Obstetric Topic 06. Physiologic changes in pregnancy (cardiovascular, respiratory system)

I. Cardiovascular System

Main Hemodynamic Changes

• Purpose: enhanced uteroplacental blood flow + maternal organ perfusion → maintain pregnancy.
• Cardiac output (CO): increases by about 30-40%.
• Timing: starts around GW6, rises in 1st trimester, peaks around GW24.
• Mechanism: CO = stroke volume x heart rate.

• Plasma volume ↑ → preload ↑ → stroke volume ↑.
• Heart rate ↑, starts around GW8 and peaks around GW28.
• Progesterone-mediated smooth muscle relaxation → vasodilation → total peripheral resistance ↓.

• Blood flow increases especially to uterus, breasts, skin and kidneys.
• Multiple pregnancy: CO rise can be greater than in singleton pregnancy.

Blood Pressure and Vascular Resistance

• BP: slightly decreases because total peripheral resistance falls.
• Lowest BP: usually midpregnancy.
• 3rd trimester: BP returns toward prepregnancy level.
• Exam point: new hypertension after 20 weeks is not physiologic → think gestational hypertension/preeclampsia.

Blood Volume and Blood Cells

• Plasma volume: increases by about 50%.
• RBC mass: increases by about 20-30% if iron supply is adequate.
• Plasma volume ↑ more than RBC mass ↑ → physiological / dilutional anemia.
• Leukocytosis: mild physiological WBC increase, especially in labor/postpartum.
• Hypercoagulability + venous stasis → increased VTE risk.

Heart Findings

• Heart: high-flow state → soft systolic murmur can be normal.
• ECG/radiology: mild left-axis deviation/horizontalization can be physiological.
• Palpitations/extrasystoles: common and often benign.
• Red flags: diastolic murmur, loud systolic murmur, cyanosis, syncope, chest pain, persistent tachyarrhythmia, pulmonary edema.

Venous System

• Enlarged uterus compresses pelvic veins/IVC → venous stasis in lower limbs and pelvis.
• Progesterone-mediated venodilation → venous pooling.
• Consequences:

• Dependent edema.
• Varicose veins in legs/vulva.
• Hemorrhoids.
• Higher thrombosis risk.

• LMWH: preferred anticoagulant if prophylaxis/treatment is indicated; does not significantly cross placenta.

Supine Hypotensive / Aortocaval Syndrome

• Mechanism: supine gravid uterus compresses IVC ± aorta → venous return ↓ → preload/CO ↓ → sudden BP drop.
• Typical after midpregnancy, especially late pregnancy.
• Symptoms: dizziness, pallor, nausea, sweating, tachycardia, hypotension, collapse.
• Fetal effect: uteroplacental perfusion ↓ → fetal bradycardia/distress.
• Treatment: turn mother to left lateral position or manually displace uterus left; fluids/oxygen if needed.
• Prevention: avoid prolonged flat supine position after about 20 weeks.

II. Respiratory System

Purpose and Drivers

• Purpose: increase maternal O2 delivery and help fetal CO2 transfer to the mother.
• Progesterone stimulates respiratory center sensitivity to CO2.
• Maternal O2 consumption increases because of uterus, fetus, placenta and higher cardiac/renal work.
• Enlarged uterus elevates diaphragm → lower resting lung volumes.

Ventilation and Blood Gases

• Respiratory rate: usually unchanged or slightly increased.
• Tidal volume: increases by about 30-40%.
• Minute ventilation: increases mainly because tidal volume increases.
• Blood gases: PaO2 mildly ↑ + PaCO2 ↓ → mild respiratory alkalosis.
• Clinical point: PaCO2 around 40 mmHg is not reassuring in pregnancy; it may mean hypoventilation.

Lung Volumes and Mechanics

• Diaphragm elevation → functional residual capacity and residual volume decrease.
• Total lung capacity: slightly decreases.
• Vital capacity: usually unchanged.
• Airway mucosa: more vascular/edematous → nasal congestion, epistaxis, difficult airway risk.

Dyspnea in Pregnancy

• Physiologic dyspnea: common; usually mild, gradual and without hypoxia.
• Mechanism: ventilation ↑ + O2 demand ↑ + diaphragm elevation.
• Red flags: dyspnea at rest, chest pain, hemoptysis, syncope, cyanosis, fever, wheeze, unilateral leg swelling, low O2 saturation.
• Anesthesia relevance: FRC ↓ + O2 consumption ↑ → faster desaturation during apnea/general anesthesia.