1. In a supine position, the heart fills to close to the limits of pericardial constraint and the pericardium may act to redistribute central blood volume from the left side of the heart back to the more compliant lung.
2. We probed whether, and through which mechanisms, a redistribution of blood from the lungs to the left heart occurs during vertical displacement and compensates for reduced venous return.
3. We investigated 16 normal volunteers with Doppler-echocardiography during 20°, 40° and 60° head-up tilting. Tilting was stopped at 10 min in 10 subjects (group 1) and at 45 min in 6 subjects (group 2).
4. At 10 min we observed a reduction in right ventricular diastolic dimension and left ventricular end-diastolic pressure, as estimated by the difference between the duration of the pulmonary venous flow during atrial contraction (Z wave) and that of the mitral A wave. We also recorded a decrease during systole (X wave) and an increase during diastole (Y wave) of the pulmonary venous forward flow velocity. These variations were evident at 20° and became progressively greater with increasing degrees of tilting. In group 2, changes at 10 min and at 45 min for any degree of displacement were similar.
5. A decrease in right ventricular dimensions (ventricular interdependence) and underfilling of the lung compartment due to volume redistribution to the periphery (diminished lung contribution to pericardial constraint) augment compliance within the pericardial space, reduce downstream pressure for pulmonary venous return and move the pulmonary venous flow predominantly to ventricular diastole, allowing diastolic filling.
6. During head-up tilting a favourable interaction between heart and lungs increases compliance within the pericardial space and facilitates redistribution of blood from the lungs, resulting in a sustained compensation for the reduced venous return.