THE EFFECT OF POSTURE CHANGE ON BLOOD-VOLUME, SERUM POTASSIUM AND WHOLE-BODY ELECTRICAL-IMPEDANCE

After standing for 1h, ten subjects (7 male, 3 female) assumed a supin e position for a further hour. Whole body bioelectrical impedance incr eased progressively during the hour spent in the supine position: afte r 60 min supine the increase was 13(6 to 32)Omega. Blood and plasma vo lumes, estimated from haematocrit and haemoglobin concentration, incre ased by 8.0(6.7 to 12.4)% and 16.7(12.3 to 20.8)% (median(range)) resp ectively after 60 min supine. Serum potassium concentration had fallen after 10 min supine (4.1(0.1)mmol l(-1); mean(SEM)) relative to the s tanding value (4.6(0.1)mmol l(-1)) and was unchanged thereafter. Serum osmolality (P=0.991) and sodium (P=1.000) and chloride (P=0.998) conc entrations remained unchanged throughout the study. The fall in serum potassium concentration in the supine position does not appear to be a simple dilutional effect consequent upon increases in blood and plasm a volume as there was no effect of postural change on serum sodium or chloride concentrations.