Expansion of the intravascular space in severe shock.

Hypovolemic shock, if untreated for a while or if severe, results in an increase in intravascular capacity due to the opening of all capillaries at once. This expansion of the vascular capacity in patients with severe or untreated shock requires much more intravenous fluids for adequate treatment than the volume of blood lost. Fluid volume requirements should be determined by results (normal blood pressure and urinary output) and by central venous pressure or pulmonary arterial pressure for maximum tolerable administration. Blood volume measurements are useless. The vascular space shrinks to normal after adequate resuscitation, creating relative hypervolemia.