Answer:
The central question about DNR/don't DNR seems to be, what level of qualify of life, no matter how short, is desireable in the face of the level of pain, overall short and long term damage, and cost.
Here's the situation: ACLS or advanced cardiac life support is generally a brutal process, and the results are not as good as you'd expect. For a young, healthy person with no history of cardiac disease, in a cardiac care unit (CCU) -- the best case scenario, both short and long term survival odds are pretty good, and ACLS is almost never skipped. However, in elderly patients with a history of cardiac disease, that code in the field away from teh hospital, the survival rate from starting ACLS to patient released from hospital is as low as 5%.
With anyone, ACLS is brutal even if it's properly carried out. You need to compress the chest to about 1.5 inches. In the young, this can cause broken ribs, cardiac damage, liver damage, and it's wildly painful -- it has to be. In the elderly, who have fragile bones, ACLS often crushes most of the bones in the chest. Breathing tubes need to be inserted. IV lines are started and a battery of medications are delivered, some quite harsh. Defibrillation -- the conversion of a cardiac arrythmia -- may be delivered. It's not all that common to survive initial CPR, much less get released from hospital in good shape. And -- again most often with the elderly, the odds of a recurrence are quite high.
Finally ACLS is all about restoring oxygen flow to the brain. If this doesn't happen very quickly, impairment inevitably occurs. This may range from something very mild up to a completely vegetative state, or of course death. Note that in a vegetative state, where the body is already mostly dysfunctional and doesn't need as much to survive, the patient can remain alive, hooked up to very expensive equipment, sometimes for years. With an often hopeless prognosis, this can easily and quite literally bankrupt most families.
Considering the pain, cost, loss of function, and shortness of time before a recurrance, many opt for a DNR.
On the other hand, as the prognosis gets less grave, the more the argument swings to no DNR. If there's a good chance, for instance, of a full recovery, and many happy years ahead, why not go for it? The trouble of course is finding a happy medium.
Signing a DNR is always a hard thing to do, and requires a LOT of consideration. However, it can be the last, final kindness one person owes another.