Heart function

My husband is 34 yrs old. He was born with Transposition of the Great Arteries and a VSD. Luckily for him he had the VSD because that is what kept him alive. He went through several bandings. and then corrective surgery at 16. Now he is in heart faiilure. His last echo showed decreased heart function. His right side was already failing now his left side is starting too also. He also has Secondary Pulmonary Hypertension. So he already has SOB and fatigue. He is on Coreg 25 mg 2x a day, along with Enalipril  and Lanoxin. The next step for him is a heart lung transplant. With all this said.. When would they consider him for the transplant list? His heart function is at 45 %. They said normal would be anything above 55 %. He has been on the Coreg now for 6 years. His O2 sats are decreasing as before corrective surgery was 65% and after was in mid 90's. Now it's in the 80"s at rest and 70's upon exertion. There is nothing that can be done about his lungs and being on oxygen does not help his O2  it only changes slightly. When should we begin to worry?

Thanks