Pathophysiology of Cardiac Tamponade: Intracardiac Pressure Curves
Tamponading pericardial fluids compress the heart throughout systole and diastole. Although the atria fill continuously, blood mainly enters the heart when blood is leaving it during the right and left ventricular ejection periods, since ventricular ejection expels blood, reducing ventricular volumes. Ejection thus transiently reduces pericardial pressure, transiently increasing transmural pressure. Ejection simultaneously aids atrial filling through enlarging the atria by pulling their “floors” (valve levels) toward the ventricular apices; this produces the normal x descent in atrial pressure curves. Pericardial volume and pressure thus vary continuously during the cardiac cycle reflecting the variations in cardiac chamber volumes. Ventricular pressure is high throughout diastole resisting filling. In early diastole, the normally rapid peak ventricular filling rate becomes radically reduced, along with the filling fraction; this increases the relative contribution to ventricular filling of end-diastolic atrial contraction. At end-diastole, the compressed ventricles are thus maximally expanded by filling, raising intra-pericardial pressure to its maximum. Although continuous atrial filling tends to expand the atria, which also tends to raise pericardial pressure, at end-systole, ventricular ejection (emptying) is complete so that the ventricles are at minimal volume permitting intrapericardial pressure to fall. In tamponade, when ventricular filling is “maximal/ the ventricles remain critically underfilled relative to their normal capacity (“underpreloaded”). They therefore operate at the low end of their Frank-Starling curves while ejecting the reduced stroke volume. canadian neightbor pharmacy
Intracardiac Pressure Curves
In full-blown tamponade, ventricular pressure curves remain flat throughout diastole at their high equilibrated level. During systole and ventricular ejection, the atrial cavitary pressure drop preserves the normal x descent in atrial pressure curves. However, continuously high ventricular diastolic pressures resist rapid ventricular filling so that atrial emptying in early diastole is resisted and ultimately aborted. Consequently the y descent of atrial pressure following atrioventricular valve opening is progressively amputated and ultimately eliminated. Moreover, high ventricular diastolic pressure favors premature mitral and tricuspid valve closure, further resisting filling with further reduction of ventricular preload reflected in the shorter fiber length of the compressed, underfilled chambers. This reduces the ventricular ejection rate which, with the progressive underfilling, progressively reduces stroke volume. In inspiration, because the right atrium and ventricle expand at the expense of the left (due to both septal shift and elevation of pericardial pressure; Fig 2), inspiratory left ventricular chamber compliance is minimal. Ultimately the mitral and aortic valves may open during expiration only with atrial systole.