Is it possible to survive a heat stroke without medical care?

No: True heat stroke is a life-thratening event in which body temperature rises to abnormally high levels plus organ failure begins. Rescue requires rapid body temperature reduction, (intravenous) hydration, and sometimes specific medications. It is doubtful that this is survivable without care. Heat syncope is a milder condition. More easily resolved.

Heat Stroke!: You may survive getting shot without medical care- but why try? Heat stroke is the most serious form of heat injury and is considered a medical emergency. If you suspect that someone has heat stroke -- also known as sunstroke -- call an ambulance immediately and give first aid until paramedics arrive. http://www.webmd.com/a-to-z-guides/heat-stroke-symptoms-and-treatment#1