The purpose of this study was to compare the effectiveness of saline load to
fludrocortisone (
florinef) as countermeasures for reduced plasma volume and
orthostatic intolerance after spaceflight. Eleven males (ages 30-50 yr) underwent a 3-day ambulatory baseline period followed by 7 days of 6 degrees head-down
bedrest, during which cardiopulmonary and arterial baroreflex sensitivity and plasma volume (PV) were determined. During pre-
bedrest and 2.5 h
after treatment on day 8, PV was also measured and subjects underwent a 15-min unsupported stand test. Treatments consisted of 8
salt tablets (1 g NaCl per
tablet) and 960 ml of water in 5 subjects and 0.6 mg (0.2 mg x 3) over 24 h in the other 6 subjects. PV decreased by 12% on day 7 of
bedrest. This was restored on day 8 by
florinef but not by saline load. The effect of
florinef on PV was paralleled by decreases in urine volume and the urinary
sodium/
potassium ratio. Reduced PV was associated with greater vascular resistance for the same drop in central venous pressure, suggesting less vasoconstriction reserve after
bedrest. Carotid baroreflex control of heart rate was attenuated after 7 days of
bedrest. Both baroreflex functions were restored by
florinef but not saline load. Only 1 of 6 subjects showed syncopal symptoms in the
florinef-treated group, whereas 4 of 5 subjects did so in the saline-load group. Acute
florinef treatment appears to have distinct advantages as a protective measure for post-
bedrest orthostatic intolerance, not only through its
salt retaining, volume-expanding mineralcorticoid effect, but possibly through its actions on baroreflex and sympathetic functions.