Dogs develop heartworm disease when bitten by an infected mosquito carrying the parasite. Untreated, heartworms eventually invade a dog's heart and his pulmonary artery. By the time heartworm disease progresses to stage 4, also known as class 4, the dog is close to death. While emergency surgery can remove some worms, many dogs still succumb to the disease.
Vena Cava Syndrome
Dogs suffering from stage 3 heart worm disease experience breathing difficulties and constant coughing. An affected dog has likely exhibited serious weight loss. When he reaches stage 4, he displays vena cava syndrome, in which he goes into shock and collapses. The urine of such dogs is dark brown. An emergency vet might try to save the dog via a "heartworm embolectomy," or removal of worms from the heart through the jugular vein. If the dog survives, he can receive long-term treatment to eradicate the remaining heartworms, along with medication for his cardiac issues.