The fall in FRC reduces the volume of oxygen left in the lungs during apnoea. Combined with greater oxygen demands, this results in the more rapid fall of maternal PaO₂ during apnoea.

The diagram shows the decrease in arterial PaO₂ after 1 min of apnoea in pregnant and non-pregnant patients. (Archer & Marx, 1974.)‏

The faster fall in PaO₂ during apnoea in pregnancy contributes to difficulties with general anaesthesia during pregnancy by reducing the time available for airway manipulation during induction.