A cannula for draining blood from a patient's heart during cardiac surgery including a main body and a pair of branches extending distally therefrom. The branches are biased to extend away from one another by pre-shaped stiffening elements, and have "D" shaped cross-sections defining flat surfaces oriented generally towards one another. The branches may be constrained in a retracted position, the flat surfaces abutting one another, and the resulting profile forming a substantially continuous extension of the main body. The branches have distal tips including drainage holes and occlusion balloons, and elbow portions including accordion-like ridges for preventing kinking. Grooves in the flat surfaces define a secondary lumen in the retracted position through which a cardioplegia delivery or an imaging device may be advanced. The cannula is directed into a trocar to constrain the branches in the retracted position, and the trocar is inserted into the right atrium. The trocar is withdrawn to deploy the branches, which automatically extend away from one another, assuming a tripartite configuration wherein the branches extend substantially transversely from the main body, and enter the superior and inferior vena cavae. The occlusion balloons are inflated to isolate the vena cavae and blood is drained therefrom for delivery to a pump-oxygenator. A cardioplegia delivery device is advanced through the secondary lumen into the coronary sinus, an occlusion balloon on a distal tip of the device is inflated to isolate the coronary sinus, and retrograde delivery of cardioplegia is achieved.