Let me share my experience. My dog is shihtzu. 14 years old . 2 years ago. he has difficulty breathing. He go upstair , try to breath and then fall down. I took him to vet for oxygen. Vet listen to heart and said it might be heart problem .When come back home, he try to breath again. So I go to vet that special on heart. They X-ray, blood test,echocardiogram,and electrocardiography. It concludes that my dog has heart problem. Enlarge,arrhythmia, valvular heart disease. Now he take 4 meds. Vetmedin, furosic, anapril,and digoxin. He live up until now.