Pathophysiology
The outer layer of the heart is made of fibrous tissue which does not easily stretch, and so once fluid begins to enter the pericardial space, pressure starts to increase.
If fluid continues to accumulate, then with each successive diastolic period, less and less blood enters the ventricles, as the increasing pressure presses on the heart and forces the septum to bend into the left ventricle, leading to decreased stroke volume. This causes obstructive shock to develop, and if left untreated then cardiac arrest may occur (in which case the presenting rhythm is likely to be pulseless electrical activity).
Read more about this topic: Cardiac Tamponade