Some embodiments of a system or method for treating heart tissue can include a catheter device that provides a user with the ability to perform a number of heart treatment tasks (before, during, and after a cardiac surgery or a percutaneous coronary intervention). In particular embodiments, the catheter device can be used to (i) precondition heart muscle tissue before the heart is isolated from the circulatory system, (ii) deliver cardioplegia into the coronary sinus during the cardiac surgery when the heart is isolated from the circulatory system, and (iii) control the blood flow through the heart after the heart is reconnected with the circulatory system. In some embodiments, the catheter device can perform some or all of: (i) intermittently occluding the coronary sinus, (ii) delivering a treatment fluid into the coronary sinus, and (iii) monitoring a flow rate of blood passing from the coronary sinus to the right atrium.