The purpose of this study is to evaluate the effect of the (angiotensin-converting enzyme) ACE inhibitor lisinopril on hemoglobin, hematocrit, and erythropoietin levels in patients with secondary polycythemia due to high-oxygen-affinity hemoglobin variants, oxygen-sensing pathway mutations, or other unexplained and potentially irreversible forms of erythrocytosis. This study aims to determine whether ACE inhibition can effectively reduce excessive erythrocytosis by modulating erythropoietin production.