With a mask, the oxygen concentration depends on how tight fitting the mask is. We often guesstimate that they allow about 40%. But with a large, tight fitting mask in a sedated dog you might actually get close to 60 or even 80%.Always remember that animals do not understand you are giving them oxygen. They may actually think that the mask is stopping them being able to breathe properly. I reckon that’s why some animals find it so stressful. And never use masks they can’t see through.If the animal starts to struggle DO NOT restrain them to keep their heads in the mask. Go for a less stressful method.By that way, that 200 ml of pus came out of that cat’s chest.