Anesthesia for Physiological Changes During Pregnancy

PHYSIOLOGICAL CHANGES DURING PREGNANCY

Pregnancy affects most organ systems (Table 40–1). Many of these physiological changes appear to be adaptive and useful to the mother in tolerating the stresses of pregnancy, labor, and delivery. Other changes lack obvious benefits but nonetheless require special consideration in caring for the parturient.

Central Nervous System Effects

The minimum alveolar concentration (MAC) progressively decreases during pregnancy-at term, by as much as 40%—for all general anes-thetic agents; MAC returns to normal by the third day after delivery. Changes in maternal hormonal and endogenous opioid levels have been impli-cated. Progesterone, which is sedating when given in pharmacological doses, increases up to 20 times normal at term and is at least partly responsible for this

observation. A surge in β-endorphin levels during labor and delivery also likely plays a major role.Pregnant patients also display enhanced sen-sitivity to local anesthetics during regionalanesthesia and analgesia, and neural blockade occurs at reduced concentrations of local anesthet-ics. The term minimum local analgesic concentra-tion (MLAC) is used in obstetric anesthesia to compare the relative potencies of local anesthetics and the effects of additives; MLAC is defined as the local analgesic concentration leading to satisfac-tory analgesia in 50% of patients (EC50). Local anesthetic dose requirements during epidural anes-thesia may be reduced as much as 30%, a phenom-enon that appears to be hormonally mediated but  may also be related to engorgement of the epiduralvenous plexus. Obstruction of the inferior vena cava by the enlarging uterus distendsthe epidural venous plexus and increases epidural blood volume. The latter has three major effects: (1) decreased spinal cerebrospinal fluid volume, (2) decreased potential volume of the epidural space, and (3) increased epidural (space) pressure. The first two effects enhance the cephalad spread of local anesthetic solutions during spinal and epi-dural anesthesia, respectively, whereas the last may complicate identification of the epidural space . Bearing down during labor further accentuates all these effects. Positive (rather than the usual negative) epidural pressures have been recorded in parturients. Engorgement of the epi-dural veins also increases the likelihood of placing an epidural needle or catheter in a vein, resulting in an unintentional intravascular injection. It is unclear whether pregnancy lowers the seizure threshold for local anesthetics.

Respiratory Effects

Oxygen consumption and minute ventilation pro-gressively increase during pregnancy. Tidal volume and, to a lesser extent, respiratory rate and inspi-ratory reserve volume also increase. By term, both oxygen consumption and minute ventilation have increased up to 50%. Paco₂ decreases to 28–32 mm Hg; significant respiratory alkalosis is prevented by a compensatory decrease in plasma bicarbonate concentration. Hyperventilation may also increase Pao₂ slightly. Elevated levels of 2,3-diphosphoglyc-erate offset the effect of hyperventilation on hemo-globin’s affinity for oxygen . The P₅₀ for hemoglobin increases from 27 to 30 mm Hg; the combination of the latter with an increase in cardiac output (see section on Cardiovascular Effects below) enhances oxygen delivery to tissues.

The maternal respiratory pattern changes as the uterus enlarges. In the third trimester, elevation of the diaphragm is compensated by an increase in the anteroposterior diameter of the chest; diaphrag-matic motion, however, is not restricted. Thoracic breathing is favored over abdominal breathing. Both vital capacity and closing capacity are minimally affected, but functional residual capacity (FRC) decreases up to 20% at term; FRC returns to normal within 48 h of delivery. This decrease is principally due to a reduction in expiratory reserve volume as a result of larger than normal tidal volumes. Flow– volume loops are unaffected, and airway resistance decreases. Physiological dead space decreases but intrapulmonary shunting increases toward term. A chest film may show prominent vascular markings due to increased pulmonary blood volume and an elevated diaphragm. Pulmonary vasodilation pre-vents pulmonary pressures from rising.

