Dilated cardiomyopathy (DCM)
The heart muscle stretches thin and the chambers balloon, so each beat pushes less blood — mostly a large-breed dog disease, with a diet-linked form under investigation.
Where hypertrophic disease thickens the wall inward, dilated disease stretches it outward. The heart gets larger and weaker at the same time, and the body compensates quietly until it cannot, at which point fluid backs up into the lungs and the dog is suddenly breathless.
In several large breeds it is straightforwardly inherited. The complication is a form seen in dogs eating certain grain-free, legume-heavy diets, which the US Food and Drug Administration began investigating in 2018; the mechanism is still not settled, and some cases improve on a diet change. Cats developed DCM from taurine-deficient food until manufacturers began supplementing it.