ICU patients need a prompt normalization of macrohemodynamic parameters. Unfortunately, this optimization sometimes does not protect patients from organ failure development. Prevention or treatment of organ failure needs another target to be pursued: the microcirculatory restoration. Microcirculation is the ensemble of vessels of maximum 100？ m in diameter. Nowadays the Sidestream Dark Field (SDF) imaging technique allows its bedside investigation and a recent round-table conference established the criteria for its evaluation. First, microcirculatory derangements have been studied in sepsis: they are mainly characterized by a reduction of vessel density, an alteration of flow, and a heterogeneous distribution of perfusion. Endothelial malfunction and glycocalyx rupture were proved to be the main reasons for the observed microthrombi, capillary leakage, leukocyte rolling, and rouleaux phenomenon, even if further studies are necessary for a better explanation. Therapeutic approaches targeting microcirculation are under investigation. Microcirculatory alterations have been recently demonstrated in other diseases such as hypovolemia and cardiac failure but this issue still needs to be explored. The aim of this paper is to gather the already known information, focus the reader’s attention on the importance of microvascular physiopathology in critical illness, and prompt him to actively participate to achieve a more comprehensive understanding of the issue. 1. Introduction The introduction in clinical practice of pulmonary artery catheter (PAC) about 40 years ago [1] allowed clinicians to measure the cardiac output (CO) at the bedside with the thermodilution technique [2]. Moreover, with an arterial and mixed venous gas analysis, arterial (CaO2) and mixed venous oxygen content (CvO2) could be easily calculated and oxygen availability (DO2) and consumption (VO2) consequently obtained by applying the following simple formulas: and . Old well-known physiologic data were available at the bedside as well as clinical parameters but their interpretation and utilization as a therapeutic target was and remains controversial to date. Shoemaker was the first clinician to try to interpret and utilize these new hemodynamic data. He was a surgeon and monitored the high risk surgical patients with PAC before, during, and after the operations [3]. He observed that patients could be divided into three groups on the basis of outcome: survived, survived with complications, and died. From the analysis of the hemodynamic data, patients with better outcome resulted to have CO,
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