The purpose of the current study was to determine for the first time the qualitative and quantitative impact of varying degrees of interatrial shunting on right heart dynamics and systemic perfusion in subjects with chronic pulmonary hypertension (CPH). Eight dogs underwent three months of progressive pulmonary artery banding, following which right atrial and ventricular end-systolic and end-diastolic pressure volume relations were calculated using conductance catheters. An 8-mm shunt prosthesis was inserted between the superior vena cava and left atrium, yielding a controlled model of atrial septostomy. Data were obtained: 1) pre-shunt or "CPH", 2) "Low-Flow" shunt, and 3) "High-Flow" shunt (occluding superior vena cava forcing all flow through the shunt). With progressive shunting, right ventricular pressure fell from 72±19 mmHg (CPH) to 54±17 (Low-Flow) and 47±17 (High-Flow) (P<0.001). Cardiac output increased from 1.5±0.3 L/min at CPH to 1.8 ±0.4 L/min Low-Flow (286±105 ml/min, 15% of cardiac output) (P<0.001), but returned to 1.6±0.3 L/min at High-Flow (466±172 ml/min, 29% of cardiac output) (P=0.008 versus Low-Flow, p=0.21 versus CPH). There was a modest rise in systemic oxygen delivery from 252±46 ml/min at CPH to 276±50 Low-Flow (P=0.07), but substantial fall to 222±50 at High-Flow (P=0.005 versus CPH, P<0.001 versus Low-Flow). With progressive shunting, bichamber contractility did not change (P=0.98), but the slope of the right atrial end-diastolic pressure volume relation decreased (P<0.04) consistent with improved compliance. This study demonstrated that Low-Flow interatrial shunting consistently improved right atrial mechanics and systemic perfusion in subjects with CPH, while High-Flow exceeded an "ideal shunt fraction".