First, I hope your mom continues to make a good recovery.
Second, these are certainly not unusual complications. Taking blood thinners after this type of surgery is normal and the risk of internal bleeding (bowels turning black) needs to be balanced with not enough thinning of the blood (which can result in a deadly blood clot). The statement that the "nurse gave her way to many blood thinners" might be out of line. I would first examine:
1. What were the actual physician's orders? Did the nurse follow those orders?
2. When was the first clue that something was wrong? Did the nurse observe it and fail to report it?
If the nurse did not follow the appropriate orders/protocol, then yes, he/she would be held responsible. If she followed the physicians instructions and reported any change in status to the physician, then we must either consider:
A. Did the physician file an appropriate order?
B. This might simply be a case of a statistical variance...meaning this was a normal risk for this type of surgery and unfortunately everything was done right, but your mom fell into that category of a bad reaction.
I cannot comment on the second surgical procedure. I would highly doubt they would have redone the same procedure without your knowlege as it would have required another 3 day to a week in intensive care, and I'm sure you would have seen her in that time. Plus, they would have had to re-open the chest wall which is incredibly invasive (and I'm sure she'd be aware of that). Rather, it is probably more possible she might have gone for a cardiac caterization to look inside the blood vessles of the heart which is a minorly invasive procedure (through the groin) that looks at the blood vessles around the heart and usually only requires an overnight hospitalization or at the very most a couple days. This would be an entirely different problem than the original valve replacement procedure. the first was a procedure to fix the valves IN the heart and the second is to clear the blood vessles that FEED the heart.
To talk to a lawyer might be premature, but I would start looking for some answers, namely:
1. What was the procedure for administering the blood thinners?
2. Was the procedure followed by both the doctor and the nurse?
3. What was the second procedure done and why?
I know it's hard to keep a straight head when it's your mom...but getting a lawyer isn't going to make her get any better faster. Yet, if you have concerns, I would ask for a family conference...administration included if you suspect any wrong doing.
I am a health care professional