Why Does My Dog Breath Heavy While Resting?

You should mention in your question about the age and breed of your dog. Heavy breathing at resting in dogs can be due to oxygen deficiency and pain. Pain can be due to injuries, infections, wounds, arthritis etc. Oxygen deficiency can be due to

1. Pulmonary edema or fluid in lungs
2. Heart diseases
3. Pneumonia
4. Lung diseases
5. Obstruction of the air passage

So, exact cause of heavy breathing can be diagnosed vet after physical examination and some laboratory works. Take your dog to vet.

It would have been better if you had informed about age and breed of your dog. Anyhow, heavy breathing in dogs without any physical activity can be due to many reasons. Most common reasons of heavy breathing in dogs are

1. Heart diseases
2. Pain
3. Lungs diseases
4. Heartworm
5. Trauma
6. Pneumonia
7. Fluid in lungs
8. Bleeding in lungs
9. Obstruction of air passage