Could be sleep apnea-type thing, but this only happens in the evening and being a greyhound, you know he sleeps all day
I was suspicious of the tramadol too, but if it gets bad, doesn't stop or keeps happening, I give him another tramadol, and that takes care of the panting for the evening. So, if the tramadol was the problem, wouldn't it get worse after another one?
I have a call into my vet, we'll see what she says. I think she will change the meds.