Pre-existing conditions are usually defined as any medical condition(illness or accident) that the insured is being treated for and/or is on medication for before the policy becomes effective. It typically deals with chronic conditions like: high blood pressure, diabetes, etc.
A heart attack may or may not be due to a pre-existing condition. It is up to the attending physician to determine whether a condition is considered a pre-existing condition or not. If the attending physician determines that a problem that started after the policy became effective was due to a pre-existing condition, then most of the plans will not cover it.
If you are interested in pre-existing condition coverage, and the visitor is below the age of 70, then you can purchase the Atlas America Plan at Free Quote: Atlas Travel
It will not cover for regular supplies and testing that you need for the management of the pre-existing condition. However, this plan covers up to $100,000 (updated August 11, 2014) for an acute onset of a pre-existing condition.
You can find more information on the plan in the Plan Brochure http://www.visitorscoverage.com/brochure/atlas-america-insurance-brochure.pdf
You can also compare and purchase any of the other visitors plans at http://www.visitorscoverage.com/visitor-insurance/
Insurance rates are regulated by law. You can't find the same insurance plan for a lower price anywhere else