[What degree of hypoxemia is tolerable for human beings?].

According to the literature, hypoxemia is considered to be severe when oxygen saturation (Sa O(2)) falls below 90 %. Frequently one can discover lower values without impairment of the patient. Especially patients with the obesity hypoventilation syndrome (OHS) will have frequent night time desaturations of significant duration below 50 % Sa O(2), but do still cope with their daytime jobs. This discrepancy can only be explained by the fact, that hypoxemia is not equivalent to tissue hypoxia. The latter is mainly being determined by oxygen delivery (DO2) which is being calculated by multiplying cardiac output (CO) and oxygen content (CaO2). Ca O(2) is determined by the product of Sa O(2) and haemoglobin (Hb) times 1.35. From this context it becomes evident, that assessing hypoxemia without considering oxygen content will frequently be misleading. The human organism has several possible ways of compensation in order to avoid tissue hypoxia. In case of acute hypoxemia that evolves within minutes the organism can shift the oxygen binding curve by changing 2 - 3-DGP erythrocytic activity. Additionally non vital organ systems might reduce their oxygen uptake. During sustained hypoxia (lasting 2 - 3 days) the Krebs cycle and the respiratory chain will express hypoxia-resistant iso-enzymes. Long lasting hypoxia can be compensated by polycythemia. Indirect data suggest, that the critical number for the oxygen content is rather low and is estimated to be somewhere around 33 % of the normal value. These mechanism of hypoxia-resistance are hardly ever maxed out in patients on critical care units.Lack of knowledge of the above described mechanisms does frequently result in diseases like ARDS which frequently develops due to excessive ventilatory pressures and excessive inspired O(2) concentrations. Georg Thieme Verlag KG Stuttgart, New York.