Is your uncle taking Lanoxin (heart medication)? Lanoxin has shown to increase the pumping strength. My father had a weak heart beat and after being on Lanoxin for about 10 years now, he's heart beat is strong (very strong per taking his pulse). Ask your uncle's doctor about Lanoxin.
The 20% you refer to is the amount of blood pumped out of the left ventricle into circulation with each heartbeat. Normal is 50 to 70% and medically referred to as ejection fraction (EF).
If the left ventricle is overworked by pumping against high resistance, i.e. high blood pressure, increased volume, etc., the left ventricle enlarges (dilates) to compensate (increase in EF), but if not properly treated the heart will increase to a size that decreases contractility and heart failure ( sometimes a silent heart attack) and that would be an EF below 30%.
Without doubt, if a heart attack, your uncle's heart attack (myocardial infarction) caused some heart cell damage. The location of the cell damage, mild or severe, etc. will reduce to contractility varying degrees of impairment and reduce EF accordingly. Sometimes the cells are not necrotic (dead) but are dormant (medically referred to as hybernating) and can be revitalized with an adequate supply of blood. This would pertain to an ischemic (lack of blood flow) heart attack. Treating the underlying cause, can reduce left ventricle size and restore normal contractility.
Other causes for an MI such arrhythmia, viral heart disease, etc. do not have a very good prognosis.
I had a silent heart attack going on 6 years (EF 13-29%), enlarged heart, etc. Medication, etc. has reduced my heart to normal size and an EF of 59%.