Expansion
Tidal volume and minute ventilation rise, functional residual capacity falls
Expansion
Increases
- Tidal volume by 30 to 40 per cent, driven by progesterone sensitising the respiratory centre
- Minute ventilation by 40 to 50 per cent
- Oxygen consumption by about 20 to 30 per cent
- Respiratory rate changes little
Decreases
- Functional residual capacity by about 20 per cent, as the enlarging uterus elevates the diaphragm
- Residual volume and expiratory reserve volume
- PaCO2 falls to about 4.1 kPa (30 mmHg), with renal bicarbonate compensation giving a mild compensated respiratory alkalosis
Unchanged: vital capacity, total lung capacity and FEV1, since the diaphragm rises but the ribcage flares outwards to compensate.
The combination of a low functional residual capacity (a smaller oxygen store) and a high oxygen consumption means pregnant women desaturate very rapidly during apnoea. Together with airway oedema and a higher risk of aspiration, this is why obstetric airway management is considered high risk and preoxygenation is essential.