I would guess that defense could be overcome if:
- the patient has easily identifiable risk factors for clotting, of which overweight is one (birth control pills are another)
- those factors and the subsequent need for anti-coagulation in this patient should have been known by the ER doc
If you look around at websites for medmal attorneys, you'll just about always see them saying phrases like "permanent and debilitating injuries". That's not a requirement of the law, but it is a usual requirement that a medmal attorney has a practical business matter. These cases are expensive to conduct, and so they want a strong possibility of a BIG payoff before they'd undertake one.
Also, since you might have a general propensity for clotting, I'd read up and see what can be done to prevent any future similar events. Good luck.

