Apnoea in pregnancy: an investigation using physiological modelling.

Oxygen desaturation during apnoea following anaesthetic induction in obstetrics is hazardous for mother and baby. This study investigated apnoea during pregnancy using computer simulation. The Nottingham Physiology Simulator was configured to replicate normal pregnant physiology. Three pregnant and three non-pregnant subjects were created, representing population variation according to published physiological values. They each commenced apnoea from four levels of pre-oxygenation from none to 99% complete denitrogenation. During apnoea, the physiological changes of rapid sequence induction were simulated. We found reduced apnoea tolerance in pregnancy, the median [range] time taken to fall to S(a)O2 < 90% after 99% complete denitrogenation being 4 min 52 s [3:43-6:17] in the pregnant subjects, vs 7 min 25 s [5:49-9:42] in the non-pregnant subjects. The time taken to fall from S(a)O2 90% to 40% was 35 s [32-45] in the pregnant vs 45 s [38-56] in the non-pregnant subjects. Two minutes of pre-oxygenation by tidal breathing provides approximately 3.5-6 min before desaturation to < 90%.