Of all the arguments against voluntary euthanasia, the most influential is the "slippery slope": once we allow doctors to kill patients, we will not be able to limit the killing to those who want to die.

There is no evidence for this claim, even after many years of legal physician-assisted suicide or voluntary euthanasia in the Netherlands, Belgium, Luxembourg, Switzerland, and the American state of Oregon. But recent revelations about what took place in a New Orleans hospital after Hurricane Katrina point to a genuine danger from a different source.

When New Orleans was flooded in August 2005, the rising water cut off Memorial Medical Centre, a community hospital that was holding more than 200 patients. Three days after the hurricane hit, the hospital had no electricity, the water supply had failed, and toilets could no longer be flushed. Some patients who were dependent on ventilators died.

In stifling heat, doctors and nurses were hard-pressed to care for surviving patients lying on soiled beds. Adding to the anxiety were fears that law and order had broken down in the city, and that the hospital itself might be a target for armed bandits.

Helicopters were called in to evacuate patients. Priority was given to those who were in better health, and could walk. State police arrived and told staff that because of the civil unrest, everybody had to be out of the hospital by 5pm.

On the eighth floor, Jannie Burgess, a 79-year-old woman with advanced cancer, was on a morphine drip and close to death. To evacuate her, she would have to be carried down six flights of stairs, and would require the attention of nurses who were needed elsewhere. But if she were left unattended, she might come out of her sedation, and be in pain. Ewing Cook, one of the physicians present, instructed the nurse to increase the morphine, "giving her enough until she goes". It was, he later told Sheri Fink, who recently published an account of these events in the New York Times, a "no-brainer".

According to Fink, Anna Pou, another physician, told nursing staff that several patients on the seventh floor were also too ill to survive. She injected them with morphine and another drug that slowed their breathing until they died.

At least one of the patients injected with this lethal combination of drugs appears to have otherwise been in little danger of imminent death. Emmett Everett was a 61-year-old man who had been paralysed in an accident several years earlier, and was in the hospital for surgery to relieve a bowel obstruction. When others from his ward were evacuated, he asked not to be left behind.

But he weighed 173kg (27 stone), and it would have been extremely difficult to carry him down the stairs and then up again to where the helicopters were landing. He was told the injection he was being given would help with the dizziness from which he suffered.

In 1957, a group of doctors asked Pope Pius XII whether it is permissible to use narcotics to suppress pain and consciousness "if one foresees that the use of narcotics will shorten life". The pope said that it was, "if no other means exist, and if, in the given circumstances, this does not prevent the carrying out of other religious and moral duties". In its declaration on euthanasia, issued in 1980, the Vatican reaffirmed that view.

The Vatican's position is an application of what is known as "the doctrine of double effect". An action that has two effects, one good and the other bad, may be permissible if the good effect is the one that is intended and the bad effect is merely an unwanted consequence of achieving the good effect. Significantly, neither the pope's remarks, nor the declaration on euthanasia, place any emphasis on the importance of obtaining the voluntary and informed consent of patients, where possible, before shortening their lives.

According to the doctrine of double effect, two doctors may, to all outward appearances, do exactly the same thing: that is, they may give patients in identical conditions an identical dose of morphine, knowing that this dose will shorten the patient's life. Yet one doctor, who intends to relieve the patient's pain, acts in accordance with good medical practice, whereas the other, who intends to shorten the patient's life, commits murder.

Dr Cook had little time for such subtleties. Only "a very naive doctor" would think that giving a person a lot of morphine was not "prematurely sending them to their grave", he told Fink, and then bluntly added: "We kill 'em." In Cook's opinion, the line between something ethical and something illegal is "so fine as to be imperceivable".

At Memorial Medical Centre, physicians and nurses found themselves under great pressure. Exhausted after 72 hours with little sleep, and struggling to care for their patients, they were not in the best position to make difficult ethical decisions. The doctrine of double effect, properly understood, does not justify what the doctors did; but, by inuring them to the practice of shortening patients' lives without obtaining consent, it seems to have paved the way for intentional killing.

Roman Catholic thinkers have been among the most vocal in invoking the "slippery slope" argument against the legalisation of voluntary euthanasia and physician-assisted dying. They would do well to examine the consequences of their own doctrines.

Copyright: Project Syndicate, 2009.