Anatomy
- Airway oedema due to \(\downarrow\)colloid osmotic pressure \(\to\) friability, difficult intubation
- Diaphragm displaced upwards
- Relaxin \(\to\) ligamentous laxity \(\to\) increased AP and lateral dimensions of thorax \(\to\) partially compensates for smaller vertical dimension
Volumes
- FRC decreases 20%, and by further 10% when supine
- TLC stable \(\to\) inspiratory capacity increases
Compliance - chest wall compliance reduced due to abdominal compression
Resistance - Higher MV compensated by bronchodilation \(\to\) usually unchanged
Dead space and shunt - both typically unchanged
Control of breathing
- Progesterone \(\uparrow\) resp drive and sensitivity to CO2 \(\to\) progressive increase in Vt throughout early pregnancy, peaking around 20 weeks. RR increases slightly (only ~2 breaths/minute).
- Respiratory alkalosis, hypocapnia
Oxygen carriage
- Right shift due to \(\uparrow\)2,3DPG \(\to\) faciliates offloading to foetus
CO2 carriage
- Hypocapnia faciliates CO2 diffusion foetus\(\to\)parent
O2 reserve
- \(\uparrow\) BMR and O2 consumption, \(\downarrow\)FRC, \(\therefore \downarrow\)O2 reserve