Been a while since I've been here but I only recently discovered shih tzus have a predisposition for heat problems, stress and stroke. I have Zippy, he's a yorkie colored and headed shih tzu basically. I've had him almost 3 years and he hadn't had an issue with outdoor temperatures until this summer.
We were doing our normal mushroom hunting thing when he began crackling while breathing, a little water, rest then continue. It started back up, crackling, we're deep in the woods, lots more happened and cut to the end of the story. Ended the trip 2 days early and got him home. Researched the issue and essentially, as I understand it, the soft pallet at the back of the mouth swells from the irritation and drops into the airway. Shih tzus have this and other predispositions that makes it tougher on them when it gets hot out. If your yorkie is mixed with shih tzu, it's a good idea to research it. Zippy will miss out on outdoor excursions when the temps rise from here on out, sadly.
I had read much better articles when it happened but the beginning of this article covers the basics, you can find the other articles on your own:
Shih Tzu Summer Heat Issues and Care Guidelines