What is and what causes cardiac tamponade?

Fluid under pressure: The heart is surrounded by an envelope called the pericardial sac. When fluid develops in the sac, it can build up pressure resulting in what's called a cardiac tamponade. The heart being a pump cannot effectively perform its function against this pressure unless the fluid is removed.

Fluid around heart: The pericardium is a "skin" surrounding the heart. It is usually empty, though can fill with fluid -from heart failure, autoimmune dz, cancer or infection- or blood from trauma. When the fluid builds up enough pressure to prevent the heart from expanding to fill with blood, tamponade results. This is life-threatening and requires immediate drainage to prevent cardiovascular collapse.