Explanation

Reddy et al describe 3 phases of hemodynamic changes in tamponade.  Phase I - The accumulation of pericardial fluid causes increased stiffness of the ventricle, requiring a higher filling pressure; during this phase, the left and right ventricular filling pressures are higher than the intrapericardial pressure.  Phase II - With further fluid accumulation, the pericardial pressure increases above the ventricular filling pressure, resulting in reduced cardiac output.  Phase III - A further decrease in cardiac output occurs, which is due to the equilibration of pericardial and left ventricular (LV) filling pressures. Pericardial effusions, which cause cardiac tamponade, can be serous, serosanguineous, hemorrhagic, or chylous The underlying process for the development of tamponade is a marked reduction in diastolic filling The amount of pericardial fluid needed to impair diastolic filling of the heart depends on the rate of fluid accumulation and the compliance of the pericardium