As I get older, I often reflect on some of the lessons I learned from my teachers during my training. I remember one mentor saying, “Our clinical practice includes lots of nonevidenced-based dogma that should be challenged.” Another mentor told me, “Strive for precision and clarity in your writing, so that you do not mislead the casual reader … and remember that most readers are casual!” This issue of Anesthesia& Analgesia includes a review article1 that brings both of those lessons to mind.
Lee and Landau1 have written a thorough, scholarly, and helpful review of aortocaval compression in pregnant women, including its implications for anesthesia practice. The article title suggests that the authors intend to question certain dogmas regarding aortocaval compression syndrome (also known as supine hypotension syndrome), yet the authors acknowledge that 8% to 10% of pregnant women experience significant hemodynamic changes (eg, symptomatic hypotension) in the supine position. In 1953, Howard et al2 observed that 18 of 160 (11%) women—who were made to lie supine for several minutes—had a decrease in systolic blood pressure of >30 mm Hg or had a systolic blood pressure <80 mm Hg, and this occurred without administration of neuraxial anesthesia. Contrary to the casual reader’s likely interpretation of the title, Lee and Landau1 do not question the occurrence of supine hypotension syndrome. Further, they acknowledge that intrinsic maternal compensatory mechanisms (ie, peripheral vasoconstriction and enhanced venous return via collateral channels) prevent symptomatic hypotension in most patients, and that this compensatory response is blunted by the sympathectomy that accompanies neuraxial anesthesia.
The authors provide a thorough review of the pathophysiology of supine hypotension syndrome, as well as the evidence for aortocaval compression in asymptomatic pregnant women during the latter half of pregnancy. They call attention to magnetic resonance imaging that demonstrates near-complete inferior venal caval compression by the gravid uterus in term pregnant women in the supine position.3 The authors correctly note that the evidence for inferior vena caval compression is far stronger than the evidence for aortic compression. It is perhaps unfortunate that—for either alphabetical or phonetic reasons—we say aortocaval compression rather than cavoaortal compression. The authors acknowledge that the severity of aortic compression may be worsened by the lack of fetal head engagement, the presence of uterine contractions, and/or the occurrence of maternal hypotension—conditions that are often present during administration of neuraxial analgesia/anesthesia in pregnant women. Studies of the impact of aortocaval compression on uteroplacental perfusion have provided inconsistent results, but the evidence of impaired uteroplacental perfusion is sufficiently worrisome for Lee and Landau1 to recommend “avoidance of the supine position in women who have documented symptoms of supine hypotensive syndrome.” They also acknowledge an association between the supine sleep position and late-pregnancy stillbirth in women with intrauterine fetal compromise.4
Some of the best evidence for the negative impact of the supine position on fetal condition comes from everyday clinical practice. Every anesthesiologist who has spent much time on a labor unit has witnessed the occurrence of fetal bradycardia when the labor nurse placed the patient in the supine position for placement of a urethral catheter shortly after administration of labor epidural analgesia. And anyone who remains unconvinced of the adverse effects of aortocaval compression on maternal cardiac output should consider the recommendation of both the American Heart Association5 and the Society for Obstetric Anesthesia and Perinatology6—namely, that in cases of maternal cardiac arrest that does not respond to initial resuscitative measures, immediate cesarean delivery should be performed if gestational age is 20 weeks or greater, aiming for delivery within 5 minutes of cardiac arrest.5,6 Rose et al7 suggested that the term perimortem cesarean should be replaced by the term resuscitative hysterotomy, to emphasize the critical importance of prompt uterine evacuation (and relief of aortocaval compression) in the resuscitation of the mother, thus offering the best chance for survival of both the mother and the infant.
In summary, Lee and Landau1 do not challenge the common occurrence of venal caval compression in term pregnant women, and they acknowledge that it may reduce maternal cardiac output, blood pressure, and uteroplacental perfusion. Their own study demonstrated decreased cardiac output and increased phenylephrine requirement in healthy women who were placed in the supine position when compared with 15° of left lateral tilt after administration of spinal anesthesia (and aggressive phenylephrine infusion to maintain baseline maternal blood pressure) for elective cesarean delivery.8 Likewise, Lee and Landau1 do not challenge the occurrence of supine hypotension syndrome in some pregnant women. They also acknowledge that “in certain clinical scenarios left uterine displacement is a crucial life-saving maneuver.”1
So what dogma do Lee and Landau1 challenge? They challenge whether it is necessary to maintain left uterine displacement after administration of anesthesia for elective cesarean delivery in healthy women with an uncomplicated pregnancy and a healthy fetus, provided maternal blood pressure is maintained at baseline with a vasopressor such as phenylephrine. Earlier studies observed neonatal depression in women who did not have uterine displacement before delivery, but those studies were performed before the advent of aggressive blood pressure control with intravenous phenylephrine, which many obstetric anesthesiologists now give by continuous intravenous infusion.
Why do Lee and Landau1 make this challenge? In daily clinical practice, a conundrum occurs when the anesthesiologist’s desire to avoid aortocaval compression conflicts with the obstetrician’s plea to reduce the amount of left uterine displacement just before skin incision for cesarean delivery. In addition, left lateral tilt is uncomfortable for some patients, in part, because the weight of the unsupported gravid abdomen pulls on soft tissues, and some patients fear that they will slide off the operating table. As a result, 15° of left lateral tilt is rarely maintained until delivery.9,10 The amount of left uterine displacement often used during cesarean delivery probably does not fully prevent or relieve venal caval compression/obstruction. Indeed, the amount of left uterine displacement often used during cesarean delivery may be described as homeopathic.
Lee and Landau1 call attention to “a compromise practiced in many institutions … [namely] to utilize left uterine displacement or table tilt following induction of anesthesia, throughout the period of Foley catheter insertion, abdominal preparation and draping, … [followed by] flattening the surgical table at the time of incision.” Clinical experience indicates that maternal hypotension and vasopressor requirements are greatest as the block ascends during the first 10 minutes after spinal drug administration. After the block is established and stable, flattening the surgical table rarely results in acute maternal hypotension.
Most honest obstetric anesthesiologists will admit that they have agreed to reduce the amount of left uterine displacement at the time of skin incision for cesarean delivery, usually at obstetrician request. This compromise seems safe in healthy women with a healthy fetus, provided maternal blood pressure is maintained at or near baseline by intravenous administration of phenylephrine. But the evidence that this compromise is equivalent to continued left uterine displacement is thin, even in healthy low-risk parturients. And clearly this compromise has not been shown to be safe in women with risk factors, such as obesity, multiple gestation, fetal growth restriction, nonreassuring fetal status, and baseline supine hypotension syndrome.
Altogether, the preponderance of evidence and clinical experience suggests that left uterine displacement remains important during induction of neuraxial analgesia/anesthesia and ascent of the block with its attendant sympathectomy in women during the latter half of pregnancy. A decision to place the patient supine should be delayed until the time of skin incision for cesarean, and requires strict attention to maternal blood pressure as well as timely delivery of the baby. Further investigations are needed before we can declare the supine position safe in high-risk patients. Additional studies should also identify the ideal method of achieving left uterine displacement in the operating room (ie, hip rotation or bump versus table tilt versus manual left uterine displacement).11
DISCLOSURES
Name: David H. Chestnut, MD.
Contribution: This author wrote the manuscript.
This manuscript was handled by: Jill M. Mhyre, MD.
REFERENCES
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