ACID BASE REGULATION IN PREGNANCY

Acid Base regulation
Increased minute ventilation leads to a decrease in PaCO
2
producing a respiratory alkalosis and a left shift of thoxyhaemoglobin dissociation curve. A 30% increase in
2-3 DPG has the opposite effect on the oxyhaemoglobin
dissociation curve with an increase of the P50 from 3.5
kPa to 4 kPa (26-30mmHg). The respiratory alkalosis is
compensated by increased renal bicarbonate excretion so
that plasma hydrogen ion concentrations remain essentially
unchanged.
Pain in labour causes maternal hyperventilation associated
with an acute left shift of the oxyhaemoglobin dissociation
curve. This increases the affinity of maternal haemoglobin
for oxygen and consequently oxygen delivery to the
fetus decreases. If labour becomes prolonged and is also
painful, basic metabolic rate increases and O
2
extraction
does as well. Under these circumstance there will be
less O
2
available to the fetus, as cardiac output cannot be
further increased to match the increased O
2
demand. In
this situation regional analgesia is useful as it prevents the
increase in BMR and further hyperventilation secondary
to the pain. Effective regional analgesia largely abolishes
the detrimental effects of a painful labour on the fetus