Summary Summary: The oxygen consumed during the metabolic process must be replenished continuously, whereby the amount of oxygen supplied exceeds the amount of oxygen consumed by the organism and the organs. The organ which uses the greatest amount of oxygen is the myocardium; that which uses the smallest amount is the kidney. Every organ, with the exception of the central nervous system and the kidney, can make more or less full use of the oxygen supplied with the arterial blood. As a general principle, a human being’s consumption of oxygen can be determined on the basis of cardiac output and the a_DO2 or via minute ventilation and the inspiratory-expiratory difference in O2-concentration. The four parameters – partial pressure of oxygen, oxygen saturation, haemoglobin concentration and oxygen concentration – constitute what is termed the oxygen status. The paO2 indicates whether the blood was oxygenated in the lung. The sO2 specifies the percentage of oxygenated haemoglobin in the haemoglobin as a whole, in arterial blood approx. 96%. In each case, approx. 0.5 - 1% of the haemoglobin is present as methaemoglobin and as carboxyhaemoglobin. For method-related reasons, a partial oxygen saturation can also be defined, if it is only the percentage of O2Hb contained in the sum of O2Hb plus HHb which is being considered. The normal arterial value will then be approx. 98%. If the concentration of chemically bound O2 is to be calculated from the sO2, then the cHb must be multiplied by what is known as the Hüfner constant, theoretically a maximum of (sO2 100%) 1.39 ml/g. The arterial oxygen status has four characteristic pathophysiological features: hypoxia as a reduction of the paO2, hypoxygenation as a decrease in the saO2 (or in the psaO2), anaemia as a reduction of the cHb and hypoxaemia as a decrease in the caO2. Arterial hypoxaemia may therefore be hypoxic, toxic or anaemic. Arterial hypoxia is caused by a drop in the paO2 to a value under the normal range of 78 - 95 mmHg. Therapeutic threshold values, that is psaO2 90% or, as the case may be, paO2 60 mmHg (optional) and 75% or, as the case may be, 40 mmHg (mandatory), apply to patients of either sex. They tell the anaesthetist what he may do (as an option) or what he must do (mandatory). Hypoxygenation with normal paO2 always suggests toxic hypoxaemia, i.e. the presence of what are termed dyshaemoglobins, that means an overload of CO with carboxyhaemoglobin (COHb) or the oxidation of the haem iron with haemiglobin (MetHb). A value of 20% DysHb may be taken as a mandatory threshold value, more sensitive than that of hypoxic hypoxaemia with 25% loss (saO2 75%). As far as capillary oxygen utilisation is concerned, anaemic hypoxaemia resulting from a reduction of the cHb is a particularly favourable special case. That is why, in the case of a patient who has not suffered previous cardiac damage, a reduction of the cHb by a half is not, in itself, an indication that tranfusion is required, as long as there is normovolaemia, normoxia and normothermia. A shift of the O2 dissociation curve in the case of hypothermia or, as the case may be, with stored blood (shift to the left), or in the case of permissive hypercapnia (shift to the right) does not generally present any problems from the clinical point of view. In the case of therapeutic hyperoxia (normo- or hyperbaric) there is a linear increase in the physically dissolved oxygen in the blood, in order to improve the supply of oxygen to tissue which may be undersupplied. Haem oxymeters are multiple-wave-length oxymeters for photometric in vitro diagnosis of all Hb derivatives, as well as of cHb. Pulsoxymeters measure the arterial, partial sO2 in vivo at the finger, by means of a photometric, continuous, non-invasive procedure. The diagnostic information obtained increases quite clearly in the following order: pO2, psO2, sO2 and cO2 ; the value of cO2 can be termed the global value of the O2 status. A diagnosis based on mixed venous blood (PA catheter) is only possible if the arterial O2 status, the total oxygen consumption and the cardiac output remain constant.Attempts to measure the average tissue-pO2 of the muscle and to interpret it as being representative of the whole organism are unacceptable.