Hemodynamic monitoring has become a real challenge in the intensive care unit. As an integrative parameter for oxygen supply/demand, venous oxygen saturation (SvO2) provided by pulmonary artery catheterization is one of the most popular parameters to assess the adequacy of cardiac output. However, technical limitations and potential iatrogenic complications constitute important limits for a widespread use. Regular central venous catheters coupled with a fiberoptic lumen for central venous oxygen saturation (ScvO2) monitoring have been proposed as a surrogate for SvO2 monitoring. The purpose of the present article is to review the physiological backgrounds of circulation, the pathophysiology of circulatory failure and subsequent venous oxygen saturation alterations, and finally the merits and the limits of the use of ScvO2 in different clinical situations. 1. Introduction Hemodynamic monitoring has become a common practice in the intensive care unit. Besides blood pressure measurement, most industrial efforts have concentrated on providing devices for cardiac output monitoring. However, adequate adaptation of these macrohemodynamic parameters is somehow challenging. Indeed, as cardiac output is an adaptive parameter, it is always difficult to judge whether a given value at a given time for a given patient is appropriate or not. Similarly, which value should be considered an appropriate goal for blood pressure, considering regional perfusion specificities (e.g., autoregulation or flow/pressure dependency), patient’s age, history of hypertension, and so on. Therefore, considering that O2 supply to the tissue is the basic objective, intensivists have been trying to find out an integrative parameter that would be more suitable to globally assess hemodynamic status of their patients. As a surrogate for evaluating O2 demand/supply adequacy, central oxygen venous saturation (ScvO2) has become a popular parameter. As explained for the dummies, oxygen venous saturation is interpreted as a bank statement at the end of the month: “if the balance is negative, you can consider two explanations: you spend too much money or you earn not enough.” The aim of the present paper is precisely to critically analyze the physiological basements for such an interpretation, the data that support its use in clinical practice, and finally the limits that should be kept in mind while using such a parameter at the bedside. 2. Physiological Background 2.1. Normal Circulation Physiology One of the main goals of blood circulation is to ensure oxygen supply to organs and tissues. The
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