Lung Transplantation: The Operation
The Lung Donor
Matching donor and recipient involves matching blood group and donor size, and avoiding any incompatible HLA antigens that might result in hyperacute or early humoral rejection. Size is very important, particularly avoiding putting large lungs into small chests, which will result in pulmonary collapse and infection.
Lung assessment differs between DCD and DBD donors.
Lung retrieval from donors following brain death
In DBD donors, where the heart is still beating, the donor undergoes bronchoscopy before retrieval surgery commences to look for evidence of infection or inflammation; bronchial aspirates are sent for Gram stain and culture to inform choice of antibiotics in the recipient. Once the operation begins the lungs are inspected externally and care is taken to ensure that all segments are fully inflated, with no evidence of atelectasis, consolidation, masses or trauma. Pulmonary vein oxygen levels are measured by aspirating blood directly from left and right upper and lower pulmonary veins. A PO2 >40 kPa is desirable.
The lungs are preserved by perfusing a low-potassium/dextran preservation solution (Perfadex), together with a prostaglandin vasodilator, via the pulmonary artery, with the lungs ventilated to aid distribution of perfusate. Following this, additional retrograde perfusion is given via the pulmonary veins to wash out clots. This may also perfuse the bronchial arteries, which arise directly from the descending thoracic aorta.
Lung donation from donors following circulatory death Retrieving lungs from DCD donors is different. Pre-operative bronchoscopy cannot be performed. Instead, once death is confirmed the donor is re-intubated and the lungs are inflated with oxygen; at this point they are no longer ischaemic. Ideally, a nasogastric tube is placed prior to treatment withdrawal and the stomach emptied to prevent reflux of gastric contents entering the lungs at the time of death.
Increasingly lungs are being placed on an ex vivo lung perfusion (EVLP) preservation machine, which circulates preservation fluid (Steen solution) through the vessels at 37°C while the lungs are insufflated with oxygen. On this apparatus the lung can be carefully evaluated, with pulmonary venous sampling to check alveolar function and bronchoscopy. The EVLP device also permits longer storage periods and may recondition lungs, allowing previously unsuitable organs to be transplanted.
Most recipients undergo bilateral lung transplant, sometimes referred to as the sequential single lung transplant. Anastomoses of artery, bronchus and a cuff of left atrium are performed at the lung hilum. Removal of all the infected material is clearly manda- tory for patients with septic lung disease. Those with pulmonary vascular disease benefit from receiving the larger vascular bed of two lungs. Patients with chronic obstructive pulmonary disease (COPD) are also best served with a bilateral lung transplant, so the single lung procedure is largely restricted to those with restric- tive or fibrotic conditions.
Traditionally, lung transplantation has been performed with the patient on cardiopulmonary bypass. This offers the advantage of haemodynamic stability as the mediastinum is being manipulated, and may allow the vascular anastomoses to be performed without clamps. The disadvantages of bypass are that it requires systemic heparinsation (which can pose significant bleeding problems if infection and inflammation has caused the lungs to adhere to the parietal pleura), renal impairment, platelet dysfunction and the use of blood products. The current trend is away from routine use of cardiopulmonary bypass. However, it is indicated if single lung ventilation causes significant hypoxia, if clamping the pulmonary artery or manipulating the mediastinum cause instability, or in patients already on extra-corporeal membrane oxygenator (ECMO) support at the time of transplant.
Single and bilateral lung transplantation
Single lung transplantation is usually performed through a posterolateral thoracotomy, particularly if cardiopulmonary bypass is not required. Bilateral lung transplantation is usually performed through a transverse incision in the fourth interspace with division of the sternum, termed a clamshell incision. This gives access to the hilar of both lungs, as well as to the heart if cardiopulmonary bypass is required. The lungs are transplanted sequentially, with the poorest functioning lung replaced first. A double-lumen endotracheal tube is placed to allow separate ventilation of each lung. In patients without pleural adhesions, there is an increasing emphasis on smaller, separate anterior thoracotomies.
The operative procedure involves dissecting the pulmonary arteries and veins free from surrounding tissue, and isolating the bronchus. Care is taken to avoid damage to the phrenic and vagal nerves. The pulmonary arteries and then the veins are ligated and divided, following which the bronchus is divided and the lung removed. In septic lung disease, such as cystic fibrosis, pneumonectomy can be a tedious and bloody affair, but good haemostasis is important, because once the new lung is in place the posterior chest wall will not be visible.
With the new lung in the hemithorax the bronchial anastomosis is completed first. Following this the pulmonary artery anastomosis is fashioned and finally a clamp is placed across the left atrium to include the origins of both pulmonary veins; these are opened as a branch patch (see Chapter 35) and a donor left atrial cuff is sewn to the recipient patch. The lung is then reperfused slowly. The procedure is repeated on the opposite side.
Reperfusion of the lung is performed keeping the artery pressures low (<20 mmHg). The initial blood returning from the transplanted lung to the heart is cold, full of ischaemic metabolites and may contain air, which causes embolism, particularly in the right coronary artery that lies most anterior. Myocardial instability at this stage is possible.
Post-operative analgesia is important, and a thoracic epidural should be routine after the full clamshell incision.
In heart–lung transplantation the organs are transplanted as a single bloc of tissue through a median sternotomy incision. The phrenic and vagal nerves are more at risk and extra care is taken to avoid damage to them; the recurrent laryngeal nerve may also be damaged as the pulmonary artery is dissected off the aorta in the region of the ligamentum arteriosum. In patients with congenital heart disease, large collateral vessels in the mediastinum make dissection more difficult. The airway anastomosis is between donor and recipient trachea; the other anastomoses are to superior vena cava, inferior vena cava and aorta.