Physiologic changes of pregnancy have a profound impact on anesthetic care; therefore a thorough understanding is necessary to provide safe and effective anesthesia to the patient. In addition, the dual-patient nature of pregnancy, involving both mother and fetus, must be considered at all times and may pose unique and sometimes conflicting concerns.

CARDIOVASCULAR

Pregnancy is associated with a 25% decrease in systemic vascular resistance and an increase in cardiac output by up to 40%, resulting in an overall mild reduction in blood pressure. Labor causes a further increase in cardiac output, which reaches maximal levels in the immediate postpartum period.¹ For high-risk patients, these changes may be cause for special consideration.

In singleton pregnancies >20 weeks gestation, aorto-caval compression by the gravid uterus can reduce venous return to the heart, thereby reducing cardiac output and uterine blood flow. Left uterine displacement (LUD) is necessary whenever the fundus extends above the level of the umbilicus.² Given that neuraxial anesthesia produces a regional sympathectomy resulting in hypotension, and that aorto-caval compression can exaggerate this, effective LUD should be performed.³

RESPIRATORY

Weight gain and edematous changes, particularly in the airway, can make mask ventilation and intubation difficult. Decreased functional residual capacity (FRC) and increased oxygen consumption also make the patient more susceptible to hypoxemia.¹

HEMATOLOGIC

Plasma volume increases by 40% to 50% and creates a compensatory mechanism during hemorrhage.¹ However, this compensation may impede early recognition of potentially significant bleeding. Since red cell mass increases by only 20%, pregnancy is associated with relative anemia, which puts the patient at a further disadvantage during hemorrhage. In contrast, the concentration of certain clotting factors and the fibrinogen level increases, which predisposes the patient to venous thrombosis and possible embolism.

GASTROINTESTINAL

It is believed that the patient has decreased gastric emptying time and decreased esophageal sphincter tone.¹ These observations, in addition to the increased intra-abdominal pressure caused by the gravid uterus as well as physiologic changes, can lead to difficult or challenging airway management. This puts pregnant patients at significantly higher risk of regurgitation and aspiration during general anesthesia (GA) or with sedation.¹ This topic is discussed in more detail later in this chapter.

HEPATIC AND RENAL

During pregnancy, cholesterol levels are higher, which increases the risk for cholestasis. In addition, a decreased level of plasma cholinesterase can cause a mild prolongation of the effects of succinylcholine. Combined with other factors that contribute to muscle weakness, it may prolong the return to effective respiratory efforts.¹ Although increased renal blood flow and glomerular filtration rate occur, this minimally affects anesthetic care.¹

CENTRAL NERVOUS SYSTEM

Due to pregnancy-related changes, pregnant women are more sensitive than nonpregnant women to both GA agents and local anesthetic (LA). This must be considered and medication doses adjusted when providing sedation, GA, or neuraxial anesthesia (NA) during pregnancy.

FETAL PHYSIOLOGY AND UTEROPLACENTAL BLOOD FLOW

Uteroplacental blood flow (UBF) increases during pregnancy, reaching at term about 700 mL/min, or 12% of cardiac output.¹ The uteroplacental vascular bed is thought to be considerably dilated, creating a low-resistance segment. This may contribute to limited autoregulation of UBF and dependency on maternal blood pressure. UBF is determined by the relationship between uterine perfusion pressure and uterine vascular resistance. Uterine perfusion pressure is directly affected by uterine arterial pressure, and inversely proportional to uterine venous pressure. Many factors can decrease UBF, and they are listed in Table 42-1.

All induction agents and opioids readily cross the placenta. Their effects on the fetus are dose dependent and influenced by the timing of administration. To minimize effects on the fetus, careful planning and coordination between the obstetrician and anesthesiologist are necessary. For example, if GA is needed for a cesarean section (C-section), all efforts must be made to minimize the induction-to-delivery time.

