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The working of heart is considered as the capacity to pump the blood to the body and is calculated as Ejection fraction
of the Left Ventricle(as it is responsible to supply blood to the other parts of body.The ejection fraction can reduce due to diseases like cardiomyopathy;myocardial infacrt(heart attack
);as a result of some specific viral infections;infective endocarditis
.etc.If the ejection fraction is around or above 60% then you are fine and can do any sort of work but if it reduces below 40% then the person cannot work upto full capacity like he will become breathless while climbing stairs easily;cannot walk long distance due to weak heart muscles.So if the heart is working 35% then it is becoming weak and the person will get easily tired and breathless even after doing work which otherwise would have been easy for him.