ObjectiveBy constraining the heart, the chest wall, lungs, and pericardium limit diastolic filling and thus have a major role in determining cardiac output. Although intermittent positive pressure ventilation and the application of positive end-expiratory pressure amplifies this constraint, no clinical method exists to assess the impact that positive end-expiratory pressure has on ventricular constraint in the newborn. In this study, we tested the hypothesis that a change in right atrial pressure (Pra) reflects the change in ventricular constraint associated with a change in airway pressure.DesignExperimental, comparative animal study.SettingRitchie Centre for Baby Health Research cardiovascular laboratory.SubjectsNeonatal (4-wk-old, n = 6) and newborn (3-day-old, n = 6) Merino/Border-Leicester cross lambs.InterventionsLambs were anesthetized (&agr;-chloralose and ketamine), ventilated, and instrumented to record Pra, thoracic inferior vena caval pressure (Pivc, saline-filled catheters), and pericardial pressure (Pper, liquid-containing balloon).Measurements and Main ResultsChanges (&Dgr;) in Pra, Pivc, and Pper were assessed while airway pressure was rapidly reduced from four set levels of continuous positive airway pressure (2.5, 5, 7.5, and 15 cm H2O) to atmospheric pressure. A strong linear relationship was observed between &Dgr;Pra and &Dgr;Pper (&Dgr;Pra = 0.90 &Dgr;Pper − 0.02,r2= .98), and between &Dgr;Pivc and &Dgr;Pper (&Dgr;Pivc = 0.86 &Dgr;Pper − 0.02,r2= .98) in both the 4-wk-old lambs and the 3-day-old lambs.ConclusionsOur experiments demonstrate that, in the newborn and neonatal lamb, &Dgr;Pra provides an accurate measure of the change in ventricular constraint that accompanies a change in airway pressure, and thus may provide a means of quantifying the magnitude of ventricular constraint imposed by positive end-expiratory pressure and mechanical ventilation during neonatal intensive care.