Renal lithium clearance in the different stages of hypertension.
This study investigated whether fractional lithium excretion (FELi), used as a marker of proximal fluid delivery, changes during different phases of essential hypertension. Forty-eight subjects were studied: 12 essential hypertensives (EH); 12 borderline hypertensives (BL); 12 normotensives with a positive family history of essential hypertension (NH) and 12 normotensives without a family history of essential hypertension (NN). Measurements were performed in the recumbent position, both in basal conditions and after a saline load (2% body weight in 1 h; 0.333 ml/min per kg body weight). In basal conditions, a moderate extracellular volume expansion was already present in the subjects. In these conditions, FELi of EH was significantly higher than that of all the other groups (P less than 0.01). After the saline load, fractional sodium excretion increased in all the groups (P less than 0.01), but to a significantly greater extent in EH (P less than 0.01). FELi rose significantly only in BL (P less than 0.05). The change in FELi of BL correlated positively (P less than 0.02) with the change in blood pressure in 10 of these subjects 3 years after this study. Moderate extracellular volume expansion may be able to either reveal or stimulate an increase of FELi in subjects with established hypertension. When a greater degree of extracellular volume expansion is induced, this increases FELi in BL and this effect may be related to the subsequent development of hypertension.