The relationship between the changes of the central blood volume and the hemodynamic responses of systemic and pulmonary vessels to passive body tilts was studied. Nine dogs were passively tilted from supine to 6°, 15°and 30°head-up(HU) or to
-6°,
-15°and -30°head-down(HD) serially for 10 minutes in each grade. Pressures of systemic artery(SAP), pulmonary artery(PAP), pulmonary arterial wedge(PAWDP), right atrium(CVP), external jugular vein(EJVP) and femoral vein(FVP) were measured. And
cardiac
output(CO), stroke volume(SV), heart rate(HR), end-diastolic volume(EDV), cardiac acceleration index(CAI) and thoracic fluid volume(TFV) were measured by a impedance cardiograph(BoMed Co.). Systemic vascular resistance(SVR) and pumonary vascular
resistance(PVR) were calculated by Ohm's principle.
At the tilts to -6°, 15° and -30°HD, TFV increased gradually by 29, 108 and 213 ml respectively, whereas it reduced by 33, 65, and 60 ml at 6°, 15, and 30° HU respectively. CO increased at -6°and -15°HD and did not change at HU. HR did not
change
at HD, but increased at HU. SAP and SVR did not show the significant changes at both tilt. PAP decreased gradually at HD and PVR increased at HU. PAWP decreased gradually at both tilts and CVP decreased gradually at HD. EJVP increased and FVP
decreased
with proportion to the tilting degree at HD. But FVP decreased proportionally, whereas EJVP showed no change at HU. CAI reduced with elevated EDV at -30°HD.
In summary, a passive body tilt produces the alteration of thoracic blood volume with progress to the direction and the magnitude of a hydrostatic pressure. It seems that the cephalic vasodilation is mediated by the cardiopulmonary baroreceptors
at
HD
tilts, and the vasoconstriction of lower extremities is mediated by the local control mechanisms as well as the cardiopulmonary receptors and arterial baroreceptors at HU tilts.
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