Normal difference between arteries and alveoli (PAO2) on oxygen is 9-15 mm hg. At respiratory failure this number increases up to 20—30 mm hg. This difference is characterised by severe level of respiratory failure and hypoxia. PAO2 gradient depends, mainly, on degree of venous blood shunting from right to left and oxygen tension in venous blood, while PvO2 depends on cardiac output, oxygen consumption and haemoglobin concentration.
The oxygen content of blood directly depends on the blood haemoglobin concentration. Each gramme of haemoglobin is capable to bind maximally 1,34 ml of oxygen. For example, at healthy person with Нb level = 150 g/l the oxygen content of blood is about 200 ml of 02/l of blood. One molecula of haemoglobin is capable to attach to itself four moleculas of oxygen:
Key factor defining a quantity of oxygen, bound to haemoglobin, is the indicator of arterial blood oxygenation (SaO2). SaO2 reflects the relationship between oxyhemoglobin and oxygen content of blood. In other words SaO2 is the ratio of oxyhemoglobin to all haemoglobin, capable to transmit oxygen:
where РНb is so-called "reduced" haemoglobin which is not bound to oxygen for any causes, but is ready to its transport. SaO2 norm at adults is 96-98 %. The relation between Pa02 and SaO2 is defined by the oxyhemoglobin dissociation curve. When PaO2 is 100 mm hg haemoglobin is almost completely packed with oxygen (SaO2 98-99 %). PaO2 level, when SaO2 is equal to 50%, is known as P50 index. It is the standard measure, that allows us to estimate the affinity of haemoglobin to oxygen; its norm range is 26-28 mm hg.