Cat breathing with open mouth
There are many possible causes of open-mouth breathing or other obvious respiratory distress, common to all of these is that they are of critical condition. A cat should never breathe with its mouth open. Dogs are often seen breathing with their mouths open, this is because they regulate their bodies by releasing heat, cats do not do this.
When a cat that is breathing with its mouth open comes to the vet, the cat is often admitted to the intensive care unit for oxygen therapy, attempts tostabilisation and support to facilitate breathing so that chest x-rays and other diagnostics and procedures can be performed. The heart is examined, as well as whether there is fluid in the chest cavity which needs to be drained and the reason why this has occurred investigated.
Pay attention to differences in your cat's breathing patterns! If you notice that your cat is having trouble breathing in any way - you should urgently go to the nearest on-call animal hospital or clinic.