Radiographic studies of the chest during changes in posture and lower body negative pressure.

Abstract

Lower body negative pressure (LBNP) is the application of subatmospheric pressure to the lower body up to the level of the iliac crests. LBNP causes a pooling of blood in the veins of the lower body by reducing the pressures in the tissues and thereby increasing the transmural pressures across the walls of these blood vessels. A change in posture from the supine to the erect position causes pooling of blood in the veins of the lower extremities by imposing a hydrostatic pressure approximately equal to the height of the column of blood within them. The physiological consequences of LBNP are qualitatively similar to those seen during a shift from the supine to the erect position. When LBNP is applied in the 40- to 60-mm-Hg range the effects are quantitatively similar. Most, if not all, of the cardiovascular effects of LBNP are attributable to a redistribution of blood from thoracic vascular reservoirs to the veins of the lower extremities (7, 10). LBNP has several possible practical applications based on its similarities to assumption of upright posture and shift of blood away from the upper body: (a) It has been used in the treatment of pulmonary edema (5). (b) It has been used to replace the tilt table as a test of orthostatic intolerance (4). (c) It may find a use as a conditioning device in a weightless environment (i.e., prolonged spaceflight). In previous studies (4, 10) on the same 5 young male subjects used in this study, we determined that LBNP at 40 mm Hg for one minute had the following effects: (a) no change in alveolar volume or vital capacity; (b) an increase of 480 cc in the functional residual volume (FRC); (c) a decrease of 30 per cent in both the pulmonary diffusing capacity (Dlco) and the pulmonary capillary volume (Vc); and (d) an increase of 513 cc in the volume of the legs. Other investigators have demonstrated reduction of cardiac output, cardiac index, central venous pressure, and stroke volume (7). The present studies were undertaken to determine what, if any, radiological changes in the thorax were associated with the above physiological changes and to compare them with those observed in shifting from the supine to the erect posture. Methods The LBNP apparatus (Fig. 1) was composed of a sheet metal tank, an air seal between the tank and the subject at the level of the iliac crest, and a source of subatmospheric pressure regulated to within plus or minus 1 mm Hg. An adjustable metal template and rubber spacer were loosely fitted about the subject. A rubber sheet was wrapped over the tank template and lower abdomen providing a self-adjusting seal at the level of the iliac crest. Inside the tank the subject was supported by an adjustable crotch support.