Physiological Changes in Pregnancy and Their Anesthetic Implications

1. Cardiovascular System

Anatomic Changes

Intravascular Volume

• Plasma volume increases 50-55% above baseline by term
• Driven by the renin-angiotensin-aldosterone axis (sodium retention + water retention) - likely triggered by rising progesterone
• Albumin falls by 25%, total protein by 10% → colloid osmotic pressure drops from 27 to 22 mm Hg
• This "physiologic dilutional anemia" is normal
• Volume returns to prepregnancy baseline at 6-9 weeks postpartum

Cardiac Output

Heart Rate and Stroke Volume

• HR increases 15-25% above baseline
• Stroke volume increases 25-30%

Systemic Vascular Resistance

• SVR decreases due to vasodilatory effects of progesterone, prostaglandins, and the low-resistance uteroplacental bed
• BP falls 5-20% by 20 weeks, then gradually recovers toward term
• Diastolic falls more than systolic → pulse pressure widens
• CVP and PCWP do not change (venous capacitance accommodates the volume increase)

Aortocaval Compression (Supine Hypotensive Syndrome)

• IVC is compressed in nearly ALL term parturients in the supine position
• Symptomatic hypotension (>15 mmHg MAP drop + HR >20 bpm rise) occurs in only 8-10%
• IVC compression → blood returns via azygos/epidural/vertebral veins (these become engorged)
• Aortoiliac compression occurs in 15-20%
• Supine position reduces CO by 10-20% vs upright
• Anesthetic relevance: neuraxial/general anesthesia abolishes the compensatory sympathetic vasoconstriction, making hypotension far more severe
• Management: left uterine displacement (15-30° left tilt or lateral position); right hip elevation by 10-15 cm

2. Respiratory System

Upper Airway

• Capillary engorgement, mucosal edema, and friability begin in the first trimester throughout the oropharynx, larynx, and trachea
• Nasal congestion and epistaxis are common
• Critical anesthetic implication: airway edema worsens with Valsalva, fluid overload, and preeclampsia; a smaller endotracheal tube (6.0-6.5 mm) is recommended; awake fiberoptic intubation threshold should be low
• Mallampati score worsens progressively through labor

Lung Volumes

O2 Consumption and Gas Exchange

• O2 consumption increases 20-35% at term (higher in labor)
• PaO2 increases (respiratory alkalosis shifts oxyhemoglobin curve)
• PaCO2 decreases to ~30-32 mmHg (progesterone-driven hyperventilation)
• pH is slightly alkalotic (~7.44), compensated by renal bicarbonate excretion (HCO3 falls to ~20 mEq/L)

3. Gastrointestinal System

• Lower esophageal sphincter (LES) tone decreases after 20 weeks of gestation
• The uterus displaces the stomach cephalad and rotates it, increasing intragastric pressure
• Gastric acid secretion increases (placental gastrin)
• Gastric emptying is essentially normal in uncomplicated pregnancy but can be markedly slowed in labor (pain, opioids, anxiety) and emergency situations
• All pregnant patients >20 weeks are considered at full-stomach risk
• Management: antacid prophylaxis (sodium citrate 30 mL PO), H2 antagonist or PPI, metoclopramide; rapid sequence induction (RSI) with cricoid pressure for general anesthesia

4. Central Nervous System and Pharmacology

MAC Reduction

• Minimum alveolar concentration (MAC) of volatile anesthetic agents decreases by approximately 25-40% during pregnancy
• Begins as early as 8-10 weeks
• Mechanism: progesterone has direct anesthetic and sedative properties; also endorphin elevation
• Implication: lower doses of volatile agents are needed to prevent awareness

Local Anesthetic Sensitivity

• Sensitivity to neuraxial and peripheral local anesthetics increases significantly
• Dose of local anesthetic should be reduced by 25-30% during any trimester
• Mechanisms: engorgement of epidural venous plexus (reduces epidural space volume), increased sensitivity of nerve membranes (progesterone effect), decreased protein binding
• Higher spread of spinal anesthesia is expected with the same intrathecal dose

Pharmacokinetics

• Increased volume of distribution (both hydrophilic and lipophilic drugs)
• Decreased protein binding (albumin and alpha-1-acid glycoprotein both fall) → increased free drug fraction
• Hepatic enzyme activity is variably affected
• Renal clearance increases (GFR up 50%)

5. Renal System

• GFR increases ~50% above baseline
• Renal plasma flow increases ~70%
• Serum creatinine and BUN fall (normal pregnancy creatinine <0.8 mg/dL; normal urea is lower than nonpregnant range)
• Glycosuria is common without hyperglycemia (reduced tubular reabsorption threshold)

6. Hematology and Coagulation

• RBC mass increases ~30%, but plasma volume increases ~50% → dilutional anemia (hemoglobin ~11-12 g/dL is normal at term)
• Leukocytosis up to 12,000-15,000/mm³ is normal
• Hypercoagulable state: factors I, VII, VIII, X, XII, and vWF all increase; fibrinogen increases dramatically (from ~300 to ~450-600 mg/dL)
• Protein S decreases; APC resistance increases
• This is physiologically appropriate for delivery but creates DVT risk
• Platelet count may slightly decrease (gestational thrombocytopenia)

7. Uterine Blood Flow

• Increases from ~100 mL/min (nonpregnant) to 700-900 mL/min at term (~10% of cardiac output)
• 80% goes to the intervillous space (placenta), 20% to myometrium
• Minimal autoregulation - vasculature is essentially maximally dilated
• Uterine blood flow = (Uterine arterial pressure - Uterine venous pressure) / Uterine vascular resistance

• Maternal hypotension (hypovolemia, spinal anesthesia, aortocaval compression)
• Vasopressors that increase uterine vascular resistance (historically alpha-agonists)
• Extreme hypocapnia (PaCO2 <20 mmHg) - direct uterine vasoconstriction + oxyhemoglobin curve shift
• Uterine hypertonus (oxytocin excess, abruptio)
• Valsalva/bearing down (increases uterine venous pressure)

Summary: Anesthetic Implications at a Glance