Oedema formation with the vasodilators nifedipine and diazoxide: direct local effect or sodium retention?

OBJECTIVE To determine whether the common side effect of ankle oedema with arteriolar vasodilators such as the calcium entry blocker (CEB) nifedipine and the potassium channel opener (PCO) diazoxide is the direct result of peripheral vasodilation or merely a consequence of renal sodium retention. DESIGN In 12 healthy sitting volunteers we studied for 3 h the effects of 20 mg nifedipine, 150 mg diazoxide intravenously, 25 mg captopril and placebo on oedema formation and sodium excretion. CONCLUSIONS Foot swelling was determined with a new accurate device (coefficient of variation 0.30%), which uses Archimedes principle to measure water displacement induced by immersion of the foot. Blood pressures were recorded with a Hawksley random-zero sphygmomanometer. RESULTS All of the active drugs decreased diastolic blood pressure (captopril by 9 +/- 2%, nifedipine by 4 +/- 3% and diazoxide by 2 +/- 2%, compared with an increase of 5 +/- 2% with placebo). Foot volume increased acutely after administration of nifedipine (by 2.6 +/- 0.4%), whereas it remained stable with placebo and the other drugs. Administration of captopril and nifedipine induced increases of fractional sodium excretion (by 20 +/- 9% and 40 +/- 20%, respectively) in contrast to the decreases with placebo and diazoxide (by 13 +/- 11% and 24 +/- 10%, respectively). Only administration of nifedipine induced significant, albeit small, increases in haemoglobin and serum albumin levels. CONCLUSIONS Administration of nifedipine increased foot volume and natriuresis simultaneously, thereby supporting the hypothesis that development of ankle oedema with CEB is a local phenomenon at the site of vasodilation. The absence of a similar increase in foot volume with diazoxide administration should be interpreted with caution because of the rather minor effect of this dose of diazoxide on blood pressure. However, it could be indicative of a different mechanism of oedema formation with PCO.