CHANGES IN PULMONARY VENOUS RETURN DURING HEAD-UP TILTING IN MAN

1. In a supine position, the heart fills to close to the limits of per icardial constraint and the pericardium may act to redistribute centra l blood volume from the left side of the heart back to the more compli ant lung. 2. We probed whether, and through which mechanisms, a redist ribution of blood from the lungs to the left heart occurs during verti cal displacement and compensates for reduced venous return. 3. We inve stigated 16 normal volunteers with Doppler-echocardiography during 20 degrees, 40 degrees and 60 degrees head-up tilting, Tilting was stoppe d at 10 min in 10 subjects (group 1) and at 45 min in 6 subjects (grou p 2). 4. At 10 min we observed a reduction in right ventricular diasto lic dimension and left ventricular end-diastolic pressure, as estimate d by the difference between the duration of the pulmonary venous how d uring atrial contraction (Z wave) and that of the mitral A wave. We al so 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 degrees and became progressively greate r vith increasing degrees of tilting. In group 2, changes at 10 min an d at 45 min for any degree of displacement were similar. 5. A decrease in right ventricular dimensions (ventricular interdependence) and und erfilling of the lung compartment due to volume redistribution to the periphery (diminished lung contribution to pericardial constraint) aug ment compliance within the pericardial space, reduce downstream pressu re for pulmonary venous return and move the pulmonary venous how predo minantly to ventricular diastole, allowing diastolic filling. 6. Durin g head-up tilting a favourable interaction between heart and lungs inc reases compliance within the pericardial space and facilitates redistr ibution of blood from the lungs, resulting in a sustained compensation for the reduced venous return.