Significance of venous oximetry in the critically ill

Critically ill patients are threatened or affected by multi-organ failure (MOF). Tissue hypoxia is one of the most important co-factors of MOF. Venous oximetry allows the critical estimation of the global oxygen (O(2)) supply-demand ratio and can be gained from mixed (S(v)O(2)) and central venous blood (S(cv)O(2)). Cellular requirements dominate the feedback hierarchy of the O(2)-metabolism. This review describes the history and validity of haemodynamic monitoring, illustrates the physiological background and clinical application of venous oximetry and presents carbon dioxide analysis as evidence of the usefulness of a multi-modal approach in cardio-respiratory monitoring. Variation of cardiac output, optimisation of arterial O(2)-saturation and adaptation of O(2)-extraction are shown to be the relevant levels of pathophysiological adaptation as well as therapeutic intervention. We portray the functional equivalence of S(cv)O(2) and S(v)O(2) and analyse their diagnostic, therapeutic and prognostic significance, providing the basis for the efficacy of venous oximetry as an important marker of critical illness. Finally, having drawn an outline of current developments for the better understanding of the oxidative balance of individual organs, we stress the importance of a synoptic O(2)-monitoring strategy as well as the need to use its beneficial, yet unfulfilled, clinical potential.