Application of orthostatic test and lower body negative pressure in investigation of circulatory reflexes in humans

Abstract

The aim of this paper is to present methods applied in analysis of tolerance to gravitational acceleration (±Gz) changes and diagnostics of the dysfunction of the autonomic nervous system (ANS) concerning cardiovascular system adaptation to the vertical body position. The head-up tilt test (HUT), head-down tilt test (HDT), and lower body negative/positive (LBNP/LBPP) pressure are widely used for evaluation of integrated ANS reflexes. Due to its simplicity, ease of application and full control over the level of excitation, HUT has become a frequently used clinical test in orthostatic hypotony. LBNP is widely used in aviation medicine to study cardiovascular regulation. In clinics, the method is applied as a reliable procedure to study results of the shift of physiological fluids in the head–legs direction. Due to the supine patient position during the test, muscle contraction (pump) and vestibular influences can be avoided and a pure cardiovascular reflex is observed. Abrupt and/or fast repeatable changes in the direction of gravitational acceleration (±Gz) can result in loss of consciousness (G-LOC). Recent results suggest that tolerance to +Gz is reduced when preceded by HDT (–Gz) (push-pull effect).