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Pregnancy Physiology Shifts
TEXTObstetric I · 10 min read
Every organ system shifts. PaCO₂ 30, CO +40%, FRC -20%, hypercoagulable, aortocaval compression after 20 weeks. Memorize, don't derive.
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3 min read9 sections- Describe CV/pulm/heme adaptations of pregnancy.
- Predict drug-handling changes.
- Identify gestational-age thresholds for anesthetic risk.
- Plan for parturient airway considerations.
Respiratory shifts
- PaCO₂ ~
30 mmHg - compensatory HCO₃ ~
20 mEq/L - pH ~7.42-7.44
Diaphragm displaced cephalad by uterus.
FRC drops 20%, residual volume drops 20%, total lung capacity slightly lower.
O₂ consumption up 20-30% from metabolic demand.
Result: rapid desaturation during apnea (1-2 min vs 5+ non-pregnant).
Cardiovascular shifts
Plasma volume rises 40-50%, RBC mass 20-30% dilutional anemia (target hemoglobin >11).
SVR drops 20% (progesterone-mediated).
MAP slightly lower T2, recovers T3.
Labor: another 30% CO rise from contractions + pain.
Post-delivery peak: autotransfusion + decompression.
High-risk MS/cardiomyopathy patients most vulnerable first 24-48 hr postpartum.
GI + airway changes
Delayed gastric emptying (especially in labor).
Higher gastric volume + lower pH.
ALL pregnant patients ≥20 weeks treated as full stomach for anesthesia.
Airway: weight gain + breast enlargement + mucosal edema.
Mallampati class often progresses through pregnancy.
Difficult intubation rate ~1:200 (vs 1:2000 non-pregnant).
Hematologic + coagulation
VTE risk 4-5× baseline.
Mild thrombocytopenia common (gestational) — usually >100k, no bleeding risk.
ITP, HELLP, preeclampsia drop platelets further.
WBC mildly elevated baseline (10-15k).
Plan for these shifts when interpreting labs — abnormal-looking values may be physiologic.
Aortocaval compression
Supine position compresses IVC (reduces venous return) and aorta (reduces uteroplacental flow).
Maternal hypotension + fetal acidosis result.
Left lateral tilt 15° (or right hip wedge) mandatory for any supine intervention.
Even if mother feels fine, fetal compromise can be silent.
Universal precaution: never leave a pregnant patient (≥20 weeks) flat supine without tilt.
Drug pharmacology in pregnancy
INCREASED VD for water-soluble drugs from increased plasma volume + total body water.
PLACENTAL TRANSFER concerns — most anesthesia drugs cross (low MW + lipid-soluble + non-ionized = crosses well).
- propofol
- fentanyl
- ketamine OK in moderate doses for non-OB surgery during pregnancy
AVOID NSAIDs in 3rd trimester (ductus arteriosus premature closure), avoid high-dose 1st-trimester benzodiazepines (weak cleft palate signal), avoid sustained-release morphine in labor (neonatal respiratory depression).
Bupivacaine 0.75% was REMOVED from OB EPIDURAL labeling in the 1980s after cardiotoxicity deaths from inadvertent intravascular boluses — use 0.5% or 0.25% for OB epidural infusions.
(HYPERBARIC 0.75% bupivacaine remains the standard for spinal anesthesia for cesarean — small intrathecal volume, no IV-bolus risk.)
Renal + hepatic shifts
Cholestasis of pregnancy late T3 — not metabolic disease per se but symptom of hepatic stress.
AST/ALT normally NOT elevated — rise warrants workup (HELLP, AFLP, hepatitis).
Endocrine + metabolic shifts
Gestational diabetes screening 24-28 weeks.
Thyroid: estrogen-mediated thyroid-binding globulin rise total T3/T4 elevated, FREE T3/T4 unchanged in normal pregnancy.
Hyperthyroidism symptoms can mimic normal pregnancy.
Adrenal: cortisol elevated 2-3× baseline by term.
Glucose handling impaired with pregnancy stress + steroid effect.
Hypoglycemia common during prolonged labor or fasting.
Practical anesthesia implications
DOUBLE pre-oxygenation in pregnant patients with predicted difficult airway.
RAMPED + LEFT TILT for any supine intervention ≥20 wk.
SMALLER ETT (6.5-7.0) on hand for anticipated airway edema, especially preeclampsia.
LMA acceptable for non-aspiration-risk procedures only (in early pregnancy <20 weeks; full-stomach status applies after).
⚠ Common pitfalls
- Treating pregnant patient like any other for airway — Mallampati progresses through pregnancy; capillary engorgement + edema.
- Forgetting that supine position after 20 weeks impairs venous return.
- Standard heparin doses without checking renal function — increased GFR alters clearance.
- Hyperventilating routinely — alkalemia decreases umbilical blood flow.
💎 Clinical pearls
- Airway in pregnancy: smaller ETT (6.0-7.0), capillary friability — avoid nasal route, smooth intubation.
- Cardiac output peaks ~32 weeks at 30-50% above baseline; further 80-100% in labor (autotransfusion).
- Plasma cholinesterase decreases ~25% — minor sux prolongation, rarely clinical.
- Mendelson's pneumonitis risk → particulate antacid + H2 blocker + metoclopramide is the classic OB pre-induction.
Recap
- Airway in pregnancy: smaller ETT (6.0-7.0), capillary friability — avoid nasal route, smooth intubation.
- Cardiac output peaks ~32 weeks at 30-50% above baseline; further 80-100% in labor (autotransfusion).
- Plasma cholinesterase decreases ~25% — minor sux prolongation, rarely clinical.
- Mendelson's pneumonitis risk → particulate antacid + H2 blocker + metoclopramide is the classic OB pre-induction.
Mark each section done to complete the module.