Minoxidil: right atrial cardiac pathology in animals and in man.

Abstract

Minoxidil is a clinically effective vasodilator with a low order of toxicity, except for a canine-specific lesion of the right atrium of the heart of dogs after they are given daily doses of 1 mg/kg or greater for 30 days. The canine right atrial lesion was generally located adjacent to the 2nd of the 3 atrial branches of the right coronary artery and had grossly red or yellowish thickening of the atrial wall. Microscopically, there was extravasation of a few intact red cells, atrophy of myocardial cells adjacent to the subepicardium, and the presence of mast cells and macrophages containing hemosiderin with subsequent proliferation of angioblasts and connective tissue cells. Cytoplasmic loss in myocardial cells of the left ventricular papillary muscle, identical in all respects to that seen after other hypotensive agents and .beta.-adrenoceptor agonists was observed in minoxidil-treated rodents and dogs. Of 158 patients who received minoxidil and died secondarily of severe hypertension, 79 were autopsied. Six cases had significant right atrial pathology not related to gross and histologic changes found in the right atrial lesions of dogs. A prospective study of the right atrium of normotensive and hypertensive patients, not treated with minoxidil, shows an increasing frequency of histologic changes, such as myocardial fibrosis and hydropic vacuolization that are related to age and the presence of hypertension.