Respiratory Changes of Pregnancy

Explain the physiological changes during pregnancy, and parturition

Respiratory changes in pregnancy are a function of two things:

• Anatomical compression of the chest
• Increased VO₂ and VCO₂

Anatomical Changes

• Diaphragm pushed upwards by ~4cm
• Increased AP and transverse diameter of the chest wall (~2-3cm)
• Large airway dilation, reducing airway resistance by ~35%

Volumes and Capacities

From conception until term:

• V_T increases by 40%
• Inspiratory capacity increases by 10%
• Expiratory capacity decreases by 30%
• Total lung capacity decreases by 5%
• Vital capacity is unchanged

From ~20 weeks until term:

• ERV decreases
• RV decreases
• FRC decreases
  • By 20% erect
  • By 30% supine

Ventilation

Progesterone stimulates respiratory centres, shifting the O₂ and CO₂ response curves to the left which causes hyperventilation and a respiratory alkalosis. From conception until term:

• MV increases by 50%
  • 10% increase in RR
  • 40% increase in V_T
• PCO₂ falls to ~26-32mmHg, with a compensatory drop in plasma [HCO₃^-] to 18-21mmol.L^-1

Labour and Postpartum

During labour:

• MV increases 70% due to pain and increased oxygen demand
• This causes hypocapnea, so cessation of uterine contractions (and the associated pain and oxygen demand) are followed by a hypoventilatory period producing desaturation

FRC and RV return to normal within 48 hours of delivery.

References

1. Kam P, Power I. Principles of Physiology for the Anaesthetist. 3rd Ed. Hodder Education. 2012.