Physiological Changes in Pregnancy

Physiological Changes in Pregnancy

Physiological Changes in Pregnancy

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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. Lung volume changes in pregnancy diagram

⭐ 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.

Physiological Changes in Pregnancy Diagram

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. Physiological Changes in Pregnancy

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.

Practice Questions: Physiological Changes in Pregnancy

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Which drug is associated with decreased fetal heart rate during labor?

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Flashcards: Physiological Changes in Pregnancy

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_____ anesthesia is the first choice for most patients with preeclampsia during labor, vaginal delivery, and cesarean section

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_____ anesthesia is the first choice for most patients with preeclampsia during labor, vaginal delivery, and cesarean section

Epidural

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