Why is it necessary for a patient to be connected to heart-lung machine during double lung transplants?

Poor lung function: Double lung transplants when done "en block" obviously need the heart lung machine. When done asynchronously require bilateral anterior thoracotomies, and one lung anesthesia. If lung function too poor to allow one lung anesthesia then en block is the only option. The transplant surgeon is the only one that can judge which technique is appropiate to his patient.