT procedure the effects of maternal anesthesia on fetal hemodynamics, intravenous anesthesia to supplement inhalational anesthesia in order to provide maternal-fetal hemodynamic stability during surgery, intraoperative fetal monitoring, maternal pharmacokinetics approach to study placental drug transfer and fetal pharmacokinetics to improve our understanding of the effects of maternal anesthesia on the fetus.

A double-blind, placebo-controlled trial of four fixed rate infusion regimens of phenylephrine for hemodynamic support during spinal anesthesia for cesarean delivery

BACKGROUND

The administration of prophylactic phenylephrine infusions in combination with fluid cohydration significantly reduces the incidence of hypotension in women having cesarean delivery under spinal anesthesia. The ideal dosing regimen for this purpose is not known. In this study, we investigated the dose of phenylephrine that, when administered as a prophylactic fixed rate infusion, is associated with the least interventions needed to maintain maternal systolic blood pressure (SBP) within 20% of baseline.

METHODS

Women undergoing elective cesarean delivery were randomly allocated to receive placebo or prophylactic phenylephrine infusion at 25, 50, 75, or 100 μg/min immediately after spinal anesthesia in combination with a 2-L fluid coload. Maternal SBP was maintained within the target range using a predetermined algorithm. The number of physician interventions, hemodynamic performance, intraoperative nausea and vomiting, and umbilical cord blood gases were compared among the groups.

RESULTS

One hundred one patients were included in the analysis. There were no differences between the placebo and phenylephrine groups in the number of interventions needed to maintain maternal SBP within the target range. Doses of phenylephrine of 25 and 50 μg/min were associated with significantly fewer interventions when compared with 100 μg/min (P = 0.004 vs 50 μg/min, P = 0.02 vs 25 μg/min). Predelivery hypotension was more frequent in the control group compared with all phenylephrine groups. Phenylephrine 75 and 100 μg/min groups were associated with a significantly higher incidence of predelivery hypertension compared with control (P < 0.001 vs 75 μg/min and 100 μg/min). There was a trend toward an increase in median magnitude of deviations of SBP above or below baseline (P = 0.006), and the bias of SBP to be above baseline (P < 0.001) with increasing rates of phenylephrine infusion. There were no differences in the incidence and severity of intraoperative nausea and vomiting and umbilical cord blood gases among the groups.

CONCLUSIONS

The use of prophylactic fixed rate phenylephrine infusions did not significantly reduce the number of physician interventions needed to maintain maternal predelivery SBP within 20% of baseline compared with placebo. However, prophylactic phenylephrine infusions reduced the incidence and severity of maternal predelivery hypotension. Phenylephrine 25 and 50 μg/min administered as a prophylactic fixed rate infusion provided greater maternal hemodynamic stability than phenylephrine 75 and 100 μg/min. Prophylactic fixed rate infusions may have limited application in clinical practice, and future studies assessing the accuracy of hemodynamic control with variable rate phenylephrine infusions are needed.

Ductus venosus velocimetry in acute fetal acidemia and impending fetal death in a sheep model of increased placental vascular resistance

We investigated whether hypoxemia without acidemia affects ductus venosus (DV) blood velocity waveform pattern in sheep fetuses with intact placenta and whether worsening acidemia and impending fetal death are related to changes in DV velocimetry in fetuses with increased placental vascular resistance. A total of 34 fetuses were instrumented at 115–136/145 days of gestation. Placental embolization was performed in 22 fetuses on the fourth postoperative day, 24 h before the experiment. The control group was comprised of 12 fetuses with intact placenta. The experimental protocol consisted of fetal hypoxemia that was induced by replacing maternal inhaled oxygen with medical air. To further deteriorate fetal oxygenation and blood-gas status, uterine artery volume blood flow was reduced by maternal hypotension. Fetuses that underwent placental embolization were divided into two groups according to fetal outcome. Group 1 consisted of 12 fetuses that completed the experiment, and group 2 comprised 10 fetuses that died during the experiment. DV pulsatility index for veins (PIV) and fetal cardiac outputs (COs) were calculated. Placental volume blood flow, fetal blood pressures, and acid base and lactate values were monitored invasively. On the experimental day, the mean gestational age did not differ significantly between the groups. In groups 1 and 2, the baseline mean DV PIV and fetal COs were not statistically significantly different from the control group. In the control group, the DV PIV values increased significantly with hypoxemia. In groups 1 and 2, the DV PIV values did not change significantly, even with worsening acidemia and imminent fetal death in group 2. During the experiment, the fetal COs remained unchanged. We conclude that fetal hypoxemia increases the pulsatility of DV blood velocity waveform pattern. In fetuses with elevated placental vascular resistance, DV pulsatility does not increase further in the presence of severe and worsening fetal acidemia and impending fetal death.

