Check out some more interesting information on this case of heartworm disease in a cat.
Heartworm can be a fatal disease in cats. Look at the normal cat x-ray (first photo below). The dark triangle is a normal cat lung field. Then note the difference in the other X-ray (second photo below). The white area is fluid that has collected in the chest. This makes it difficult for the cat to breathe. There is so much fluid that the lungs are actually floating!
Please put your cats on heartworm prevention!