The physician must consider what's in the best interest of the patient. As lawyers, we suggest that people express their wishes regarding extraordinary medical treatment with written directives and have a copy put into their medical records. The CMA Code of Ethics, furthermore, stipulates that you provide "appropriate care." At the point at which the attempt to resuscitate is no longer appropriate, the procedure could be considered a violation of human dignity or a violation of a person's right to bodily integrity.
(see our cardiologist's response to this question in the cardiology category)