The oxygen trail: measurement

Tissue hypoxia may be defined as abnormal oxygen utilization such that cells are experiencing anaerobic metabolism. Tissue hypoxia can be defined biochemically by low levels of ATP, high levels of NADH, or decreased oxidized cytochrome aa3. It is possible to measure these biochemical markers in the laboratory setting with, for example, nuclear magnetic resonance spectroscopy. However, this is not as yet a clinical option. There is no 'gold standard' for the diagnosis of clinical hypoxia. We can detect the gross consequences of tissue hypoxia, such as organ dysfunction and metabolic markers of anaerobic metabolism (e.g. lactic acidosis). We have also become familiar with the measurement of both global and regional oxygen dispatch and consumption. However, organ dysfunction and metabolic acidosis consistent with established tissue hypoxia commonly exists in the presence of normal and even supra normal global measures of oxygen dispatch and consumption. Therefore, we should ideally make measurements at the end of the oxygen trail, i.e. cellular oxygen delivery and effective utilization.