Am I obligated to provide care to everyone I encounter who goes into cardiac arrest?

There are certain situations in which health-care professionals have legal duties to the public. For example, if you are employed to provide care (e.g. as a physician, nurse, or paramedic), and in the course of your employment a medical emergency develops, you are obligated to perform in accordance with your employment duties. 

However, if you are off-duty and an emergency develops, you generally do not have a legal duty to provide care. Some states do have laws that may establish duty based on licensure; but, this is not common.

It is important to remember that if you do begin to provide assistance, you should not withdraw care until someone else takes over the responsibility. By deciding to intervene, you voluntary established a contract with the victim, and ceasing care prematurely could be construed as abandoning the patient.

Most health-care professionals are eager to provide emergency care, so the important thing is to make sure that the care you give is reasonable and competent. Be certain to turn the care of the victim over to someone who can accept this responsibility (e.g. paramedics). Whether or not you have ACLS training is relevant to your professional decisions regarding what care you should properly give; it is not, however, a mandate to provide this care.