Why would seroma develop after open heart surgery?

Usually does not: A seroma is a collection of clear fluid that can develop after surgery but is most commonly seen following breast surgery and some types of plastic surgery. Serum leaks into tissue causing swelling. It is different from a hematoma which is blood and contains red blood cells. I can't recall seeing this as a significant complication of open heart surgery except when fluid accumulates in the legs.

Where?: Any incision may have some bleeding- -and a hematoma may develop. This can become a seroma over time as the blood is resorbed. Or some lymphatic s are disrupted and fluid escapes into tissue. Tissue juices and serum can remain in a wound as well. Most are temporary. All are resolvable . See the surgeon.