Pregnancy — Arterial Blood Gases

pH

• Respiratory Alkalosis → pH increases
• Complete renal compensation → pH normalizes

PO2

• Alveolar PO₂ increases according to alveolar gas equation

• As PaCO₂ decreases from 40 → 30mmHg
• PAO₂ = 150 – 37 = 113mmHg
• However there is increased O₂ consumption (15~30% above normal) in the hypermetabolic state → PaO₂ ~ 102mmHg

pCO₂

• Progesterone in pregnancy causes increased chemoreceptor sensitivity to CO₂
• Minute ventilation increases 50%
  • 25% increase in tidal volume
  • 25% increase in respiratory rate
• Dead-space increases 45%
• However Vd/Vt unchanged
• CO₂ production increases due to hypermetabolic state and foetal metabolism, however this is compensated by chemoreceptor sensitization

• therefore P_ACO₂ decreases
• P_ACO₂ approximates P_aCO₂ = approx 30mmHg → respiratory alkalosis

HCO₃^–

• Full metabolic compensation for chronic respiratory alkalosis occurs after 3 days according to the formula.

• This is due to decreased HCO₃^– reabsorption in the renal tubules
• In alkalosis, inhibition of renal secretion of hydrogen ions and ammonium
  • glomerular bicarbonate filtration exceeds tubular acid secretion
  • Excess bicarbonate is not converted to CO₂ and H₂O and reabsorbed → eliminated → Loss of base from system

Base Deficit

• reflects the renal loss of HCO3^–

Sakurai 2016

Answer by Sakurai