Key result
Graded tilt testing shows the hydrostatic indifference point sits higher than the volume indifference point.
Why the study?
Does inflation of medical antishock trousers affect the hydrostatic and volume indifference points during graded tilt in male volunteers?
Does inflation of medical antishock trousers affect the hydrostatic and volume indifference points during graded tilt in male volunteers?
The volume indifference point is located lower in the abdomen than the hydrostatic indifference point, indicating a more profound gravitational influence on blood distribution than previously thought based on venous pressure alone.
Indicates stronger gravitational blood volume shifts than venous pressure models suggest; leaves open effects on antishock garment use or orthostatic protocols.
The hydrostatic indifference point (HIP; where venous pressure is unaffected by posture) is located at the level of the diaphragm and is believed to indicate the orthostatic redistribution of blood, but it remains unknown whether HIP coincides with the indifference point for blood volume (VIP). During graded (± 20°) head-up (HUT) and head-down tilt (HDT) in 12 male volunteers, we determined HIP from central venous pressure and VIP from redistribution of both blood, using ultrasound imaging of the inferior caval vein (VIPui), and fluid volume, by regional electrical admittance (VIPadm). Furthermore, we evaluated whether inflation of medical antishock trousers (to 70 mmHg) affected HIP and VIP. Leaving cardiovascular variables unaffected by tilt, HIP was located 7 ± 4 cm (mean ± SD) below the 4th intercostal space (IC-4) during HUT and was similar (7 ± 3 cm) during HDT and higher (P < 0.0001) than both VIPui (HUT: 22 ± 16 cm; HDT: 13 ± 7 cm) and VIPadm (HUT: 29 ± 9 cm; HDT: 20 ± 9 cm below IC-4). During HUT antishock trousers elevated both HIP and VIPui [to 3 ± 5 cm (P = 0.028) and 17 ± 7 cm below IC-4 (P = 0.051), respectively], while VIPadm remained unaffected. By simultaneous recording of pressure and filling of the inferior caval vein as well as fluid distribution, we found HIP located corresponding to the diaphragm while VIP was placed low in the abdomen, and that medical antishock trousers elevated both HIP and VIP. The low indifference point for volume shows that the gravitational influence on distribution of blood is more profound than indicated by the indifference point for venous pressure.
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Petersen et al. (2014) studied this question. Graded head-up and head-down tilt and medical antishock trousers was evaluated on Location of hydrostatic indifference point (HIP) compared to volume indifference point (VIP) (p=<0.0001). Graded head-up and head-down tilt in male volunteers revealed the hydrostatic indifference point is located significantly higher than the volume indifference point (P < 0.0001).
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