Cardiac Tamponade

Cardiac tamponade occurs when fluid, blood, pus, clots or gas accumulates within the pericardial space and raises pressure around the heart. The pressure interferes with ventricular relaxation and filling during diastole. This produces obstructive shock: the myocardium may still contract, but too little blood can enter the ventricles to support forward flow. The pericardium does not respond to added volume in a linear way. A slowly enlarging effusion can stretch the pericardium and remain relatively well tolerated, sometimes exceeding 1 L before a major pressure rise. In contrast, rapidly accumulating blood may cause tamponade with only about 100 to 200 mL in a previously normal pericardial sac. Once the pressure-volume curve reaches its steep portion, a seemingly small additional volume can cause abrupt deterioration. This is the “last-drop” phenomenon. The lower-pressure right atrium and right ventricle are usually affected first. Impaired right-sided filling reduces pulmonary blood flow and, through ventricular interdependence, limits left-ventricular filling as well. Stroke volume and cardiac output fall. The body initially compensates with sinus tachycardia and systemic vasoconstriction, but these mechanisms cannot overcome the external compression....