Compensatory reactions during lower body negative pressure (LBNP) exposure, head-up tilt (HUT) and +Gz tolerance.

NASA: The effects of lower body negative pressure (LBNP) on acceleration tolerance and head-up tilt were investigated. Healthy male subjects underwent LBNP protocols of various pressures and acceleration tolerance tests; some subjects additionally underwent head-up tilt tests. All subjects were measured for hemodynamic changes and plasma hormonal level changes. The results of these studies showed acceleration tolerance to depend on plasma renin activity, left ventricular ejection time, cardiac output, preejection period, plasma aldosterone levels, and heart rate. The significance of this result is discussed.^ieng

Influence of endurance exercise performance on hemodynamic and hormonal responses to lower body negative pressure (LBNP) and +Gz tolerance in the aspect of individual sensitivity to motion sickness.

NASA: A possible relationship between endurance exercise training, susceptibility to motion sickness, and orthostatic tolerance was investigated. Male subjects underwent acceleration tolerance tests, lower body negative pressure, and Coriolis tests. During the experimental protocol, hemodynamic parameters were measured including heart rate, stroke volume, blood pressure, and cardiac output, and blood was drawn and analyzed for various hormones. Specific results are presented and discussed.^ieng

Application of multivariate statistical analysis to estimate +Gz tolerance based on the changes of hemodynamic parameters during lower body negative pressure (LBNP).

The application of lower body negative pressure (LBNP) is very useful method for simulation of +Gz stress and for evaluation of orthostatic reaction. The different physiological changes that occur during LBNP test and +Gz acceleration test are similar. Lategola and Trent found that supine LBNP exposure at the level of -50 mmHg may be equivalent to +2Gz in producing the changes of heart rate (HR). Polese and coworkers compared hemodynamic changes occurring during upright and supine LBNP at the levels to -70 mmHg with identical measurements made during accelerations to +2Gz, +3Gz, and +4Gz in the same subjects. They noted for example that HR changes during upright LBNP exceeded HR supine levels. Peak values of HR during +3Gz and +4Gz significantly exceeded HR levels during both kinds of LBNP, but HR values at +2Gz were equivalent to those at -40 mmHg of upright and -70 mmHg of supine LBNP. So, the present study was undertaken to evaluate adaptating responses to LBNP stimulus at the level of -60 mmHg, regulatory mechanisms of the circulatory system (central and peripheral) and to look for the possibility of +Gz tolerance prediction based on the changes of some hemodynamic parameters during LBNP.

Relationship between atrial natriuretic peptide (ANP), renin (PRA), aldosterone (PAC), hemodynamic responses to lower body negative pressure (LBNP) and +Gz tolerance.

The redistribution of a certain thoracic blood volume to the lower parts of the body and decrease of the venous return of blood to the heart during lower body negative pressure leads to the central hypovolemia and the deactivation of cardiopulmonary and arterial baroreceptors. Many compensatory mechanisms are involved during central hypovolemia, which is also reflected by the changes in the secretion of different vasoactive hormones. Due to this fact the LBNP stimulus is widely used for the investigation of regulatory (compensatory) mechanisms in cardiovascular system providing deeper understanding of orthostatic reaction. Recently several papers were published on application of this experimental model for +Gz acceleration tolerance assessment. The purpose of this study was evaluate the possible dependence between the changes of ANP secretion, renin-angiotensin-aldosterone system activity, the changes of some hemodynamic parameters during the model of gravitational stress i.e. LBNP exposure and +Gz acceleration tolerance.