If the heart must be stopped for the operation, cardioplegia cannulas are also required. Antegrade cardiopledgia (forward flowing, through the heart's arteries), retrograde cardiopledgia (backwards flowing, through the heart's veins), or both types may be used depending on the operation and surgeon preference. For antegrade cardiopledgia, a small incision is made in the aorta proximal to the arterial cannulation site (between the heart and arterial cannulation site) and the cannula is placed through this to deliver cardiopledgia to the coronary arteries. For retrograde cardiopledgia, an incision is made on the posterior (back) surface of the heart through the right ventricle. The cannula is placed in this incision, passed through the tricuspid valve, and into the coronary sinus. The cardiopledgia lines are connected to the CPB machine.