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To discuss the role of microcirculatory abnormalities in critically ill patients and the link between systemic hemodynamics and microvascular perfusion.Microcirculatory alterations have been repeatedly observed in patients with severe sepsis, but recent findings show that these also occur in patients with severe heart failure and in those submitted to high-risk surgery. More severe and more persistent alterations are observed in patients with a poor outcome. Even though a minimal cardiac output and arterial pressure is mandatory to sustain the microcirculation, this level is not yet well defined and seems to be submitted to high individual variability. Above this level, microcirculation and systemic circulation are relatively dissociated, so that microcirculatory alterations can be observed even when systemic hemodynamics are within satisfactory goals. In addition, the response of the microcirculation to therapeutic interventions is often dissociated from systemic effects. However, microcirculatory perfusion can be affected by cardiac output and arterial pressure when these are critically altered.Microvascular alterations frequently occur in critically ill patients and these may be implicated in the development of organ failure and are associated with outcome. The link between systemic hemodynamics and microcirculation is relatively loose.