Can you provide a complete lecture on physiological changes, um, in pregnancy? Um, but... Do not over-explain anything. Just, um, all the systems and one point in whether which thing increases, which decreases, and the reason behind it.
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Physiological Changes in Pregnancy
Source: Morgan & Mikhail's Clinical Anesthesiology, 7e
1. Cardiovascular System
| Parameter | Change | Reason |
|---|---|---|
| Blood volume | +35% | Increased plasma + RBC mass; supports uteroplacental perfusion |
| Plasma volume | +55% | Aldosterone/estrogen-driven Na+ and water retention |
| Cardiac output | +40% | HR + SV increase; peaks mid-2nd trimester |
| Stroke volume | +30% | Increased preload from expanded blood volume |
| Heart rate | +20% | Sympathetic drive + progesterone effect |
| Systolic BP | -5% | Vasodilation outweighs increased CO |
| Diastolic BP | -15% | Progesterone relaxes smooth muscle; low peripheral resistance |
| Peripheral vascular resistance | -15% | Progesterone-mediated vasodilation + placental arteriovenous shunt |
| Pulmonary vascular resistance | -30% | Pulmonary vasodilation prevents pressure rise |
Key point: Aortocaval compression by the gravid uterus in the supine position (after 20 weeks) can drop CO by up to 40% - the "supine hypotension syndrome." Left lateral tilt fixes it.
2. Respiratory System
| Parameter | Change | Reason |
|---|---|---|
| Oxygen consumption | +20-50% | Increased metabolic demand (fetus, uterus, maternal work) |
| Minute ventilation | +50% | Progesterone stimulates respiratory center (acts as direct stimulant) |
| Tidal volume | +40% | Primary driver of increased minute ventilation |
| Respiratory rate | +15% | Minor contribution |
| FRC | -20% | Diaphragm elevation by gravid uterus; reduced expiratory reserve volume |
| PaCO2 | -15% (28-32 mmHg) | Hyperventilation - progesterone-driven |
| HCO3- | -15% | Compensatory renal excretion to prevent severe alkalosis |
| PaO2 | +10% | Hyperventilation; slightly higher than normal |
| Airway resistance | -35% | Bronchodilation from progesterone |
Key point: FRC falls + O2 consumption rises = rapid desaturation during apnea. Pre-oxygenation before intubation is mandatory in pregnant patients.
3. Hematological System
| Parameter | Change | Reason |
|---|---|---|
| Hemoglobin | -20% | Dilutional - plasma expands more (+55%) than RBC mass (+20-30%), creating "physiological anemia of pregnancy" |
| Platelets | -10% | Hemodilution + increased platelet consumption |
| Clotting factors (I, VII, VIII, IX, X, XII) | +30-250% | Estrogen effect; prepares for hemostasis at delivery |
| Protein C, S | Decreased | Net hypercoagulable state - 5x increased DVT risk |
| Fibrinogen | Increases significantly | Acute-phase reactant; enhanced synthesis |
Key point: Pregnancy is a procoagulant state - needed for delivery hemostasis but causes DVT/PE risk.
4. Renal System
| Parameter | Change | Reason |
|---|---|---|
| GFR | +50% | Increased CO raises renal plasma flow; starts in 1st trimester |
| Serum creatinine | Decreased (0.4-0.8 mg/dL normal) | GFR increase dilutes creatinine; "normal" creatinine in pregnancy may mask impairment |
| BUN | Decreased | Same reason |
| Glucosuria | Present despite normal glucose | GFR overwhelms tubular reabsorption threshold |
| Urinary frequency | Increased | Pressure from uterus + increased GFR |
| Renal pelvis/calyces | Dilated (right > left) | Progesterone relaxes ureteral smooth muscle + uterine compression |
5. Central Nervous System
| Parameter | Change | Reason |
|---|---|---|
| MAC (minimum alveolar concentration) | -40% | Progesterone (sedating at high levels) + elevated β-endorphins |
| Local anesthetic requirement | -30% | Engorged epidural veins reduce epidural space volume; hormonal sensitization |
| Pain threshold | Altered | β-endorphin surge during labor |
Key point: MAC returns to normal by day 3 postpartum.
6. Gastrointestinal System
| Parameter | Change | Reason |
|---|---|---|
| Gastric emptying | Delayed | Progesterone relaxes gastric smooth muscle; later - mechanical displacement by uterus |
| Lower esophageal sphincter tone | Decreased | Progesterone effect |
| Intragastric pressure | Increased | Gravid uterus displaces stomach upward |
| Gastric pH | More acidic (increased volume) | Elevated gastrin from placenta |
Key point: The combination of decreased LES tone + increased intragastric pressure + delayed emptying = very high aspiration risk. All pregnant patients beyond 1st trimester are treated as full stomach.
7. Hepatic System
| Parameter | Change | Reason |
|---|---|---|
| Plasma cholinesterase (pseudocholinesterase) | -25-30% | Hemodilution; reduced synthesis - prolongs succinylcholine action |
| Albumin | Decreased | Hemodilution; reduced synthesis relative to expansion |
| Alkaline phosphatase | Increased (up to 4x normal) | Placental isoform - not liver disease |
| ALT/AST | Normal | Any elevation suggests pathology (pre-eclampsia, HELLP, cholestasis) |
| Serum bilirubin | Slight increase | Reduced bile flow due to progesterone |
8. Endocrine System
| Parameter | Change | Reason |
|---|---|---|
| Progesterone | Marked increase (20x at term) | Corpus luteum then placenta; relaxes smooth muscle, stimulates breathing, sedation |
| Estrogen | Markedly increased | Placenta; drives blood volume expansion, clotting factors |
| hCG | Peaks at 10-12 weeks | Placenta; maintains corpus luteum early on; responsible for morning sickness |
| Cortisol | Increased (total) | Increased CBG + increased synthesis; free cortisol also increases |
| Thyroid hormones (total T3/T4) | Increased | hCG stimulates TSH receptor + increased TBG (estrogen) - free T3/T4 remains normal |
| Insulin resistance | Increased | Human placental lactogen (hPL) antagonizes insulin; physiological gestational diabetes |
| PTH-related protein | Increased | Placenta-derived; maintains fetal calcium delivery |
9. Musculoskeletal / Other
| Parameter | Change | Reason |
|---|---|---|
| Lumbosacral lordosis | Increased | Center of gravity shifts anteriorly |
| Symphysis pubis & sacroiliac joints | Widened/laxity | Relaxin (from corpus luteum/placenta) loosens ligaments |
| Diaphragm | Elevates ~4 cm | Uterine enlargement |
| Skin (spider angiomas, palmar erythema) | Appear | Elevated estrogen |
| Weight gain | 11-16 kg average | Fetus, placenta, amniotic fluid, uterus, breast tissue, expanded blood volume, fat stores |
Quick Summary Mnemonics
What INCREASES: CO, HR, SV, blood volume, O2 consumption, minute ventilation, TV, GFR, clotting factors, fibrinogen, aldosterone, cortisol, hCG, progesterone, estrogen, ALP, weight
What DECREASES: FRC, PaCO2, HCO3-, Hb, platelets, SVR, PVR, BP (diastolic more), albumin, pseudocholinesterase, MAC, serum Cr/BUN, LES tone
- Morgan & Mikhail's Clinical Anesthesiology, 7e, Chapter 40
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