Cats often hide illness for a long time and may therefore seem fairly normal until their breathing problems have become significant. The cause can be in the airways or lungs, but fluid around the lungs and heart disease are also important possibilities. These conditions cannot be reliably told apart at home.
A cat that breathes with its mouth open, clearly uses its belly to breathe, sits with its neck stretched out or cannot settle needs help quickly. Avoid holding your cat to count breaths or look in its mouth if it is already struggling to breathe. Place it calmly in the carrier and keep stress and heat to a minimum.
Seek emergency veterinary care for laboured breathing. Call the clinic before you arrive if you can. At the hospital, stabilisation with as little handling as possible is often the priority before more extensive diagnostics are carried out.