For each cardiac chamber, its transmural pressure—intracardiac pressure minus pericardial pressure—is a principal determinant of its filling. (Transmural pressure is a true filling [distending] pressure that contributes to ventricular preload.) Normal pericardial pressure is lower than the right atrial mean and right ventricular diastolic pressures so that right atrial transmural pressure (right atrial pressure minus pericardial pressure) is normally higher than its cavitary pressure. In tamponade, rising pericardial pressure progressively reduces— and ultimately can make phasically negative—the average transmural pressure of first the right and subsequently the left cardiac chambers. Survival necessitates the ensuing parallel rise in diastolic pressures, first in the right side of the heart and later the left side of the heart, critically reducing all chamber transmural pressures, and with them all filling. canadian drug mall
Like most tamponade-induced abnormalities of pressure and flow, transmural pressures are reciprocally reduced and increased during the respiratory phases for the left vs the right heart. Thus, inspiration increases right heart filling at the expense of the left heart with reversal in expiration. In critical tamponade, when cardiac output usually has fallen by at least 30%, transmural pressures are, on average, zero (typically between 15 and 30 mm Hg within the pericardium and between 15 and 30 mm Hg within the heart in euvolemic patients) so that respiratory reciprocation becomes a principal physiologic mechanism contributing at some level to cardiac input and output. A significant component of respiratory reciprocation is the marked shift of the ventricular septum into the left ventricle when inspiration fills the right heart at the expense of the left with reversal on expiration. Clinically, respiratory reciprocation is expressed as pulsus paradoxus.