Mrs. Melnyk’s case is not an isolated one: As medical technology becomes more advanced, it also becomes more difficult for doctors to discern the line between life and death.

PET scans, which have been widely used since the 1990s, are a good example. Rather than solely showing body anatomy like an MRI or CT scan would, a PET scan can actually detect cellular activity in tissues. In a study published last year in The Lancet, 13 out of 41 patients in a persistent vegetative state showed detectable brain activity on PET scans, results that the investigators thought were consistent with “minimal consciousness.” The investigators were not, of course, able to tell what those 13 patients were thinking, just that some cells in their brains were active and doing … something. “Minimal consciousness” is an optimistic term; the PET scans may have been detecting only the play of random signals across neural wires that had long since failed to relay coherent thought. One year later, four of those 13 patients with detectable brain activity had died, and the remaining nine had either stayed “minimally conscious” or recovered “a higher level of consciousness,” though the authors did not elaborate on what that meant.

This is an interesting finding, but it also complicates how doctors might approach the end of life. Families and clinicians could wonder if all patients in a coma should have a PET scan, or whether patients whose scans show residual brain activity should be kept alive longer than they would have been otherwise. What we don’t know is the most important thing: whether any of the patients in the Lancet study regained the kind of lives they would find meaningful.

In a similar vein, an emerging resuscitation technology called extracorporeal membrane oxygenation, or ECMO, is complicating not how doctors pinpoint a time of death, but how and when we admit that death is inevitable. ECMO is a portable machine the size of a microwave oven that pulls blood out of the body, bubbles oxygen into it, and then circulates the oxygen-rich blood back into patients whose hearts or lungs have failed. Used by a growing number of hospitals around the world, ECMO can keep patients alive in the intensive-care unit while they await surgery or recover from a cardiac arrest, and has been credited with some miraculous saves. Recently, for example, a Canadian hospital reported that 47 percent of patients who received ECMO after in-hospital cardiac arrest had a good neurological outcome, an unheard-of success rate.

But as more and more institutions adopt ECMO, they will also have to confront the ethical issues that come with it. ECMO can sustain life for weeks and even months, but not indefinitely. Eventually, the synthetic tubing that connects the ECMO machine to the body can get clogged or infected, or obstruct blood flow. That means the technology is best used as a bridge to more definitive fixes like heart transplantation. But if that eventual fix becomes impossible—for example, because the patient is too ill—then doctors may need to admit ECMO is a bridge to nowhere and let the patient go. There is no formal stopping point for ECMO, which means it can be up to the physician to decide which day is the right last day for a given patient. Moreover, unlike most patients on life support, some ECMO patients can be awake and interactive. That means physicians may need to look a conscious patient in the eye and tell them the time has come. Once the ECMO machine is turned off, death will likely come within minutes.