Microcirculatory alterations in critically ill patients. 
The recent onset of orthogonal polarization spectral (OPS) imaging techniques has allowed the direct visualization of the microcirculation at the bedside of critically ill patients.  A systematic review with particular emphasis on recent findings and implications in pathophysiological processes is presented.  Using OPS techniques various investigators have observed microcirculatory alterations in critically ill patients, and especially in patients with severe sepsis and septic shock.  These alterations include a decrease in vessel density and an increased proportion of non perfused or intermittently perfused capillaries, and these alterations can be fully reversed by the topical application of acetylcholine.  Similar alterations are observed in patients with septic and cardiogenic shock.  Persistent microvascular alterations are associated with the development of organ failure and death.  In addition, the reversal of these alterations during resuscitation procedures is highly predictive of outcome.  Unfortunately, the effects of many therapeutic interventions usually performed in critically ill patients are still not well defined, even though evidence coming from animal experiments is sometimes available.  In particular, the role of fluid resuscitation, red blood cell transfusions, inotropic, vasoactive and anesthetic agents should be investigated.  Microcirculation plays an important role in the pathogenesis of shock and organ dysfunction, especially in sepsis.  The microcirculatory effects of various therapeutic interventions have still to be reported.  OPS technique may become a valuable tool to monitor patients with circulatory failure.