Relationship of Pericardial to Pleural Pressure During Quiet Respiration and Cardiac Tamponade

Pericardial pressure was studied in chronic experiments on dogs with relation to intrathoracic pressure in the normal state and in cardiac tamponade. In the normal resting animal, the two pressures were in good agreement throughout the respiratory cycle. The cardiac cycle produced superimposed pressure fluctuations amounting to 30% of the respiratory swings. Adding saline to the pericardial sac produced sigmoid pressure-volume curves; tamponade required 100 to 330 ml of fluid. Although during tamponade the pericardial pressure greatly exceeded the intrathoracic pressure, the pericardial pressure invariably fell with inspiration, generally by the same amount as the intrathoracic pressure. The cardiac cycle produced greater pressure fluctuations (50% of the respiratory pressure fluctuations). The superior vena caval pressure always fell with inspiration in the normal state and during tamponade. Hypovolemia did not change the resting pericardial pressure, but decreased the slope of the pressure-volume curve measured during cardiac tamponade. Hypervolemia increased the resting pericardial pressure considerably, and increased the slope of the pericardial pressure-volume curve. The results suggest that pleural pressure is a reasonable approximation of the pericardial pressure in normal dogs. There appears to be no substantial evidence that pulsus paradoxus is due to failure of the pericardial sac to transmit inspiratory reductions of pleural pressure.