to OP:
Has nobody mentioned positioning yet? FLEXED position, pain inhibition due to abdominal incision, poor basal expansion, been ventilated and anaesthetised in supine – collection of secretions etc, is analgesia correctly controlled? Immediate treatment approach would be get into a decent position to improve expansion, check fluid balance, chat with docs, saline nebs, flirt with nurses, eat chocolates, acbt with supported cough etc etc… re-auscultate, check to see if sats are improving,etc. If they’re really crappy you can try the bird. Then liaise with docs again re: problems and get them to sort out fluid balance and any other issues.