In the general practice I go to, one of the doctors refuses to prescribe contraceptives because of his personal beliefs. So I would imagine that similar biases could apply to any withdrawal of life support measures if the doctor's personal belief did not coincide with this.
Could you comment on this?
If a hospital/doctor was aware of an AHD and chose not to follow it, could the patient or their family sue the hospital?
I would like to preface this by saying that I'm not a lawyer, nor trained in law specifically and as such this is only my opinion.
A Dr does not have to perform a procedure or provide a medication that they believe does not align with their morals. The link below discusses a range of ethical issues predominately facing the American College of Ob's and Gob's (aka obstetrics and gynaecology) but also faces GP's, Emergency Physicians and other Dr's.
http://www.ama-assn.org/amednews/2008/01/21/prsc0121.htm
These are actually very common issues. The most important thing is that un-biased/scientific information is presented and that the Dr's personal beliefs are not forced upon the patient. Implicit in this is that the ability for referral is possible. You can imagine in a one horse (read 'Dr') town this could be effectively impossible.
An example that I personally know about is in a tourist town (population ~55000) in Qld the only place to get clean 'sharps' (clean needles for IV drug users) was from the Pharmacist. This is great for harm minimisation except the Pharmacist thought it appropriate to tell the person's family that they were procuring them, which of course led to needle sharing (spread of disease) and use of blunt needles (permanently damages veins). So you can see how difficult it is to maintain a moral position (I'm sure the Pharmacist believed they were doing the right thing) and also provide the best standard of care.
This brings me to negligence.
Very simply to be negligent, as a Dr, you have to perform an act or omission that causes a loss to a patient. For example if you give a medication and fail to warn that you could go blind (INFORMED CONSENT is very important but not the topic here) and the patient goes blind then there is a potential negligence case. If however you had warned about the risk (to a COMPETENT patient) or no-one knew that blindness was a potential problem (ie acted within what was acceptable medical standard at the time) or the patient didn't go blind (ie did not suffer a loss) then it is unlikely a negligence case would be successful.
I know this is long-winded but I'm trying to demonstrate that if you continued life support for a patient, potentially against their ADH, then it would be VERY difficult to prove negligence. This is due to both criteria not being fulfilled, ie most likely, the Dr would be providing a normal and acceptable treatment and then how would you prove a loss from being kept alive. If you argued, for example that a leg amputation from gangrene was a loss/suffering that was preventable if the patient was allowed to die then the Dr would argue that the saving of life is a gain that outweighs the loss of a limb (and is also 'normal' practice).
There is also a very fine line between euthanasia and allowing someone to pass away, and euthanasia carries a potential gaol term. According to law there is no difference between an 'act' or an 'omission'; so if you 'act' to give a bit too much morphine or you 'omit' to give an antibiotic for pneumonia it's the same theoretically. In practice this is not the case thankfully and if any lawyers are reading this don't get any ideas because one day you or your family will appreciate that as Dr's we have a loophole and can assist/allow nature to take it's course.
Something that does need to be made clear, in my experience of working at >5 public hospitals and numerous nursing homes is that the right to die a dignified death is highly regarded by staff and that the problem usually arises from family members who don't see the person regularly. A noteable example, for me anyway due to my area of speciality, is if the patient is depressed/psychotic etc and the reasoning behind wanting to die is based on delusional/nihilistic premise. As in this situation there is a need for mental health intervention, essentially it means they lack CAPACITY.
Sorry this is so long and may not have directly answered your question but this is a very difficult area and I am by no means an expert. This is, by and large, the domain of Ethicists; so if there are any out there please contribute.