The diverse effects of vasopressors on the fetoplacental circulation of the dual perfused human placenta

BACKGROUND

We studied the effects of 5 vasopressors on fetal arterial perfusion pressure (FAP) in vitro using the dual perfused, single isolated cotyledon, human placental model.

METHODS

In 29 separate experiments, epinephrine (75 mg), norepinephrine (75 mg), ephedrine (50 mg), phenylephrine (2 mg), and methoxamine (40 mg) were introduced into the 250-mL reservoir serving the maternal perfusion circuit to determine the effect of each drug on FAP. The duration of drug exposure for each placental cotyledon was approximately 180 minutes.

RESULTS

After 180 minutes, FAP (mean +/- sd) increased significantly with ephedrine from 64 +/- 3 to 172 +/- 71 mm Hg (P < 0.001) and with phenylephrine from 81 +/- 4 to 132 +/- 11 mm Hg (P = 0.003). No changes in FAP were seen with epinephrine, norepinephrine, and methoxamine.

CONCLUSIONS

In the dual perfused, single isolated cotyledon, human placental model, exposure of the maternal circulation to ephedrine and phenylephrine caused an increase in FAP, whereas exposure to norepinephrine, epinephrine, and methoxamine did not. The pharmacodynamic mechanisms underlying these differences have yet to be explained. Thus, the clinical implications of the findings are as yet unclear.

Significance of retrograde diastolic uterine artery blood flow during regional anesthesia in instrumented pregnant sheep

BACKGROUND

We studied the interactions between uterine and placental hemodynamics during maternal hypotension in chronically instrumented fetal sheep. In addition, we investigated maternal hemodynamic characteristics, fetoplacental hemodynamics and fetal acid-base status when a retrograde diastolic uterine artery blood flow pattern is present during maternal hypotension.

METHODS

Invasive maternal and fetal hemodynamic parameters, uterine (Q(UtA)) and placental (Q(UA)) volume blood flows and acid-base values were examined in 24 chronically instrumented sheep at baseline and during epidural-induced maternal hypotension at 117-132 (term 145) days of gestation. Uterine artery blood flow velocity waveforms were obtained by Doppler ultrasonography.

RESULTS

Maternal hypotension decreased Q(UtA) without affecting Q(UA). During hypotension, eight out of 24 sheep demonstrated a retrograde diastolic blood flow velocity waveform pattern in the uterine artery. Maternal systolic, diastolic and mean arterial blood pressures were significantly lower in the retrograde group than in the antegrade group. No statistically significant differences in Q(UtA), Q(UA) and fetal blood gas values were detected between the two groups during hypotension.

CONCLUSIONS

An acute decrease in uterine artery volume blood flow during maternal hypotension is not compensated by increased placental volume blood flow. A retrograde diastolic blood flow pattern in the uterine artery is related to lower maternal arterial pressures, especially during diastole. A uterine artery retrograde diastolic blood flow pattern does not have any additional detrimental short-term effects on fetal acid-base status.

Divergent effects of ephedrine and phenylephrine on cardiovascular hemodynamics of near-term fetal sheep exposed to hypoxemia and maternal hypotension

BACKGROUND

We hypothesized that the administration of ephedrine and phenylephrine for maternal hypotension modifies cardiovascular hemodynamics in near-term sheep fetuses.

METHODS

At 115-136 days of gestation, chronically instrumented, anesthetized ewes with either normal placental function or increased placental vascular resistance after placental embolization were randomized to receive boluses of ephedrine (n = 12) or phenylephrine (n = 12) for epidural-induced hypotension after a short period of hypoxemia. Fetal cardiovascular hemodynamics were assessed by Doppler ultrasonography at baseline, during hypotension and after vasopressor treatment.

