How does ventilation change in pregnancy, and what drives it?

Minute ventilation increases by ~40%, driven almost entirely by an increase in tidal volume (~500 to 700 mL) - the respiratory rate stays unchanged.

The increase is due to progesterone sensitising the respiratory centre to carbon dioxide and the increased metabolic rate. The woman perceives this hyperventilation as physiological dyspnoea (breathlessness).

Clinical relevance. The progesterone-driven rise in ventilation explains physiological breathlessness and the respiratory alkalosis of pregnancy.

More Detail

Ventilatory changes in pregnancy

Pregnancy increases ventilation to meet the higher oxygen demand and clear fetal CO2.

The change

• Minute ventilation (tidal volume x respiratory rate) increases by ~40%.
• This is due almost entirely to an increase in tidal volume (from ~500 to ~700 mL) - the respiratory rate remains unchanged.

What drives it

• Progesterone sensitises the respiratory centre to carbon dioxide, lowering the CO2 threshold for ventilation.
• The increased metabolic rate (oxygen consumption rises ~20%).

Consequence

• The increased ventilation is perceived as shortness of breath (physiological dyspnoea) by many pregnant women - usually mild and not limiting (distinguish from pathological dyspnoea).
• It also produces a mild respiratory alkalosis (next card).