I think another factor in whether a certain treatment is considered “ordinary” or “extraordinary”, is that the same treatment could have a totally different risk/benefit profile, based on who you are; your age, your medical conditions, etc.
Some debilitated patients aren’t even offered the option of open heart surgery, because the chance of the dying from the surgery itself, is much higher than the chance the surgery will help them at all.
Many people who have DNR orders, are those who have some kind of terminal condition such as end-stage cancer, that means they’re probably (1) not going to survive CPR anyway, or (2) may survive, but in a very impaired condition, dependent on mechanical ventilation for the rest of their lives – and AFAIK, mechanical ventilation IS considered “extraordinary” care by the Church.
On the other hand, I suspect most medical providers, even secular ones, would be very uncomfortable with following a DNR/DNI order on, say, an 18 year old with asthma who may need to be on ventilator for 2 or 3 days during a bad attack, but who are probably going to be fine after that.
Also, note that even people who DO refuse mechanical ventilation often make an exception for general anesthesia, because that is usually given through the same kind of tube used to hook people up to ventilators.
ETA: I’d also note that I actually know people who work for Catholic hospitals, and sometimes treat Jehovah’s Witnesses. Such patients are allowed to refuse blood transfusions, though some hospitals do require them to sign a form that releases the hospital from responsibility for anything bad that happens if they don’t get it, including death. (Some people also refuse blood from non-religious grounds too, some people are still afraid of getting HIV from blood, for example.)
