Field Trials Canada

Veterinary Voices UK has a good article on dealing with heat stroke. 

Is it true that if we use cold water on heat stroke pets they will go into shock?

One of the most common things we still hear is that we can only use tepid water on a pet with heat stroke, incase they get some complications like hypothermic overshoot, peripheral vasoconstriction hindering a cooling response, and cardiogenic shock...

We have heard not to use cold water in case it causes shock... this rarely happens!

But guess what? In a recent study over 26% of dogs presented with heat stroke died, with flat faced breeds making up nearly half of heat stroke cases seen in the study.

You should:

• Get someone to call the local veterinary practice and tell them you're going to travel down with a heat stroke patient
• Pour, hose or if possible immerse the pet in very cold water (this should obviously be done under constant supervision, ensuring the head is fully above water and immersion should not be attempted if the animal is too large, or you are unable to do so without hurting yourself) NB: If using a hose pipe, make sure it has run through until cold, as they can often contain water that is extremely hot in the tubing initially
• Do not drape in towels and leave them in situ. Keep the cold water flowing.
• Move to a cool, shaded area
• Prepare to transport to vets in a cold, air conditioned car

In studies they found that:

• International consensus from sports medicine organisations supports treating EHS with early rapid cooling by immersing the casualty in cold water.
• Ice-water immersion has been shown to be highly effective in exertional heat stroke, with a zero fatality rate in large case series of younger, fit patients.
• Hyperthermic individuals were cooled twice as fast by Cold Water Immersion as by passive recovery.

No complications occurred during the treatment of three older patients with severe heat stroke were treated with cold‐water immersion.

Cold water immersion (CWI) is the preferred cooling modality in EHS guidelines and the optimal method applicable to UK Service Personnel

Studies suggest using either ice-water or cold-water immersion

The best intervention is PREVENTION, but if you find yourself with an animal with heat stroke, using cold water either by pouring, hosing or ideally (if safe) immersion then this may help reduce their temperature to safe levels while you transport to a veterinary practice.

Read more here.