Background To determine the transmural pressure-dimension relations of the right atrium (RA) and right ventricle (RV) before and after pericardiectomy, six open-chest dogs were instrumented with pericardial balloons placed over the RA and RV free walls.Methods and Results RA appendage dimensions and RV free-wall segment lengths were measured using sonomicrometry. Intact-pericardium RA and RV transmural pressures were calculated by subtracting the pericardial pressures (measured using balloons) from the cavitary pressures. Pooled data from six animals with pericardium intact indicate that at RA and RV cavitary pressures of 5, 10, and 15 mm Hg, RV pericardial pressure was 4.3+-0.3, 8.6+-1.0, and 13.3+-1.5 mm Hg, respectively, and RA pericardial pressure was 4.8+-0.3, 9.6+-0.6, and 14.6+-0.6 mm Hg, respectively (mean+-SD). With calculated unstressed dimensions, the cavity dimension data were normalized to strain (in percent). We determined that in the dog, RV strain would increase by 14% and RA by 68% to maintain cavitary pressure at 10 mm Hg on pericardiectomy. To compare these results with clinical data, RV (n=7) and RA (n=6) transmural pressures were measured using balloons in patients (age, 19 to 76 years) undergoing cardiac surgery. RA transmural pressure of six patients was 1.0+-1.5 mm Hg when central venous pressures (CVPs) ranged from 3 to 16 mm Hg. RV transmural pressure equaled 1.2+-1.9, 2.3+-1.9, and 3.4+-2.0 mm Hg when CVP was 5, 10, and 15 mm Hg, respectively.Conclusions Pericardial constraint (as evaluated by the ratio of pericardial to intracavitary pressures when CVP is 10 mm Hg) accounted for 96% of RA cavitary pressure in the dog and 89% in humans and at least 86% of RV cavitary pressure in the dog and 77% in humans.(Circulation. 1994;90:2492-2500.)