The effect of posture change on blood volume, serum potassium and whole body electrical impedance.

After standing for 1 h, ten subjects (7 male, 3 female) assumed a supine position for a further hour. Whole body bioelectrical impedance increased progressively during the hour spent in the supine position: after 60 min supine the increase was 13(6 to 32) omega. Blood and plasma volumes, estimated from haematocrit and haemoglobin concentration, increased by 8.0(6.7 to 12.4)% and 16.7(12.3 to 20.8)% (median(range)) respectively 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 standing 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) concentrations 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 plasma volume as there was no effect of postural change on serum sodium or chloride concentrations.