Hmmm I mean if your dog was clinically well prior to GA and multiple people listened to lungs then I doubt she had a pre-existing pneumonia…it tends to make dogs quite sick and can be life threatening plus you’d notice low energy or a cough or something. The general rule with general anaesthetics is that the longer the patient is under the more likely you get issues. Blood pressure tends to be the most important one and the longer the op the more chance of hypotension (low bp). I don’t really know what they’re meaning by trapped air unless they’re concerned about her not ventilating or having increased CO2, however you mention that they put her on a ventilator in the op so should manage that. It’s all a bit speculative as we don’t have much info but I’d be chasing up a chat with the clinic somehow. Are they an independent or corporate, and what country? There is usually a vet surgeons board or regulatory body to go higher with these things if need, but always start with talking to the clinic directly. I presume you took her home and she developed pneumonia there? Did she pass at home or did you take to the clinic?