Klippel-Trénaunay syndrome (KTS) is a rare and complex congenital syndrome defined as the triad of cutaneous capillary malformation, bone and soft tissue hypertrophy, and venous and lymphatic malformations. KTS is thought to be due to a somatic mutation in phosphatidyl-inositol 3 kinase. It belongs to a group of syndromes termed the PI3CA-Related Overgrowth Spectrum (PROS) disorders. Because of the rarity and clinical heterogeneity of these disorders, management is patient specific, and best evidence guidelines are lacking. The most common clinical complications are thromboembolism, thrombophlebitis, pain, bleeding, and high-output heart failure. Surgery is recommended for hemangiomas and chronic venous insufficiency. The early identification of children with PROS disorders has allowed treatment with mTOR inhibitors which have been shown to be effective. The recent development of a direct PI3K inhibitor (alpelisib) has shown promise in preventing abnormal growth and long-term complications of KTS. This report documents a case of high-output heart failure due to the vascular malformations associated with KTS in a 57-year-old male patient and discusses current literature regarding the management of KTS with inhibitors of mTOR and PI3KCA.