Part of what they're looking for is likely the chemical signs of a heart attack - if heart muscle is dying off, it's generally detectable in the blood. I suspect part of what they were looking for is BNP: "When your heart is damaged, your body secretes high levels of BNP into your bloodstream to try to ease the strain on your heart." (http://www.mayoclinic.org/diseases-conditions/heart-disease/...)