Effects of pregnancy on the respiratory system
Two independent changes
Two independent changes: a hormonal one (progesterone) and a mechanical one (the uterus). They affect different variables.
| Variable | Change | Driver |
|---|---|---|
| Tidal volume | inc ~30-50% | Progesterone |
| Minute ventilation | inc ~20-50% | Progesterone |
| Respiratory rate | Unchanged or minimally inc | - |
| FRC | dec ~20% | Uterus |
| ERV | dec ~20-25% | Uterus |
| RV | dec | Uterus |
| TLC | dec slightly (~5%) | Uterus |
| VC, FEV1, FVC, FEV1/FVC, PEF | UNCHANGED | - |
| O2 consumption | inc 20-30% | Fetus, uterus, work of breathing |
- *Spirometry does not change in normal pregnancy -> an abnormal FEV1 or FVC in a pregnant woman is disease, never physiology*
- Dyspnoea of pregnancy affects 60-70% of women, often from the first trimester - i.e. before the uterus is large enough to matter mechanically. It is the progesterone.
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