Thank you for the information. There are a few different things that could be going on with your dog.
1) Heart disease
. Although your dog did not have a murmur, heart disease does not always come with a murmur. Radiographs and echocardiograms (ultrasound of the heart) can help to see if the heart is of abnormal size or if there is abnormal flow thru the heart. If the heart is enlarged, indicating heart disease, this pushes upward on the trachea. This can cause coughing. Also if the heart is pumping insufficiently, the blood begins to back up. Fluid begins to build up in the lungs and wheezing and coughing can be heard. Exercise intolerance is also seen because the dog cannot catch its breath.
2) Hypertension. If your dog's blood pressure is high, it certainly can cause an increased respiratory rate, exercise intolerance, and coughing. Your vet can simply take your dog's blood pressure to make sure that your dog is not having hypertension.
3) Cancer. It is possible your dog has a mass hiding somewhere. Being an older dog, some elevations in liver enzymes can be normal. But if they are significantly higher, it may indicate liver disease or even a mass lurking in the liver. An ultrasound can be done to look for any suspicious areas, if any are found a biopsy can be done. Certainly there can be chest masses too, or lung masses which might be accounting for her panting, coughing and wheezing. Xrays could easily rule this out.
4) Cushings disease. This is an endocrine disease. It is a dysfunction of either the pituitary or adrenal glands. Basically too much cortisol, a steroid, is produced in an animals body. This can cause exercise intolerance, constant panting, pot bellied appearance with upper body muscle
wasting, increase drinking and urinating
. Alot of signs are progressive over the years. Sometimes on bloodwork, you can see a few telltale signs that maybe further diagnostic testing should be done. A liver enzyme called alkaline phosphatase (ALKPH), cholesterol, and glucose may all be elevated.
So where do you go now? I think ruling out heart disease might be wise. Chest xrays with blood pressure might be the best place to start. I would consult with your vet to see he would be willing to do this.
I hope this helps! Good luck with your dog! I hope he feels better soon!