Flattening of the IVC is most commonly described in patients with abdominal trauma leading to hypovolemia and hypoperfusion of the bowel ("shock bowel."). The IVC is considered "flattened" if its AP diameter is less than 9 mm at the level of the renal veins. Associated imaging findings of the "hypovolemia complex" include decreased caliber of the abdominal aorta, increased enhancement of the bowel mucosa, and prolonged intense enhancement of the adrenal glands.
In pediatric patients the constellation of findings seen with post traumatic shock is called the "hypoperfusion complex." Imaging findings include fluid-filled, dilated bowel; increased enhanced of the bowel wall, mesentery, kidneys, pancreas, adrenal glands, aorta, and IVC; decreased caliber of the aorta and IVC. Presence of these findings indicates a poor prognosis.
Eisenstat RE, Whitford AC, Lane MJ, et al. The "Flat Cava" sign revisited: What is its significance in patients without trauma? AJR Am J Roentgenol 2002;178(1):21-5.
O'Hara SM, Donnelly LF. Intense contrast enhancement of the adrenal glands: another abdominal CT finding associated with hypoperfusion complex in children. AJR Am J Roentgenol 1999;173(4):995-7.