Heat exhaustion is a condition that is caused by being exposed to high temperatures for a long period of time. It is a heat-related illness and is often accompanied by dehydration. Heat exhaustion may also be known as sunstroke. The main symptoms include a rapid pulse, heavy sweating, both of which are caused by your body overheating after being exposed to high temperatures. Heat exhaustion is preventable. It is estimated that heat exhaustion and heat-related illnesses in general affects millions of people in the United States at some point in their lives.