Girly,
Pre race, the heat factors in for more any hyper type horse that washes out or gets crowd excited and leaves his race in the paddock.
Post race the debilitating/dehydration factor, weight loss will be greater than if it was much cooler. They all use Lasix, except CD?,so the buckets with electrolytes will be ready for them afterwards.
Sight may wish to add the expertise of a vet.
Mike