The drugs prescribed were to treat the cough. The vet performed heart worm test and white blood cell count there in office and said the white blood cells were fine and no heart worms. The vet said if the cough is still around a week from now, and/or Bailey doesn't seem to be feeling any better he wants to do an ECG. He explained Bailey might just have a naturally slow heart rate, and it might be decreased because he is a little under the weather. Bailey has always been an athletic dog, so like you stated lvoss, that might be the case. The vet spent a lot of time listening to his chest, said there was no murmur or swooshing sounds, which was good.
Truthfully I am unsure of his heart rate at previous visits. It was never brought up as a concern before.
What concerns me most is how sudden his symptoms came about. He is getting older, but this wasn't gradual. Within a week the coughing got bad and he was tired and out of breath only 1/3 of the way into his normal walk.