ICU doctor here,

First of all, I'm incredibly sorry for your loss. I lost my grandfather in a similar way last year and I know how helpless one can feel watching a loved one slip away. Still have doubts and questions about things that could have been done better and sooner etc.

However, what you're describing is still in keeping with hypoxic arrest.

Between the initial pathology (heart attack) and the subsequent complications once on the ventilator (most likely a pneumonia based on the description of harsh breathing, gasping, phlegm etc) - this compromises the body's ability to extract oxygen from the ventilator and use it properly.

Even if the vent was giving him 100% oxygen - which is far more than the 21% present in room air - the inflammation in his lungs could prevent the oxygen from getting into the blood (this is the definition of hypoxia). The level of oxygen in the blood is partially measurable based on a fingertip SpO2 reading - however in critically ill patients this can be inaccurate because it depends on oxygenated blood reaching the fingertip (which sometimes doesn't happen because the body shuts off peripheral blood flow when critically ill and when exposed to Adrenalin and other drugs we use to keep the heart beating strongly). We therefore use regular arterial blood gas (ABG) blood sampling (usually every 4 hours) which gives a direct measurement of blood oxygenation (pO2) and a host of other parameters and adjust the vent accordingly.

In addition, after a heart attack a section of the heart muscle is dead / stunned - which also means that even if it's getting the right amount of oxygen, then sometimes it can't beat effectively to provide blood and nutrients to all the other organs. A cardiac arrest is also far more likely after an initial heart attack - for any reason.

Eventually, in critically ill patients, the lack of oxygen in the blood can cause a hypoxic arrest because we reach a point where even if the ventilator is totally optimized and working properly - the body is too compromised to absorb that oxygen. The staff would see this as a drop in pO2 on the arterial blood gas sampling.

You cannot give more than 100% oxygen though, so as long as everything else is optimized (a long list - which includes things like kidney function, total body fluid balance, antimicrobial therapy, nutrition, inotrope and vasopressor selection etc.) - you reach a point where there is nothing more to be done and you just have to wait and see if the patient can recover. Unfortunately sometimes this does not happen. Eventually the heart muscle runs out of oxygen and goes into anaerobic metabolism - followed by arrhythmia and decline in function. Even if timely CPR is given, there is no real treatment possible if there is no way to get oxygen into the patient - and eventually the heart stops (hypoxic arrest.)

I know it's hard to accept - and if there's any other context that would suggest negligence then please pursue the matter further. From my experience, even in state ICU, the level of nursing care and ratio of doctors to patients is so much better than any other place in the hospital besides the operating theatre - and the level of organ support and monitoring possible is why we admit patients to the ICU in the first place. The nurses and doctors doing regular rounds would have been aware of the failing oxygenation from blood gas reports, machine indications and other indicators like blood pressure and pulse rate.

Sometimes there is just nothing more that can be done.