Horace Gilbert Smithy Jr. (July 19, 1914 – October 28, 1948) was an American cardiac surgeon who in 1948 performed the first successful mitral valve repair (mitral valvulotomy) since the 1920s. Smithy's work was complicated because it predated heart-lung machines or open heart surgery. Though his procedure did not become a definitive treatment for valvular heart disease, he introduced the technique of injecting novocaine into the heart to avoid arrhythmias during surgery, and he showed that it was feasible to access and operate on the heart's valves.