Applied Resp – Other Physiology

Master answer

Respiratory Changes In Pregnancy

Anatomical

• Diaphragm ascends ~4cm due to foetus
• AP and lateral diameters of thorax increase 2~3cm due to relaxin effect on chest
  wall
• Thoracic circumference increases 5~7cm (AP and lateral diameters increased 2~3cm)
• Airway dilatation → decreased resistance (35%) and increased deadspace (45%)
• Increased cardiac output, intravascular volume, and decreased oncotic pressure
  • Increased plasma volume → increased pulmonary blood volume → decreased compliance
  • Venous engorgement → Upper airway oedema

Mechanical

• Lung volume changes occur from 20 weeks
• TLV decreases 5% from baseline
• FRC decreased 20% due to both ERV and RV
• Significant postural change in FRC of 70%
• Progesterone → increased sensitivity to CO2 → Minute ventilation increases 50%
  • TV increases (25%)
  • RR increases
  • Results in hypocarbia and compensated respiratory alkalosis
    • pCO2 ~ 26-32mmHg
    • HCO3 ~ 18-20mmol/l
  • Further increase in MV during contractions
• Inspiratory capacity increased by 10%
• Closing capacity unchanged
• Increased O2 consumption during labour (60%)

After birth

• FRC and RV return to normal within 48 hours
• TV returns to normal within 5 days

Sakurai 2016

Answer by Sakurai