Why Mild Respiratory Alkalosis Occurs in Pregnancy
Pregnant women develop mild respiratory alkalosis because progesterone increases minute ventilation by 20-40% above baseline, which exceeds the increased oxygen demands of pregnancy, resulting in lower arterial CO2 levels with compensatory renal bicarbonate excretion. 1
Primary Mechanism: Progesterone-Driven Hyperventilation
The fundamental cause is hormonal:
- Progesterone directly increases respiratory center sensitivity, driving minute ventilation up by 20-40% above baseline by term 1, 2
- This hyperventilation is not triggered by central chemosensitive areas or peripheral chemoreceptors, but rather by progesterone's direct effect on the respiratory control centers 3
- The degree of hypocapnia correlates closely with arterial progesterone levels, as demonstrated in studies comparing follicular phase, luteal phase, and pregnancy 3
Metabolic Context
The respiratory alkalosis serves a physiological purpose:
- Oxygen consumption rises 20-33% above baseline by the third trimester due to increased metabolic demands of the fetus, placenta, and maternal organs 2, 4
- The increased minute ventilation exceeds even these elevated oxygen demands, resulting in net CO2 elimination and alkalosis 5
- This creates a favorable gradient for CO2 transfer from fetal to maternal circulation across the placenta 6
Compensatory Response
The body adapts to maintain pH homeostasis:
- Renal excretion of bicarbonate occurs as compensation, resulting in a "fully compensated respiratory alkalosis" that is considered normal 7
- Both arterial blood and cerebrospinal fluid demonstrate marked respiratory alkalosis during pregnancy and even into the puerperium 3
- Plasma bicarbonate levels decrease as the kidneys excrete excess base to normalize pH 6
Clinical Significance on Arterial Blood Gas
On ABG, mild fully compensated respiratory alkalosis is the normal finding in pregnancy 7:
- Low PaCO2 (typically 28-32 mmHg vs. non-pregnant 35-45 mmHg)
- Low bicarbonate (18-21 mEq/L vs. non-pregnant 22-26 mEq/L)
- Normal or near-normal pH (7.40-7.45)
Important Clinical Caveats
Severe maternal alkalosis can potentially harm the fetus in pregnancies with borderline fetal reserve, as it may decrease oxygen delivery to the fetoplacental unit 8. However, the mild physiological alkalosis of normal pregnancy is protective and beneficial.
Respiratory rate itself remains unchanged in pregnancy 7, so tachypnea (>20 breaths/minute) should prompt evaluation for pathology rather than being attributed to normal pregnancy physiology 7.
While rare, severe hyperventilation can occur, presenting with profound dyspnea and tachypnea from as early as 18 weeks gestation, though this represents an extreme of the physiological spectrum rather than pathology 9. In exceptional cases, acute exacerbation of chronic respiratory alkalosis can cause severe hypokalaemia through intracellular potassium shifts 10.