Pericardial Effusion

The normal pericardial sac contains only a small amount of serous fluid. An effusion is an excess collection within that sac, but the measurement alone does not tell you how sick the patient is. A rapidly accumulating small effusion can cause tamponade; a large effusion that developed slowly may be tolerated because the pericardium has had time to stretch. The bedside question is therefore not simply, “How large is it?” It is, “Is the fluid now preventing the heart from filling?” As intrapericardial pressure rises above chamber diastolic pressure, ventricular filling falls, stroke volume drops, and tachycardia and vasoconstriction temporarily preserve cardiac output. With further deterioration, venous congestion and systemic hypoperfusion become apparent. This pressure effect also exaggerates ventricular interdependence. During inspiration, increased right-sided filling pushes the interventricular septum towards the left ventricle, so left-sided filling and systolic pressure fall. This is the basis of pulsus paradoxus: an inspiratory decrease in systolic blood pressure greater than 10 mmHg. Tamponade is not an echocardiographic label alone. It requires pericardial fluid and haemodynamic compromise. A patient may have right-sided chamber collapse on...