The combination of decreased FRC and increased oxygen consumption promotes rapid oxygen desaturation during periods of apnea. Preoxygenation (denitrogenation) prior to induc-tion of general anesthesia is therefore mandatory to avoid hypoxemia in pregnant patients. Closing volume exceeds FRC in some pregnant women when they are supine at term. Under these condi-tions, atelectasis and hypoxemia readily occur. The decrease in FRC coupled with the increase in minute ventilation accelerates the uptake of all inhalational anesthetics. The reduction in dead space narrows the arterial end-tidal CO ₂ gradient.

Capillary engorgement of the respiratory mucosa during pregnancy predisposes the upper airways to trauma, bleeding, and obstruction. Gentle laryngos-copy and smaller endotracheal tubes (6–6.5 mm) should be employed during general anesthesia.

Cardiovascular Effects

Cardiac output and blood volume increase to meet accelerated maternal and fetal metabolic demands. An increase (55%) in plasma volume in excess of an increase in red cell mass (45%) produces dilutional anemia and reduces blood viscosity. Hemoglobin concentration, however, usually remains greater than 11 g/dL. Moreover, in terms of tissue oxygen delivery, the reduction in hemoglobin concentration is offset by the increase in cardiac output and the rightward shift of the hemoglobin dissociation curve (see the section on Respiratory Effects). A decrease in systemic vascular resistance by the second trimes-ter decreases both diastolic and, to a lesser degree, systolic blood pressure. The response to adrenergic agents and vasoconstrictors is blunted.

At term, blood volume has increased by 1000–1500 mL in most women, allowing them to easily tolerate the blood loss associated with deliv-ery; total blood volume reaches 90 mL/kg. Average blood loss during vaginal delivery is 400–500 mL, compared with 800–1000 mL for a cesarean sec-tion. Blood volume does not return to normal until 1–2 weeks after delivery.

The increase in cardiac output (40% at term) is due to increases in both heart rate (20%) and stroke volume (30%). Cardiac chambers enlarge and myocardial hypertrophy is often noted on echo-cardiography. Pulmonary artery, central venous, and pulmonary artery wedge pressures remain unchanged. Most of these effects are observed in the first and, to a lesser extent, the second trimes-ter. In the third trimester, cardiac output does not appreciably rise, except during labor. The greatest increases in cardiac output are seen during labor and immediately after delivery (see the section on Effect of Labor on Maternal Physiology). Cardiac output often does not return to normal until 2 weeks after delivery. Decreases in cardiac output can occur in the supine position after week 20 of pregnancy. Such decreases have been shown to be secondary to impeded venous return to the heart as the enlarging uterus compresses the inferior vena cava.

Approximately 5% of women at term develop the supine hypotension syndrome (aortocavalcompression), which is characterized by hypoten-sion associated with pallor, sweating, or nausea and vomiting. The cause of this syndrome appears to be complete or near-complete occlusion of the inferior vena cava by the gravid uterus. When combined with the hypotensive effects of regional or general anesthesia, aortocaval compression can readily pro-duce fetal asphyxia. Turning the patient on her side typically restores venous return from the lower body and corrects the hypotension in such instances. This maneuver is most readily accomplished by placing a wedge (>15°) under the right hip. The gravid uterus also compresses the aorta in most parturients when they are supine. This latter effect decreases blood flow to the lower extremities and, more importantly, to the uteroplacental circulation. Uterine contrac-tion reduces caval compression but exacerbates aor-tic compression.

Chronic partial caval obstruction in the third trimester predisposes to venous stasis, phlebitis, and edema in the lower extremities. Moreover, compres-sion of the inferior vena cava below the diaphragm distends and increases blood flow through the para-vertebral venous plexus (including the epidural veins), and to a minor degree, the abdominal wall.

Lastly, elevation of the diaphragm shifts the heart’s position in the chest, resulting in the appear-ance of an enlarged heart on a plain chest film and in left axis deviation and T wave changes on the elec-trocardiogram. Physical examination often reveals a systolic ejection flow murmur (grade I or II) and exaggerated splitting of the first heart sound (S₁); a third heart sound (S₃) may be audible. A few patients develop small, asymptomatic pericardial effusion.

