Is a cat breathing with its mouth open sick?

There are the following main reasons for cats breathing with their mouths open. Firstly, the weather is hot and the cat opens its mouth to expose its tongue to dissipate heat. Secondly, there may be a respiratory disease causing nasal blockage or lung problems. Thirdly, cats suffering from cardiovascular system disease may show signs of oxygen deprivation and thus open their mouths to breathe.