Maternal Physiology In Pregnancy (Table 32.1)

TABLE 32.1 Physiologic Changes Associated With Pregnancy

System	Parameter Changes	Anesthetic Implications
Respiratory	Anatomy Upper airway edema Mucosal capillary dilation Enlarged tongue and epiglottis Capacities/volume ↑Tidal volume by 45% ↑Inspiratory reserve volume by 5% ↔Vital capacity ↔Closing capacity ↓Total lung capacity by 5% ↓Functional residual capacity by 20% ↓Expiratory reserve volume by 20% ↓Residual volume by 15% Mechanics ↑Minute ventilation by 45% ↑Alveolar ventilation by 45% ↔FEV₁ ↔FEV₁/FVC Blood gases ↑Pao₂ by 5–10% (100–105 mm Hg [13.3–14.0 kPa]) ↔pH ↓Paco₂ by 10% (28–32 mm Hg [3.7–4.2 kPa]) ↓HCO₃ (18–22 mEq/L) Oxygen consumption ↑Oxygen consumption by 20% ↑P50 at term to 30 mm Hg	↑risk of epistaxis ↑risk of difficult laryngoscopy and intubation Rapid desaturation Risk of hypoxemia in the supine or Trendelenburg position ↓time to onset of volatile anesthetics
Cardiovascular	↑Cardiac output by 50% ↑Stroke volume by 25% ↑Heart rate by 20–25% ↓Systematic vascular resistance by 20% Aortocaval compression may lead to supine hypotension syndrome	↑risk of arrhythmia, most notably AF, pSVT, and pVT Blood loss may be more difficult to estimate Left uterine tilt after 20 wk of gestation
Hematology	↑Blood volume by 45% ↑Plasma volume by 55% ↑Red blood cell volume by 25% Coagulation factors ↑Factors VII, VIII, IX, X, XII, fibrinogen ↔Prothrombin ↔or ↓ Platelet count ↓Factors XI, XIII ↓Total protein (albumin, globulin)	↑risk of VTE ↓pseudocholinesterase activity rarely causes prolongation of muscle relaxant activity ↓protein binding ↑volume of distribution
Central nervous system	↓MAC Epidural venous engorgement ↑CSF pressure	↓Local anesthetic requirement ↑Risk of PDPH with accidental dural puncture
Gastrointestinal	Cephalad mechanical displacement of stomach ↓Gastric emptying during labor (to 18 h postpartum) ↓Lower esophageal sphincter pressure	↑risk of aspiration Rapid sequence induction in latter half of second and third trimesters
Hepatic	↑AST, ALT, LDH, bilirubin ↑Alkaline phosphatase ↑Gallbladder volume and ↓ motility	↑risk of gallbladder disease requiring nonobstetric surgery
Renal	↑Glomerular filtration rate by 50% ↑Renal plasma flow by 75%	Normal BUN and Cr may mask impaired renal function
Musculoskeletal	↑joint laxity ↑anteroposterior diameter of chest wall Exaggerated lumbar lordosis	↑cephalad spread of local anesthetics Chest wall may hinder laryngoscopy ↑risk of chronic back pain regardless of neuraxial analgesia/anesthesia use

AF, atrial fibrillation; ALT, alanine aminotransferase; AST, aspartate aminotransferase; CSF, cerebrospinal fluid; FEV₁, forced expiratory volume in one second; FVC, forced vital capacity; LDH, lactate dehydrogenase; MAC, minimum alveolar concentration; PDPH, postdural puncture headache; pSVT, paroxysmal supraventricular tachycardia; pVT, paroxysmal ventricular tachycardia; VTE, venous thromboembolism.

