Respiratory alkalosis of pregnancy
A mild respiratory alkalosis (a raised blood pH caused by breathing out too much carbon dioxide) is normal in pregnancy, because progesterone increases tidal volume.
Remember
Progesterone makes pregnant women breathe more deeply, so tidal volume rises and the arterial carbon dioxide tension (PaCO2, the pressure of carbon dioxide dissolved in arterial blood) falls. That gives a normal, mild respiratory alkalosis.
In pregnancy, progesterone stimulates the brain's respiratory centres (the central ventilatory drive), so women breathe more deeply. The tidal volume (the volume of air moved in and out with each normal breath) rises, and so does minute ventilation (the volume of air breathed each minute).
More carbon dioxide is breathed out, so the arterial PaCO2 falls. Carbon dioxide acts as an acid in the blood, so losing extra of it makes the blood slightly more alkaline. That is a mild respiratory alkalosis. The normal pH range in pregnancy is 7.40 to 7.45, and the kidneys compensate by lowering serum bicarbonate, which keeps the change mild.
The change is physiological rather than a disease, and it is helpful because it enhances gas exchange with the placenta. A lower carbon dioxide pressure in the mother's blood favours movement of carbon dioxide from the foetus into the maternal circulation.
Later in pregnancy the growing uterus pushes the diaphragm upwards (splinting it), which can make deep breaths harder, so many well women feel a little breathless. Splinting is mechanical and does not cause the alkalosis. Peak flow (the fastest rate of air flow in a forced breath out) and FEV1 (the volume breathed out in the first second of a forced breath) should remain unchanged.
Key facts
- Cause
- progesterone raises ventilatory drive, giving a larger tidal volume and a lower arterial pCO2.
- Blood gas
- arterial pCO2 is lower than in non-pregnant women and the pH is slightly higher, with a normal range in pregnancy of 7.40 to 7.45.
- Type of alkalosis
- respiratory, because the primary change is a low PaCO2 rather than a raised bicarbonate.
- Benefit
- a low maternal carbon dioxide pressure favours movement of carbon dioxide from the foetus into the maternal circulation.
- Unchanged
- peak flow and FEV1 should remain unchanged, which matters when managing asthma in pregnancy.
- Symptom
- mild breathlessness is common in well pregnant women, but sudden or severe breathlessness needs assessment.
Exam traps
- A faster respiratory rate is not the main way minute ventilation rises in pregnancy, because the rise comes mostly from a larger tidal volume.
- Functional residual capacity (the volume of air left in the lungs after a normal breath out) does fall in late pregnancy, but that is a change in lung volume rather than in the air breathed each minute, so it does not explain the lower carbon dioxide.
- A reduced drive to breathe does not explain the alkalosis of pregnancy: it would let carbon dioxide build up and make the blood more acidic, but progesterone raises the drive to breathe.
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