RESULTS

During hypotension, fetal PO(2) decreased and proximal branch pulmonary arterial and pulmonary venous vascular impedances increased. Additionally, in the embolized fetuses, the time-velocity integral ratio between the antegrade and retrograde blood flow components of the aortic isthmus decreased. These parameters were restored to baseline conditions by ephedrine but not by phenylephrine. With phenylephrine, weight-indexed left ventricular cardiac output and ejection force decreased in the non-embolized fetuses, and the proportion of isovolumetric contraction time of the total cardiac cycle was elevated in the embolized fetuses.

CONCLUSIONS

After exposure to hypoxemia and maternal hypotension, ephedrine restored all fetal cardiovascular hemodynamic parameters to baseline. Phenylephrine did not reverse fetal pulmonary vasoconstriction or the relative decrease in the net forward flow through the aortic isthmus observed in fetuses with increased placental vascular resistance. Moreover, fetal left ventricular function was impaired during phenylephrine administration.

Vasopressors in obstetrics: what should we be using?

PURPOSE OF REVIEW

Historically, ephedrine has been recommended as the best vasopressor in obstetrics because animal studies showed it caused less reduction in uterine blood flow compared with alpha-agonists. Recent clinical evidence, however, suggests that this is not as important as initially thought. This review evaluates current data with a focus on spinal anesthesia for cesarean section.

RECENT FINDINGS

Ephedrine and phenylephrine have been most investigated. Advantages of ephedrine include familiarity, long history and low propensity for uteroplacental vasoconstriction. Ephedrine, however, has limited efficacy, is difficult to titrate, causes maternal tachycardia and depresses fetal pH and base excess. Advantages of phenylephrine include high efficacy, ease of titration and the ability to use liberal doses to maintain maternal blood pressure near normal and then prevent nausea and vomiting without causing fetal acidosis. Phenylephrine, however, may decrease maternal heart rate and cardiac output and few data are available on its use in high-risk cases. Combination of a phenylephrine infusion and rapid crystalloid cohydration is the first method described that reliably prevents hypotension.

SUMMARY

When current evidence is considered, in the authors' opinion, phenylephrine is the vasopressor that most closely meets the criteria for the best vasopressor in obstetrics.

Ephedrine and phenylephrine for the treatment of maternal hypotension in a chronic sheep model of increased placental vascular resistance †

BACKGROUND

We hypothesized that ephedrine and phenylephrine are equal with respect to uterine and placental haemodynamics and fetal acid-base status after exposure to maternal hypoxaemia and hypotension in a chronic sheep model of increased placental vascular resistance (R(UA)).

METHODS

At 114-135 days gestation, chronically instrumented fetal sheep underwent placental embolization leading to increased R(UA). Twenty-four hours after embolization, the ewes were anaesthetized and randomized to receive boluses of ephedrine (n=7) or phenylephrine (n=6) for epidural-induced hypotension after maternal hypoxaemia. Uterine (Q(UtA)) and placental (Q(UA)) volume blood flows and uterine vascular resistance (R(UtA)) and R(UA) were recorded. Uterine (PI(UtA)) and umbilical artery (PI(UA)) pulsatility indices were obtained by Doppler ultrasonography. Fetal arterial blood samples were analysed for acid-base values and lactate concentrations.

RESULTS

During hypotension, Q(UtA), fetal pH, BE, and Po(2) decreased whereas R(UtA), PI(UtA), R(UA), and fetal lactate concentration increased. With ephedrine, Q(UtA), R(UtA), PI(UtA), R(UA), and fetal Po(2) returned to baseline. Fetal pH, BE, and lactate concentration did not change from hypotensive values. With phenylephrine, Q(UtA) remained lower (P=0.007) and R(UtA) (P=0.007), PI(UtA) (P=0.013), and R(UA) (P=0.050) higher than at baseline. Fetal Po(2) returned to baseline and fetal pH and BE did not change from hypotensive values. However, fetal lactate concentration increased further (mean difference 1.49, 95% confidence interval 0.72-2.26 mmol litre(-1); P=0.004).

CONCLUSIONS

In a chronic sheep model of increased placental vascular resistance, compared with ephedrine administration, phenylephrine administration was associated with impaired uterine and placental haemodynamics and increased fetal lactate concentrations.