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The volume indifferent point: an approach for explaining orthostatic intolerance
Author(s) -
Jarvis Sara,
Sheriff Don,
Pawelczyk James
Publication year - 2007
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.21.6.a877
Subject(s) - orthostatic intolerance , orthostatic vital signs , venous return curve , tilt (camera) , medicine , head down tilt , anatomy , pelvis , cardiology , blood pressure , hemodynamics , mathematics , geometry
During standing or head‐up tilt (HUT), gravity redistributes ~700 mL of blood to the dependent regions (legs, buttocks, and pelvis). A quantitative approach to relate these changes in blood volume distribution to cardiac filling remains lacking. In a hydrostatic gradient there exists a hydrostatic indifferent point (HIP), a point where pressure is balanced against the mechanical properties of the circulation and where pressure does not change, regardless of posture (Gauer and Thron, 1965). Absent a tilt‐induced change in compliance, the HIP should be coincident with a volume indifferent point (VIP). Since cardiac filling is determined by the hydrostatic gradient between the right atrium and the HIP, a more inferior location of the HIP/VIP would lead to diminished venous return and decreased filling pressure, contributing to orthostatic intolerance. By employing segmental impedance to regress regional blood volume shifts against the distance of each segment from the feet, we present preliminary evidence that quantifies the location of the VIP in humans. During HUT, impedance increases above the VIP and decreases below it. The VIP, or the point where impedance does not change, is located in the abdominal compartment at approximately 2/3 of an individual’s height. Moreover, the VIP appears to be reproducible on separate days and is not influenced by the magnitude of the tilt angle. Supported by NASA NNJ04HF45G