Hemodynamic mechanisms based on the “Five Differences” framework in critical illness

Critical illness is marked by life-threatening organ dysfunction with different characteristics. Although typical forms of critical illness have multiple subtypes, they tend to converge toward ischemic and hypoxic shock as the shared final stage. Understanding the hemodynamic mechanisms is difficult because of the complexity of interacting components. This article outlines five defining features of the progression of critical illness that I have named the “Five Differences” framework and describes four stages through which understanding of the hemodynamics of critical illness have evolved, that has shifted from a predominantly physical model to a biologically oriented paradigm. Furthermore, this article highlights the central role of hemodynamic instability and injury of critical units in the pathogenesis of critical illness, driven by the host/organ unregulated response. The Five Differences framework aims to provide a theoretical foundation for understanding the hemodynamic mechanisms that guide targeted clinical management of critical illness.