Nonuniform regional deformation of the pericardium during the cardiac cycle in dogs. We hypothesized that local contact forces between the pericardium and the heart cause regional variation in pericardial deformation during the cardiac cycle, reflecting volume changes of the underlying cardiac chambers. To test this, we measured regional pericardial area over the right atrium (RA) and right ventricle (RV) with orthogonal pairs of sonomicrometers in six open-chest dogs. At a left ventricular end-diastolic pressure of 5 mm Hg, RV pericardial area paralleled RV volume, that is, shrinkage during ejection by 10 +/- 8% and expansion during filling. RA pericardial area was reciprocally related to RV pericardial area, with average expansion during ventricular ejection of 2 +/- 2%, thus paralleling RA volume during RV ejection. With volume loading, RV pericardial shrinkage during ejection increased to 14 +/- 6%, but the RA pericardial area change was no longer reciprocal (0 +/- 3% change during RV ejection). Elimination of contact forces by cardiac tamponade resulted in both marked attenuation of RV pericardial area changes and synchronization of the RV and RA pericardial area pattern; that is, both shrank during RV ejection. In two additional dogs, measurement of pericardial area over left ventricle and atrium showed similar results. We conclude that dynamic pericardial contact forces cause regional variation in pericardial deformation, which reflects volume changes of the underlying chambers. These findings imply that the influence of the pericardium on filling and ejection may be more complex than previously recognized, varying both by chamber and dynamically over the course of the cardiac cycle. 