Cardiac tamponade: A life-threatening situation in which there is such a large amount of fluid (usually blood) inside the pericardial sac around the heart that it interferes with the performance of the heart. If cardiac tamponade is left untreated, the result is dangerously low blood pressure, shock, and death. The excess fluid in the pericardial sac acts to compress and constrict the heart. Cardiac tamponade can be due to excessive pericardial fluid, a wound to the heart, or rupture of the heart. Also known as pericardial tamponade.