Pressure-Volume Relationship. Various pressures as a function of pericardial effusion volume VPERI. These pressures include pericardial pressure (PPERI), mean diastolic atrial (PRA and PLA) and ventricular (PRV and PLV) pressures. At 800 ml, there is a 2 mmHg increase in pericardial pressure and equalization to right diastolic chamber pressures. At 950 ml, pulsus paradoxus first appears. At 1050 ml, chamber pressures equalize to within 2 mmHg of each other and chamber collapse is observed at 1100 ml, with 34% of the mean cardiac cycle marked by collapse of the right atrium. The figure insert plot (top left) shows the transmural pericardial pressure vs. pericardial volume for data points derived from Reddy et al.  in which a fixed mean pleural pressure is assumed, and a nonlinear least-squares fit to the data (see text for details).