Renal & Gastrointestinal Effects

Renal plasma flow and the glomerular filtra-tion rate increase during pregnancy, and as a result serum creatinine and blood urea nitrogen may decrease to 0.5–0.6 mg/dL and 8–9 mg/dL, respectively. A decreased renal tubular threshold for glucose and amino acids is common and often results in mild glycosuria (1–10 g/d) or proteinuria (<300 mg/d), or both. Plasma osmolality decreases by 8–10 mOsm/kg.

Gastroesophageal reflux and esophagitis are common during pregnancy. Gastric motility is reduced, and upward and anteriordisplacement of the stomach by the uterus promotes incompetenceof the gastroesophageal sphincter. These fac-tors place the parturient at high risk for regur-

gitation and pulmonary aspiration. However, neither gastric acidity nor gastric volume changes signifi-cantly during pregnancy. Opioids and anticholiner-gics reduce lower esophageal sphincter pressure, may facilitate gastroesophageal reflux, and delay gastric emptying.

Hepatic Effects

Overall hepatic function and blood flow are unchanged; minor elevations in serum trans-aminases and lactic dehydrogenase levels may be observed in the third trimester. Mild elevations in serum alkaline phosphatase are due to its secretion by the placenta. A mild decrease in serum albu-min is due to an expanded plasma volume, and as a result, colloid oncotic pressure is reduced. A 25–30% decrease in serum pseudocholinesterase activity is also present at term but rarely produces significant prolongation of succinylcholine’s action. The break-down of ester-type local anesthetics is not apprecia-bly altered. Pseudocholinesterase activity may not return to normal until up to 6 weeks postpartum. High progesterone levels appear to inhibit the release of cholecystokinin, resulting in incomplete emptying of the gallbladder. The latter, together with altered bile acid composition, can predispose to the forma-tion of cholesterol gallstones during pregnancy.

Hematological Effects

Pregnancy is associated with a hypercoagulable state that may be beneficial in limiting blood loss at delivery. Fibrinogen and concentrations of factors VII, VIII, IX, X, and XII all increase; only factor XI levels may decrease. Accelerated fibrinolysis can be observed late in the third trimester. In addition to the dilutional anemia (see the section on Cardiovascular Effects), leukocytosis (up to 21,000/µL) and a 10% decrease in platelet count may be encountered dur-ing the third trimester. Because of fetal utilization, iron and folate deficiency anemias readily develop if supplements of these nutrients are not taken.

Metabolic Effects

Complex metabolic and hormonal changes occur during pregnancy. Altered carbohydrate, fat, and protein metabolism favors fetal growth and devel-opment. These changes resemble starvation, because blood glucose and amino acid levels are low whereas free fatty acids, ketones, and triglyceride levels are high. Nonetheless, pregnancy is a diabetogenic state; insulin levels steadily rise during pregnancy. Secretion of human placental lactogen, also called human chorionic somatomammotropin, by the pla-centa is probably responsible for the relative insulin resistance associated with pregnancy. Pancreatic beta cell hyperplasia occurs in response to an increased demand for insulin secretion.

Secretion of human chorionic gonadotropin and elevated levels of estrogens promote hypertrophy of the thyroid gland and increase thyroid-binding globu-lin; although T4 and T3 levels are elevated, free T4, free T3, and thyrotropin (thyroid-stimulating hormone) remain normal. Serum calcium levels decrease, but ionized calcium concentration remains normal.

Musculoskeletal Effects

Elevated levels of relaxin throughout pregnancy help prepare for delivery by softening the cervix, inhib-iting uterine contractions, and relaxing the pubic symphysis and pelvic joints. Ligamentous laxity of the spine increases the risk of back injury. The lat-ter may contribute to the relatively high incidence of back pain during pregnancy.