If a large number of worms are present they can literally fill up the right atrium and caudal vena cava. (lbah.com)
The poorly oxygenated blood returns to the heart through the femoral vein, then the iliac veins, and eventually the caudal vena cava (labeled as Vein in this picture). (lbah.com)
As the stomach begins to turn, a cascade of pathological events begin to occur: compression of the caudal vena cava and portal vein (blood flow to the heart), reduced cardiac output and myocardial ischemia, shock, perfusion to critical organs is reduced and fatality if not corrected quickly. (vrc-nova.com)