and ({dot{D}}_{mbox{textsc{mathrm{o}}}_{2}})

is shown in Figure 1^[8]. The oxygen transport system normally operates to maintain ({dot{V}}_{mbox{textsc{mathrm{o}}}_{2}})

in conditions where ({dot{D}}_{mbox{textsc{mathrm{o}}}_{2}})

varies widely. If global ({dot{D}}_{mbox{textsc{mathrm{o}}}_{2}})

decreases then o₂ER increases to maintain adequate oxygen supply. If ({dot{D}}_{mbox{textsc{mathrm{o}}}_{2}})

continues to decrease, a point is reached where the o₂ER is maximal and cannot increase further. This point is called the ‘critical ({dot{D}}_{mbox{textsc{mathrm{o}}}_{2}})

’, and it is the point below which energy production in cells becomes limited by the supply of oxygen. When the decrease in ({dot{D}}_{mbox{textsc{mathrm{o}}}_{2}})

is caused by anaemia the Hb concentration at the critical ({dot{D}}_{mbox{textsc{mathrm{o}}}_{2}})

is termed the ‘critical Hb concentration’. Any further reduction in ({dot{D}}_{mbox{textsc{mathrm{o}}}_{2}})

will result in tissue hypoxia, conversion to anaerobic metabolism and the production of lactic acid. The critical ({dot{D}}_{mbox{textsc{mathrm{o}}}_{2}})

is not a fixed value, but varies between organs and is dependent on the metabolic activity of the tissue.

The lack of a fixed threshold for critical ({dot{D}}_{mbox{textsc{mathrm{o}}}_{2}})

indicates the need to monitor ({dot{D}}_{mbox{textsc{mathrm{o}}}_{2}})

and ({dot{V}}_{mbox{textsc{mathrm{o}}}_{2}})

on an individual basis preferably for individual organs. At present our ability to monitor this is limited and inadequate. Global measurements of ({dot{D}}_{mbox{textsc{mathrm{o}}}_{2}})

and ({dot{V}}_{mbox{textsc{mathrm{o}}}_{2}})

⁠, and global markers of tissue hypoxia (plasma acidosis and lactate concentration) are useful, but are insensitive and non-specific. Regional indices, such as gastric tonometry and near infrared spectroscopy (NIRS), are indirect methods of measuring tissue oxygenation, which are still being evaluated. The value for critical ({dot{D}}_{mbox{textsc{mathrm{o}}}_{2}})