Effects of aging on leg vein filling and venous compliance during low levels of lower body negative pressure in humans.

To evaluate the fluid shift and leg venous compliance during orthostatic stress with advancing age, 12 aged and 5 young healthy males were subjected to graded lower body negative pressures (LBNP) of -5, -10, and -15 mmHg. Cardiovascular variables were monitored continuously, and leg venous compliance was determined by venous occlusion plethysmography. Neither heart rate nor mean arterial pressure changed significantly in any subject during LBNP. A progressive decrease in the thoracic fluid volume index and a gradual increase in the leg fluid volume index, indicating a fluid shift towards the lower body were observed significantly in the young group (p<0.05), while these changes were not significant in the aged group. A linear reduction in peripheral venous pressure could be seen during graded LBNP in all subjects, but the reduction rate was smaller in the aged group. Baseline leg venous compliance was reduced in the elderly (p<0.05). During LBNP, venous compliance decreased in all subjects, but the decrease was significantly smaller in the aged group (p<0.05). It is suggested that the smaller fluid shift and smaller decreased leg venous compliance in aged people during gravitational stress were mainly due to the vascular and ventricular stiffness induced by an age-related reduction in visco-elasticity of the peripheral venous and ventricular walls.