Microcirculation in the critically ill. Part I: Review and physiology of septic patients

Severe sepsis and septic shock involve circulatory, inflammatory and metabolic disorders eventually resulting in a disruption of cellular energy. Microcirculatory disturbances are common in septic patients. Microcirculation is the primary site of oxygen and nutrients exchange to cells. Direct observation using Sidestream Dark Field (SDF) imaging has allowed direct visualization of microcirculatory failure in critically ill patients. Septic shock is characterized by weak or vulnerable microcirculatory units and heterogeneity of microcirculatory flow. Multiple mechanisms may contribute to these alterations, including endothelial dysfunction, altered glycocalyx, impaired inter-cell communication and adhesion and rolling of white blood cells and platelets. Many therapeutic interventions routinely used in the treatment of critically ill patients seem to result in limited changes in microcirculatory perfusion, irrespective of systemic hemodynamics, due to the heterogeneous nature of these changes and the potentially involved mechanisms. Therefore, microcirculatory alterations and their presence in states of shock, especially in septic shock, can represent diagnostic and severity stratification tools and may be a target for therapeutic intervention (microcirculatory resuscitation), besides suggesting a prognostic role.