heart level

my husband had a heart attack 1 1/2 years ago.  He damage one-third of his heart and had two stents put in  his artery.  Since then he has been doing great. last week he had a echo cardiogram and his doctor said that he has a level (I'm not sure what the level is called) of 38, and if it goes down to 30-35 he will need a defribrillator.  My husband is 52 yrs old and is not overweight and exercises every other day.  I don't understand why his level has gone down.  His doctor asked him if his ankles of feet were swelling, which they are not, but I know the doctor is somewhat concern of his level.