Respiratory Changes in Pregnancy

Structural changes:- Diaphragm elevated 4 cm (uterus pushes up) - Thoracic cage widens (subcostal angle increases from 68° to 103°) - These changes compensate — overall tidal volume increases despite elevated diaphragm Lung volumes: - Tidal volume (TV): INCREASES 40–50% (most important change) — more air per breath - Residual volume (RV): decreases 20% — air remaining after maximum exhalation - Functional residual capacity (FRC): decreases 20–25% - Total lung capacity (TLC): slightly decreases (elevated diaphragm) - Vital capacity: unchanged or slightly increased - Respiratory rate: unchanged or minimally increased (12–16/min) - Minute ventilation: INCREASES 40–50% (TV × RR) — primary driver Gas exchange: - Progesterone stimulates respiratory center → increases sensitivity to CO2 → hyperventilation - Result: maternal PaCO2 decreases to 27–32 mmHg (normal non-pregnant: 35–45) - Compensatory metabolic acidosis: bicarbonate (HCO3−) decreases to 18–21 mEq/L - Result: mildly alkalotic ABG: pH 7.40–7.47 - Maternal PaO2 increases slightly (to 104–108 mmHg) - This creates a gradient favoring O2 transfer to fetus (fetal pO2 remains lower) Dyspnea of pregnancy: - Common complaint (~70% of pregnant women), especially 2nd trimester...