Modelling the effect of ambient oxygen fraction on hypoxaemia during apnoea

Hypoxaemia during apnoeic oxygenation complicates tests for brainstem death and exposes the patient's organs to the risk of anoxic damage. This study investigates the effect on hypoxaemia of varying the ambient oxygen fraction during apnoea.

Methods and results

The Nottingham Physiology Simulator is a validated simulation of advanced, iterative physiological models [1]. The model was set up as a 70 kg adult with normal physiological values other than: pulmonary venous admixture 20%, alveolar deadspace fraction 20% of tidal volume and functional residual capacity 21. The patient's lungs were ventilated with 100% oxygen for 2 min and the patient was then apnoeic with an open airway exposed to 21, 50, 80 or 100% oxygen. Arterial oxygen and carbon dioxide tensions (PaO₂, PaCO₂) were recorded continuously until arterial oxygen saturation fell to 50%. The changes in PaO₂ and PaCO₂ are shown in the figure on the previous page.

Figure 1

Provision of very high ambient oxygen fractions greatly extends the safe duration of apnoea. As oxygen fraction is increased, increasingly large effects are achieved

Discussion

Provision of very high ambient oxygen fractions greatly extends the safe duration of apnoea. As oxygen fraction is increased, increasingly large effects are achieved.