Physiological Changes in Pregnancy

Respiratory System Changes - Breathing for Two

• Airway: Mucosal engorgement & edema (↑ Mallampati, ↓ glottic view; difficult intubation risk ↑).
• Ventilation:
  • Minute Ventilation (MV) ↑ by 50% (Tidal Volume ↑ 40%, Respiratory Rate ↑ 15%).
  • Progesterone: primary respiratory stimulant, sensitizes respiratory center to $CO_2$.
• Lung Volumes & Capacities:
  • Functional Residual Capacity (FRC) ↓ by 20% (Expiratory Reserve Volume ↓, Residual Volume ↓) due to diaphragmatic elevation.
  • Inspiratory Capacity (IC) ↑; Vital Capacity (VC) unchanged.
• Arterial Blood Gases (ABG):
  • $PaCO_2$ ↓ to 28-32 mmHg (compensated respiratory alkalosis).
  • $PaO_2$ ↑ to 100-105 mmHg.
  • Bicarbonate ($HCO_3^-$) ↓ (renal compensation).
• Oxygen consumption ($VO_2$) ↑ by 20% at term.

⭐ FRC reduction (by 20-30%) is a key change, predisposing parturients to rapid desaturation during apnea or induction of anesthesia.

Cardiovascular System Changes - Heart's Heavy Load

• Cardiac Output (CO): ↑ 30-50% (peaks early 3rd trimester/immediately postpartum).
  • Early: ↑ Stroke Volume (SV) by 25-30%.
  • Later: ↑ Heart Rate (HR) by 15-25% (↑ 15-20 bpm).
• Systemic Vascular Resistance (SVR): ↓ ~20% (progesterone, PGs).
• Blood Pressure (BP):
  • DBP ↓ (nadir 24-28 wks), SBP slight ↓/unchanged.
  • MAP ↓.
• Aortocaval Compression (Supine Hypotension):
  • From ~20 wks, gravid uterus compresses IVC & aorta.
  • Leads to ↓ venous return, ↓ CO, hypotension.
  • Management: Left Uterine Displacement (LUD).
• Heart Sounds & ECG:
  • Common: Systolic ejection murmur (flow), S3.
  • ECG: Left axis deviation, T-wave flattening/inversion (III, aVF).
• Uterine Blood Flow: ↑ significantly (to 700-900 mL/min).

⭐ Cardiac output is highest immediately postpartum, making this a vulnerable period.

Hematological Changes - Blood & Clot Tales

• Plasma volume ↑ 40-50%; Red cell mass ↑ 20-30%.
• Physiological (dilutional) anemia: Hb often 10-11 g/dL.
• Hypercoagulable state:
  • ↑ Fibrinogen (by 50%), Factors VII, VIII, X.
  • ↓ Protein S; Fibrinolysis ↓ (↑ PAI-1, PAI-2).
  • Platelets: Normal or mild gestational thrombocytopenia.
• Implications: ↑ DVT/PE risk.

⭐ Fibrinogen levels increase by approximately 50%, a key factor in pregnancy-induced hypercoagulability_._

Gastrointestinal & Renal Changes - Gut & Kidney Shifts

• Gastrointestinal (GI):
  • ↓ Lower Esophageal Sphincter (LES) tone (progesterone), delayed gastric emptying → ↑ Aspiration risk (📌 Progesterone's Gut Problems)
  • ↑ Intragastric pressure (gravid uterus)
  • Nausea & Vomiting in Pregnancy (NVP) common; Gallbladder stasis → ↑ gallstone risk
  • Alkaline phosphatase ↑ (placental)
• Renal System:
  • ↑ Renal Plasma Flow (RPF) & ↑ Glomerular Filtration Rate (GFR) by ~50% → ↓ serum creatinine, BUN
  • Physiological hydronephrosis (progesterone, compression) → ↑ UTI risk
  • Glucosuria common; Proteinuria up to 300 mg/day normal

⭐ Progesterone-induced ↓ LES tone and delayed gastric emptying significantly elevate aspiration risk under anesthesia. ​

Nervous System & Pharmacology - Sensitive & Swift

• Anesthetic Sensitivity ↑:
  • Volatile MAC: ↓ 25-40%.
  • Neuraxial blocks: ↓ dose needed (↑ sensitivity, ↓ space volume).
• Hormonal Effects:
  • Progesterone: sedative, ↑ ventilatory drive.
  • Endorphins: ↑ pain threshold.
• Anatomical Changes:
  • Epidural vein engorgement, ↓ CSF volume → ↑ local anesthetic spread.
• Pharmacokinetics:
  • ↓ Plasma cholinesterase (minor succinylcholine effect).
  • ↑ Vd, ↓ protein binding → ↑ free drug fraction. 📌 Mnemonic: MAC Down, Spread Wide (MAC, Dose, Spread)

⭐ MAC for volatile anesthetics is significantly reduced (by 25-40%) in pregnancy.

High‑Yield Points - ⚡ Biggest Takeaways

• Cardiac output ↑ by 30-50% and SVR ↓; aortocaval compression is a key concern.
• FRC ↓ by 20-30% and O₂ consumption ↑, risking rapid desaturation during apnea.
• Minute ventilation ↑ by 50% (respiratory alkalosis); airway edema increases difficult intubation risk.
• MAC of volatile anesthetics ↓ by 25-40%; sensitivity to neuraxial blockade ↑.
• ↓ LES tone and delayed gastric emptying ↑ aspiration risk.
• Hypercoagulable state (↑ VTE risk) and physiological dilutional anemia are present.