Am I responsible for medical bills my ex-husband incurred?

Ouch, you know those wedding vows said you took him "in sickness and in health. In most states, that makes you responsible for paying his necessary medical expenses. So the only important questions are "was the medical care necessary or elective?" and "did you start the divorce process before you obtained the care?" In some states, if you start the divorce, you end your legal responsibility. I would urge you to discuss this with your divorce attorney.

Necessary medical treatment will be considered a marital debt, subject to allocation in a divorce.

The expenses are reviewed in relation to why they were incurred and how close they were incurred to the end of the marriage. So, it is likely he will be responsible for most if not all of them.