How does herceptin cause cardiac dysfunction?

It can cause heart muscle damage which leads to heart failure (the heart's inability to adequately pump blood throughout the body). It's a monoclonal antibody that targets HER-2. Some cancer cells "over express" HER-2 on their surfaces. My guess (you should check this out w/ an MD) is that it also attaches to HER-2 on the surface of heart muscle cells.