I hope I can help you understand it a little better. I will certainly try. Normally I would suggest you talk to the doctor for these answers, but obviously that's not possible (and I won't go into how wrong that is, since we both already know

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Think of when you've had surgery. The doctor and anesthesiologist should have explained to you that there are possible complications to the procedure. Most of them are quite rare, and normally the benefits of a surgery far outweigh the potential risks. Most people do very well. A very rare few will be the unlucky ones who suffer the serious complications. Think of it as winning the lottery only it's a prize you want no part of. What I mean is, lots and lots (and lots) of people buy lottery tickets. Only a few win money. Lots and lots and lots (hundreds of thousands) of people have pneumonia - only a few develop ARDS, and usually there are other risks, such as the patient is very young or very old, or has a chronic illness. There do not have to be underlying problems, but it's very rare for there not to be. Does that make sense? It is possible for an otherwise healthy person to get pneumonia, get sicker and sicker, require ventilatory support, and develop ARDS. It's incredibly unlikely however. The vast majority of people with pneumonia are successfully treated and recover fully. Only a small percentage even require ICU care. Even then, most don't develop ARDS. It's something that happens when the body is very stressed and just can't cope with it well. ARDS is a type of respiratory failure. It's a horrible complication that no one is really sure why happens. Obviously I have no idea why it happened to your dd's friend, but it's likely that no one else does either. It's just a very sad thing. What I CAN tell you for sure, is that there is NO WAY your dd could have known. Whatever made her friend sick in the first place is what contributed to her going into respiratory failure and developing ARDS. Doctors can't even tell when or to whom it will happen. Everyone can do everything right and it can still happen. I can't do anything to help your dd's grief over the loss of her dear friend, but I hope I can remove any idea of guilt - there is nothing she could have done. Some patients just "decompensate" and no one can tell which ones they're going to be, until it happens, and then we do the best we can to stablilze them, but sometimes it just isn't possible. Another person with the exact problem that your dd's friend had might recover. It's impossible to tell why she suffered the tragic complications that she did. Everyone's an individual and reacts to things differently.
Sorry this is long, I just wanted to try to explain it a bit better...that no one could have forseen this complication, it is something that happened to her after she was in ICU, and your dd did everything she could to help her friend. And yes, ARDS can cause the other organ systems to fail. They are all intricately related. If the lungs fail, it affects the kidneys, the heart is affected as blood going to the lungs is congested in the right side of the heart, etc. When more than 3 organ systems fail, recovery is almost impossible. For example, a patient with kidney failure can be put on dialysis and stabilized, but if the heart and liver also fail, it becomes a much more complicated issue.
I have to go now, but I wanted to reassure you of two things - Frist- all those times you or your family were sick it is incredibly unlikely that you would have developed ARDS. It's just as possible that your kidneys could have failed or that you could have suffered a rare allergic reaction to a medication. Worry about things you have control over -Wear your seatbelt, don't smoke, eat healthy, etc. You can't do anything to prevent a rare complication. Which brings me to my second thing, and I said it already, but I will say it again there is NOTHING your dd or anyone could have done to know things would progress so quickly and so critically. Help your dd to remember the happy times with her friend. Maybe she can make a donation in her name to a charity or other thing that would bring her comfort. I can't imagine how stressful it was for her togo through not only the illness and death of her friend, but also the other problems with the family that you shared. It would be worth looking into grief counseling, either through your church/temple/place of worship, or through the hospital. Where I work, there are grief conuseling services for parents, siblings and children who have experienced a tragic loss. Grief is a normal process but it still hurts, and there's nothing wrong with getting help to go through it. Sometimes just talking to others who've been though the smae thing is helpful. No one else can really understand what it feels like. I will continue to keep your dd in my prayers.
Please feel free to PM me if I can try to explain it better or you have more questions.
Laurie