29th June 2023

Respiratory

Throughout pregnancy, there is a steady increase in oxygen consumption due to increased metabolic needs as well as the necessity of an alteration in the blood buffering system to optimise gas exchange in the placenta.

Tidal volume increases to enable this demand to be met, with an increase in rib cage circumference. Minute ventilation increases by about 45% due to the increase in tidal volume; there is usually no change in respiratory rate. At the end of pregnancy, due to diaphragmatic elevation, there is a fall in functional residual capacity.

PaO2 increases and PaCO2 decreases to enable gas exchange at the placenta. A chronic, fully compensated, respiratory alkalosis is therefore normal with pH 7.42-7.46 and a serum bicarbonate of 18-22.