Respiratory changes during pregnancy

• Greater resistance to flow
• Bag-valve-mask ventilation becomes more difficult
• Greater risk of airway obstruction with sedation

• The diaphragm is pushed up by 4cm
• Diaphragmatic excursion increases by about 2cm
• Rib cage expands: subcostal angle of the ribs at the xiphoidal level
  increases from 68.5° at the beginning of pregnancy to 103.5° at term
• Anatomical dead space increases by about 45% due to increased airway diameter late in pregnancy

• Tidal volume increases  by ~ 30-50%
• Respiratory rate increases  to 15-17
• Minute volume increases by 20-50%.
• TLC is reduced by about 5%
• IC is increased by about 10%
• FRC is decreased by about 20%

• Chest wall compliance decreases due to increased fat and abdominal content
• Lung compliance remains the same

Resistance

• Resistance to air flow in the lower airways increases in early pregnancy, and decreases in late pregnancy
• FEV1/FVC is stable over the course of pregnancy

• PaCO₂ decreases
• PaO₂ increases

• Maternal erythrocytes produce higher levels of 2,3-DPG
• p50 remains normal (by the combined effects of 2,3-DPG increase and alkalosis)

Central respiratory control

• Progesterone-associated chronic respiratory alkalosis develops (due to increased sensitivity to CO₂)

Demands on the respiratory system

• Increased body mass (increased weight plus one extra organism on board) = increased total body oxygen demand (by about 21%)  and increased ventilatory requirements for the clearance of the excess CO₂
• During labour, the oxygen consumption increases by about 60%.