Author(s): A. Taylor Thomas, Emily Naoum

Respiratory system
1. Capillary engorgement of the mucosa may occur throughout the respiratory tract, beginning early in the first trimester and increasing throughout pregnancy. Historically, a 6.0- to 6.5-mm (inner diameter) endotracheal tube has been recommended for intubation to decrease the possibility of airway trauma in the setting of increased tissue friability. However, the use of larger tubes may be possible in most patients if required. Fluid retention may lead to an enlarged tongue, explaining the increased prevalence of Mallampati class 3 and 4 airways in term parturients compared to the general population. In addition, the airway examination may evolve during the course of labor resulting in a further increase in airway class. Hypertensive disorders of pregnancy are associated with higher Mallampati class airways. Lastly, because of mucosal engorgement, nasotracheal intubation may cause epistaxis and is best avoided in pregnant women.
2. Minute ventilation increases by 45% to support the high oxygen requirements of the mother and fetus and is driven by a proportional increase in tidal volume. Pulmonary resistance is decreased by 50% to accommodate these changes. As pregnancy progresses, elevation of the diaphragm from the gravid uterus leads to a 20% decrease in functional residual capacity (FRC). The resultant decrease in oxygen reserve mandates adequate preoxygenation prior to induction of general anesthesia.
Cardiovascular system
1. Cardiac output increases progressively by 50% from the end of the second trimester to term. This increase is primarily due to increased stroke volume, with a small contribution from heart rate. A grade II systolic ejection murmur at the left sternal border is commonly heard due to stroke volume augmentation in response to increased intravascular volume. During labor, contractions of the engorged uterus provide a 300- to 500-mL autotransfusion into the maternal circulation, further increasing cardiac output. Cardiac output is highest immediately postpartum, increasing 80% to 100% above prelabor values due to further autotransfusion from the placenta and loss of inferior vena cava (IVC) compression by the fetus. Despite this, blood pressure is not elevated at term due to decreased peripheral vascular resistance.
2. Supine hypotension may occur after 20 weeks of gestation when the gravid uterus compresses the IVC of the patient in the supine position. This decreases venous return, resulting in maternal hypotension and decreased uteroplacental blood flow. Left uterine displacement of at least 15 degrees may mitigate this problem.
Hematology
1. Blood volume increases markedly throughout pregnancy. Because plasma volume increases (up to 50% by 34 weeks of gestation) more than red cell mass, a relative dilutional anemia occurs.
2. The pregnant patient is hypercoagulable throughout gestation. The concentration of most coagulation factors increases (except factors XI and XIII), as does platelet production, activation, and consumption. This hypercoagulable state helps to limit blood loss at delivery, though also increases the risk of thrombotic complications.
Nervous system
1. The minimum alveolar concentration (MAC) for inhalational anesthetics is decreased up to 40% during pregnancy. The etiology is unclear but may be related to alterations of hormone and endorphin concentrations during pregnancy, which result in an increased pain threshold. However, this increased sensitivity to volatiles may not translate to an enhanced effect on the brain nor amnesia; therefore targeting a reduced MAC is not recommended because using a reduced MAC may increase the incidence of awareness under anesthesia. The speed of induction of general anesthesia is increased in pregnancy secondary to increased cardiac output for intravenous agents, and increased minute ventilation and decreased FRC for inhalational agents.
2. The local anesthetic dose required for regional anesthesia is reduced in the parturient compared to the nonpregnant patient. Reasons for this include the following:
  1. A decrease in cerebrospinal fluid (CSF) protein results in a greater proportion of free and active drug.
  2. Elevated CSF pH increases the unionized fraction of the local anesthetic.
  3. Distention of epidural veins during pregnancy results in decreased lumbar CSF volume, enhancement of local anesthetic spread, and reduction of the segmental dose requirement for spinal anesthesia.
3. Sympathetic nervous system (SNS) activity increases from as early as 6 weeks of gestation despite decreases in diastolic pressure and total peripheral resistance. The parturient is highly dependent on the SNS for hemodynamic control, which is reflected by the significant decrease in blood pressure seen after regional anesthesia. SNS activity may return to normal by 24 to 48 hours postpartum.
4. The apex of lumbar lordosis is shifted cephalad with reduced thoracic kyphosis while supine. This contributes to the increased cephalad spread of spinal anesthesia.
Gastrointestinal system. Because of the relaxation of the lower esophageal sphincter and mechanical displacement of the stomach by the gravid uterus, gastric reflux, and heartburn are present in 30% to 50% of pregnant patients. It is not clear when during the pregnancy the risk for aspiration increases, although the difference between intragastric pressure and lower esophageal sphincter tone is decreased as early as the first trimester. Gastric emptying is not delayed during pregnancy, but it is slowed during labor, particularly following the administration of opioids. When a general anesthetic is planned, a nonparticulate antacid should be given routinely; a histamine (H₂) blocker and metoclopramide should be considered. Patients in the latter half of the second and third trimesters should have a rapid sequence induction for general anesthesia.
Renal system. Renal plasma flow and glomerular filtration increase by up to 50%, leading to increased creatinine clearance and decreased blood urea nitrogen and serum creatinine levels.
Musculoskeletal system. Increased lumbar lordosis and intra-abdominal pressure secondary to the enlarging uterus can stretch and compress the lateral femoral cutaneous nerve and produce a sensory loss over the
anterolateral thigh (“meralgia paresthetica”). Carpal tunnel syndrome is also common and its etiology is likely multifactorial. Increases in the hormone relaxin result in progressive ligamentous laxity during pregnancy, peaking at 37 weeks. This allows for widening of the pubic symphysis and an increase in the anteroposterior and transverse diameters of the lower rib cage seen in parturients.

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