In the Accountable Care Organizations (ACOs) to whom is the physician accountable?

Hint: It is not the patient, at least not in the structure or intent of the ACOs. See here for Paul Hsieh 's discussion of this issue.

Dr.Hsieh succinctly nails it here:

... under ObamaCare, your doctor will be increasingly pressured into sacrificing your individual medical interests for a nebulous “social justice.”

Exactly

He references some key quotes from physicians and physician organizations who favor and have been lobbying for the substitution of the pursuit of an elastic and nebulous collective good for the long standing fiduciary duty of the physician to the patients.

A now-famous article in the 1998 Annals of Internal Medicine recommended that “devotion to the best medical interests of each individual patient be replaced with an ethic of devotion to the best medical interests of the group...” The American College of Physicians ethics charter now states that physicians should balance traditional principles of patient welfare and patient autonomy with “social justice” to achieve “a just distribution of finite resources.” A 2011 New England Journal of Medicine article urged abandoning “the primacy of patient welfare” in favor of “collectively caring for a defined population within a fixed annual budget.”

Read the entire piece. It is excellent. Dr.Hsieh has been working tirelessly to support the concept of freedom and individual rights particularly in regard to the individual rights of doctors and the practice of medicine. Read more from him here.