Why

Lung transplantation, or pulmonary transplantation is a surgical procedure in which a patient's diseased lungs are partially or totally replaced by lungs which come from a donor. While lung transplants carry certain associated risks, they can also extend life expectancy and enhance the quality of life for end-stage pulmonary patients.

Double-lung

A double-lung transplant, also known as a bilateral transplant, can be executed either sequentially, en bloc, or simultaneously. Sequential is more common than en bloc.^[2] This is effectively like having two separate single-lung transplants done.
The transplantation process starts after the donor lungs are inspected and the decision to transplant has been made. An incision is then made from under the patient's armpit, around to the sternum, and then back towards the other armpit; this is known as a clamshell incision. In the case of a sequential transplant the recipients lung with the poorest lung functions is collapsed, the blood vessels tied off, and cut at the corresponding bronchi. The new lung is then placed and the blood vessels reattached. To make sure the lung is satisfactory before transplanting the other a bronchoscopy is performed. When the surgeons are satisfied with the performance of the new lung, surgery on the second lung will proceed. In 10% to 20% of double-lung transplants the patient is hooked up to a heart-lung machine which pumps blood for the body and supplies fresh oxygen.

To read more http://en.wikipedia.org/wiki/Lung_transplantation