Table 1.

Comparison of LBNP and HUT as tools to induce orthostatic challenge.

Items	HUT	LBNP
Lower body pooling	Yes	− 40 mmHg
Heart rate increase similar at	70°	− 50 mmHg
Systolic pressure	↓	↓
Diastolic pressure	↑	↓
Gravity-induced	Yes	No
Arterial resistance	↑	↑
Thoracic, pelvic, leg blood volumes	↑	↑
Visceral blood volume	↑	↓
Transmural pressure gradient	From the heart to lower body	Throughout lower body subjected to suction
Hydrostatic pressure gradient between thorax and abdomen or main arteries	Yes	No
Endothelial activation	Release activator	Release activator
Shear stress	↑	↑
Hormonal changes	NO↑ H2S↓	NO↑ H2S↓
Epinephrine	↑	↑↑

↑, increase; ↓, decrease; HUT, head-up tilt test; LBNP, lower limb negative pressure test; NO, nitric oxide; H₂S, hydrogen sulfide.