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Effects of increased inspired oxygen concentration on tissue oxygenation: theoretical considerations
1Anaesthetic Department, St James's University Hospital, Leeds, UK.
Background And Objectives:
Breathing increased fractional oxygen concentration (FiO2) is recommended for the treatment of tissue ischaemia.
Methods:
The theoretical benefits of increasing FiO2 on tissue oxygenation were evaluated using standard physiological equations. Assuming constant oxygen consumption by tissues throughout the length of a capillary, the oxygen content at 20 arbitrary points along a capillary was calculated. Using mathematical representations of the haemoglobin dissociation curve and an iterative approach to include the dissolved oxygen component of oxygen content, the oxygen partial pressure (PO2) profile along a capillary was estimated.
Results:
High FiO2 concentrations cause large increases in PO2 at the arteriolar end of capillaries but these large PO2 values, caused by the extra dissolved oxygen, rapidly decline along the capillary. At the venular end of the capillary (the area of tissue most likely to be hypoxic), breathing oxygen causes only a modest improvement in PO2.
Conclusion:
Increasing FiO2 to treat tissue hypoxia has clear benefits, but a multimodal approach to management is required.
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