The shift in the relationship between intrapericardial fluid pressure and volume induced by acute left ventricular pressure overload during cardiac tamponade.

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Abstract
We hypothesized that a process leading to an acute increase in cardiac size would change the relationship between intrapericardial pressure and fluid volume during cardiac tamponade, resulting in a change in the time of onset of right ventricular diastolic collapse (RVDC) as seen on the two-dimensional echocardiogram. Five spontaneously breathing dogs were instrumented to measure ascending aortic and right atrial blood pressures and intrapericardial pressure (IPP). A hydraulic occluder was placed around the proximal descending thoracic aorta. Each animal underwent six consecutive episodes of cardiac tamponade, three in the presence alternating with three in the absence of aortic constriction. The onset of RVDC was recorded and the volume infused into the pericardial space was measured. In the presence of aortic constriction, the relationship between pericardial pressure and incremental pericardial fluid volume was shifted so that IPP was an average of 3.4 mm Hg higher at any given intrapericardial fluid volume (p less than .001). At the onset of RVDC, the mean IPP was higher and the intrapericardial fluid volume was lower during aortic constriction than under control conditions (p less than .001 for both comparisons). Thus, a rapid increase in left ventricular volume in the presence of an otherwise unimportant pericardial effusion may increase intrapericardial fluid pressure sufficiently to cause RVDC.