Can you use the open drop / Bell jar method, to administer isoflurane on a cotton gauze and once the mice don't show righting reflex or reaction to toe pinching, plus the breathing has slowed down, you do cardiac perfusion (takes atleast 3-4 mins) and then euthanize the animal.

Is that a correct method to follow? Because I find that the mouse starts hyperventilating once I start the surgery for cardiac perfusion.