Measurement of Hepatic Venous Pressure Gradient.

Portal hypertension is defined as an increased pressure in the portal vein, but is better evaluated by the pressure gradient between the portal vein and the inferior vena cava, with a normal value lower than 5 mmHg. This gradient represents the real perfusion pressure within the portal and hepatic circulation, which is, under normal conditions, a high-flow/low-resistance system, considering the high portal blood flow (between 700 and 1000 mL/min). The various causes of an increased portal venous resistance, which are discussed in specific chapters in this book, are characterized by changes in the anatomic architecture (fibrous scars delineating nodules, distal venous thrombosis, collagenization of the space of Disse, and loss of the normal elasticity of the sinusoidal endothelium), changes in splanchnic hemodynamics (increased splanchnic blood flow), and changes in the intra-hepatic vascular resistance (vasoconstriction of the sinusoids related to the transformation of stellate cells into myofibroblasts). [ABSTRACT FROM AUTHOR]