Why is cardiac tamponade lethal?

"Cardiac tamponade is usually the result of penetration of the pericardium, which is the thin, double-walled sac that surrounds your heart. The cavity around your heart can fill with enough blood or other bodily fluids to compress your heart. As the fluid presses on your heart, less and less blood can enter. Less oxygen-rich blood is pumped to the rest of your body as a result. The lack of blood getting to the heart and the rest of your body can eventually cause shock, organ failure, and cardiac arrest."

from Janet Barwell & Marijane Leonard