Some clues that can help us understand when a pig's respiratory problems are worsening to the point that oxygen may be needed:
1.) respiratory rate, or number of breaths per minute. In a pig there is a HUGE range of "normal" rate quoted in the literature (40-150 breaths per minute) so it just confuses things. Get used to his rate of his breathing at rest by watching him and NOTICE when it gets faster and stays that way for hours- that is cause for concern and going back to the vet.
For what it is worth, in my own experience I am looking at all my pigs now who are quiet resting or eating and they are all hovering around 70-80 breaths per minute. I had a pig on oxygen at the hospital a few months ago and everyday his respiratory rate was around 110-130 at rest.
Look for change, look for increased respiratory rate when the pig is at rest to show you things are getting worse and oxygen could be needed.
2.) respiratory effort, or how hard he is working to breath. You can figure this out by watching his sides move in and out as he breaths. A pig in respiratory distress will have deeper indentations /more movement where the sides are moving in and out with each breath. That is a warning sign that they are working too hard to breath, if you see that breathing movement increase. That is a very good indicator that a pig may need oxygen quickly- if they are working too hard to breath the body eventually gets exhausted, can't work that hard anymore and so the respiratory rate slows down->pig doesn't get enough oxygen in blood to survive ->pig codes and you find him passed away in pen.
3.) color of mucus membranes- this is just plain hard for most people who aren't used to looking to do. Medical/veterinary people who are used to assessing this can look at the pigs lips/tongue/mouth lining and see that it is more pale or less pink than usual. But many people have a hard time with this if they haven't done it before and been taught how. If you see your guinea pig with the inside of his mouth looking white with very little pink- THIS IS AN EMERGENCY. he is in respiratory and distress and could die in minutes if not put on oxygen. If you suspect you are seeing less pink inside his mouth as you check him a few times a day (and I suggest you do check him a few times a day this way) then he needs to go back to the vet.
Pink mucous membranes in pig (crazy tooths, I know, ignore)
Pale whitish OXYGEN EMERGENCY mucous membranes in dog:
A pig staying overnight or a few days at the vets in an oxygen tent is not as expensive as some might fear. If money is an issue call the vet ahead to find out the cost so you have a plan in place, but in my experience the fee for oxygen therapy is as little as $40-$80 a day. It is all the other stuff they may want to do (mainly numerous chest X-rays) that gets expensive fast. It is better to take an oxygen requiring pig to the vet and get him on the oxygen, even if you can't afford all the other things they want to do- oxygen is life or death- do not hesitate to bring him because you may have to tell the vet no to a bunch of expensive tests. If getting him oxygen is the best you can do, DO IT or he will die.
So just a heads up- I have seen oxygen save 5 different pig's lives. Sometimes a guinea pig just needs that extra help while we are waiting for the antibiotics to kick in. These are herd and prey animals so they have evolved to REALLY hide signs of illness which makes things hard for us owners. But if you start looking at these three things every time you check on your pig (respiratory rate, respiratory effort, mucous membrane pinkness) you have a really good chance of catching a decline in his ability to breath on his own and should be able to get to the vet's in time.