High doses of LA deposited in close proximity to the uterine artery, which is possible with a paracervical block, can trigger vasospasm and cause adverse effects on the fetus. When LA is absorbed in large quantities systemically, “ion trapping” can enhance the transfer of LA to the distressed and acidotic fetus, causing a greater proportion of LA remaining on the fetal side of the placenta and a larger total amount in the fetal blood.⁴

SYSTEMIC ANALGESIA AND MONITORED ANESTHESIA CARE

Alternative Therapies for Labor Analgesia

A number of nonpharmacologic therapies have been used for labor analgesia, including massage, heat or cold application, aromatherapy, music or audio therapy, biofeedback, hypnosis, transcutaneous electrical nerve stimulation, acupuncture, acupressure, hydrotherapy, and emotional support provided by family members and labor and delivery (L&D) personnel. Although the effectiveness of these treatments is unclear, many women find them beneficial.⁵

Inhalational Agents

Subanesthetic concentrations of inhalational agents (e.g. nitrous oxide or sevoflurane) have been used for labor analgesia. However, volatile anesthetics have environmental pollutant effects, as well as side effects like dose-dependent sedation, nausea, and potentially negative effects on the airway; therefore their use is limited.⁶

Parenteral Agents

The most commonly used systemic medications for labor analgesia are opioids. Dose-related side effects may include drowsiness, nausea, vomiting, delayed gastric emptying, and respiratory depression. In addition, since opioids cross the placenta, they affect the fetus, causing a decrease in fetal heart rate (FHR) variability and depressed respiratory status.^7,8 Parenteral medications may be given as intermittent bolus dosing or through patient-controlled analgesia delivery systems. A limited list of commonly used parenteral medications is provided in Table 42-2.

REGIONAL ANESTHESIA

A brief description of the pain pathways during the first and second stages of labor, as well as during C-section, is provided in this section (also see Table 42-3).⁹ An understanding of innervation is useful to visualize what nerve roots must be targeted when using regional techniques for analgesia or surgical anesthesia.

Neuraxial administration of LA and adjuvant medications is often provided to the patient in order to achieve both analgesia and surgical anesthesia. Typically, neuraxial techniques are preferred for cesarean delivery, as they allow both maternal participation in the birth and presence of the support person.¹⁰ In addition, these techniques minimize drug exposure to the fetus and avoid the need for airway manipulation (see Table 42-4).

LOCAL ANESTHETIC AGENTS AND ADJUNCT MEDICATIONS

Bupivacaine, ropivacaine, and lidocaine are the most widely LA used for NA for both labor analgesia and C-section. LA agents are chosen based on their duration of action, toxicity profile, and propensity toward sensory and motor blockade. Several adjuvant medications, including opioids, epinephrine, sodium bicarbonate, and clonidine, may be used to affect the duration, onset, and quality of anesthesia or analgesia.^10–11

GENERAL ANESTHESIA

GA may be used for cesarean delivery or nonobstetric surgery (NOBS) in the pregnant patient. Because of physiologic changes of pregnancy, airway management may be more challenging, especially at term and during labor. GA is required when neuraxial techniques are contraindicated or the patient refuses NA or is unable to cooperate. In addition, GA is necessary if there is insufficient time to perform NA, if NA fails, or for fetal indications. GA renders the patient unconscious and requires airway management, as it provides analgesia, autonomic stability, and amnesia, and it also offers significant muscle relaxation. The choice of medications depends on the patient’s comorbidities and surgical goals.

Nil per Os Guidelines

For elective surgical cases in healthy patients, fasting is recommended to reduce the risk of aspiration during sedation or induction of GA. American Society of Anesthesiologists (ASA) fasting guidelines are provided in Table 42-5.¹¹

For the patient, similar nil per os (NPO) guidelines are suggested; however, physiologic changes in pregnancy cause a delay in gastric emptying, which can be seen as early as 8 to 12 weeks gestation.¹² Because of this, patients (especially those beyond 12–16 weeks gestation) are considered at higher risk for aspiration than nonpregnant patients, regardless of their NPO status.¹⁴ During labor, ice chips and sips of clear liquids are permissible in low-risk patients, but solids must be avoided. Prior to administering an anesthetic for a procedure, aspiration prophylaxis is needed, with nonparticulate antacids (sodium citrate). In addition, coadministration of H2-receptor antagonists and/or metoclopramide can be considered; however, it may take 30 to 40 minutes to take full effect.¹³

Since the introduction of neuraxial labor analgesia (NLA) nearly 70 years ago, it has grown in acceptance as the most effective modality for pain control in labor. The evolution of the labor epidural through the introduction of low-dose anesthetics, advanced infusion modalities, and newer technologies continues to improve the safety and quality of NLA. In the United States, approximately 60% of women in labor request and receive labor epidurals.