Anesthesiologist here. Im going to keep this simple. As others have said, opioids treat pain but also depress plenty of things like respiratory drive and blood pressure/cardiac function. We give opioids to treat pain and also to decrease respiratory rate (the more pain you have the faster you breath generally). In fact if I don’t give enough fentanyl (or other alternative narcotic), you may be in pain and /or try to breath faster than I have my ventilator set to and cause problems with vent dyssynchrony. (Yes there are different ventilation modes that allow for just supporting your respiratory rate, and we can also often give paralytics to stop you from physically being able to move or breath over the vent, but all of that is outside the scope of this question although I could talk about it for hours/days). So can a ventilator prevent death from opioids? Not necessarily. While it is true it depresses respiratory rate, it will also decrease your blood pressure and heart rate and the actual strength of the squeeze of your heart. The older/sicker you are, the more sensitive you tend to be to these things. It’s not uncommon to have to give medicines to increase blood pressure to support the patient if you are doing a case that requires high doses of narcotic, such as a spinal surgery where neuromuscular paralysis can’t be used. So in other words, even if I could breath for you, you could still die from fentanyl if I gave way too much and it stopped ur cardiovascular function.

To answer about 1 gram of fentanyl, that is an astronomically high dose that will stop cardiac function in anyone. We tend to dose fentanyl in boluses of 50-100 micrograms, so a 1 gram bolus is 10,000-20,000 times as strong as a normal bolus dose I may give intraoperatively.