Hypoxaemia during open-airway apnoea: a computational modelling analysis.

Hypoxaemia during open-airway apnoea, e.g. during brainstem death testing, may cause organ damage. The effect of ambient oxygen fraction on the extent of hypoxaemia has not been established. We validated the Nottingham Physiology Simulator in this context by reproducing the methodologies and results of four published clinical studies. We then used the simulator to examine the effects of different ambient oxygen fractions (0.21-1.0) and shunt fractions (1-30% of cardiac output) during apnoea. Increasing ambient oxygen fraction from 0.9 to 1.0 more than doubled the time to haemoglobin desaturation at all shunt fractions, and extended apnoea longer than when the ambient oxygen fraction was increased from 0.21 to 0.9. When ambient oxygen fraction and shunt fraction were large, arterial oxygen tension transiently increased during apnoea. A very high ambient oxygen fraction and a patent airway are likely to delay dangerous hypoxaemia during apnoea.