Heat stroke can affect dogs and cats on warm days, not just the hottest days. Some dogs have particular problems staying cool (Boxers or other short nosed dogs, overweight animals, those with heart or respiratory disease).
Rapid panting can lead to breathing difficulties due to airway obstruction, and also ineffective heat loss, which makes a hot dog even hotter. This is a very serious problem, and needs urgent attention to avoid mortality.
Prevent it in the first place by exercising your dog in the morning or evening when it is cool, providing shade, cool water and a fan if you are indoors. NEVER leave a dog or cat alone in a car on a hot day. They can die in a matter of minutes. Leaving the window open is not enough, the car is a metal box which turns into an oven very quickly. Just don’t risk it.