Ashok Muniappan
INDICATIONS/CONTRAINDICATIONS
Single lung transplantation is suitable for the two most common causes of end-stage lung disease, chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis (IPF). The first successful single lung transplant was performed for IPF in 1983 at Toronto General Hospital. Initially, patients with COPD were considered to be poor candidates for single lung transplantation, as it was thought that preferential ventilation of the native emphysematous lung would complicate allograft function. This concern was overstated and only a few years later single lung transplantation for COPD was proven to be feasible. Single lung transplantation was the most common form of pulmonary transplantation until the late 1990s, when bilateral (or double) lung transplantation became more common. In 2011, the most recent year with available registry data, bilateral lung transplants were performed 3.8 times as often as single lung transplants.
The most significant attribute of single lung transplantation is its ability to maximize donor supply and utilization. Presently, the donor pool for pulmonary allografts is severely constrained, with a severe mismatch in supply and demand. Single lung transplantation affords two recipients an opportunity to undergo transplantation from a single suitable donor, maximizing a valuable resource.
Another attribute of single lung transplantation is that it is technically simpler and associated with less early postoperative morbidity than bilateral lung transplantation, when patients are selected appropriately. This is relevant for the elderly or frail patient, for whom a prolonged anesthetic time results in increased postoperative complications.
Presently, 10% to 15% of patients listed for transplantation die on the waiting list, due to lack of donor availability. Listing patients for both single and bilateral lung transplantation, when appropriate, reduces waitlist deaths. This is especially relevant for a patient with IPF, for whom disease progression can be quite rapid and a prolonged waiting time is to be avoided. A single lung transplant may be lifesaving, if it means more prompt transplantation.
Single lung transplantation is contraindicated in patients with cystic fibrosis or other conditions with suppurative lung disease, such as bronchiectasis. Bilateral pneumonectomy is necessary to prevent contamination of the pulmonary allograft by native lung sepsis. Although there are a few reports of cystic fibrosis patients undergoing single lung transplantation and contralateral pneumonectomy (either synchronous or previously), the vast majority of patients are best served by bilateral lung transplantation.
Severe pulmonary hypertension is another contraindication to single lung transplantation. A single allograft may not provide enough unloading of the strained right heart. Significant right heart dysfunction can complicate recovery from single lung transplantation. In addition, the single lung allograft may be exposed to almost the entire cardiac output when significant pulmonary hypertension remains in the native lung, increasing the risk of primary graft dysfunction. Although it is uncertain what degree of pulmonary hypertension precludes safe single lung transplantation, a mean pulmonary artery (PA) pressure greater than 50 mm Hg is often cited as a contraindication.
Extended donor criteria including age >55 and smoking history >20 pack-years are increasingly used to expand the donor pool. There is some uncertainty as to the appropriateness of using marginal lungs in single lung transplantation. Both early and late outcomes may be compromised when a single marginal lung is transplanted. There are possibly interactions with the recipient’s diagnosis and condition as well and it remains to be seen how extended criteria lungs function in both single and bilateral transplant settings.
PREOPERATIVE PLANNING
Beyond routine preoperative assessment, a potential single lung recipient requires careful cardiovascular evaluation. Severe pulmonary hypertension, as revealed on echocardiography and right heart catheterization, is a contraindication to single lung transplantation. Moreover, evidence of significant left or right heart dysfunction on echocardiography can predict the requirement for cardiopulmonary bypass (CPB) and appropriate cannulation strategy is selected ahead of time. A right-sided transplant may be preferable in a patient expected to require CPB, as cannulation of the aorta and atrium is straightforward after right thoracotomy. CPB is almost never necessary for single lung transplantation in patients with obstructive lung disease and is necessary in about 15% to 25% of patients with fibrotic disease. Regardless, a CPB circuit and perfusionist should be on stand-by for all single lung transplants.
A quantitative ventilation–perfusion (V/Q) scan determines whether there is differential perfusion between the native lungs. If the option is available, the less perfused lung is transplanted, barring any other requirements for laterality (e.g., chest size, pathology in native lung, etc.). When the recipient is suffering from obstructive lung disease, the native left lung is less prone to hyperinflation after right lung transplant. In contrast, patients with fibrotic lung disease may be better suited to left lung transplantation, as the liver is not a fixed impediment to allograft expansion. In spite of these observations, there are no hard and fast rules to guide selection of the side to transplant and reasonable outcomes can be expected regardless of side transplanted.
Donor lung characteristics may also dictate the laterality of single lung transplantation. When pneumonia or pulmonary contusion is severe in one lung, the contralateral lung may still be suitable for transplantation. Thorough radiographic assessment, including liberal use of computed tomography, helps assess donor lungs, especially when extended criteria are used. Bronchoscopy determines whether or not hemorrhage or purulence localizes to one lung. Even with such testing, suitability of a lung may not be determined until after donor sternotomy. Blood gas analysis of pulmonary venous blood tests whether or not one lung has improved function and is performed if there is any question of which side to procure.
SURGERY
Positioning and Incision
Single lung transplantation may be performed through either a posterolateral thoracotomy or a muscle-sparing anteroaxillary incision. While a full lateral decubitus position is required for posterolateral thoracotomy, an anteroaxillary approach is performed with the patient positioned 60 degrees forward from the horizontal axis. For a posterolateral incision, the latissmus is divided and the serratus is retracted and vice versa for an anterolateral or anteroaxillary approach. Exposure and draping of the ipsilateral groin, especially when left-sided transplantation is performed, permits femoral cannulation for CPB. A fourth intercostal space thoracotomy facilitates aortic cannulation and access to the pulmonary hilum.
Technique
Single lung ventilation is established through a left-sided double-lumen endotracheal tube. Significant hypoxemia with single lung ventilation may develop, which may be an indication for CPB. Careful anesthetic management avoids CPB in almost all patients with obstructive lung disease and in the majority of patients with fibrotic disease undergoing single lung transplantation. On occasion, clamping of the PA of the lung to be explanted is performed to improve oxygenation, as this eliminates the shunt through the deflated lung.
Native lung pneumonectomy is performed with careful hilar dissection. Particular attention is paid to preserving the phrenic nerve, as postoperative diaphragmatic paralysis complicates recovery from transplantation. Likewise, the vagus nerve (and recurrent nerve on the left side) should be preserved to minimize the risk of postoperative aspiration. The inferior pulmonary ligament is completely divided and the superior and inferior pulmonary veins are isolated extrapericardially. The PA is isolated and test clamped to determine whether the contralateral lung permits sufficient oxygenation and reasonable hemodynamics. Right and left heart function may severely deteriorate with PA clamping and necessitate CPB. A reduction in cardiac output of 1 to 1.5 L/min with PA clamping suggests that CPB is necessary.
The truncus PA branch is isolated and divided (Fig. 2.1). The PA beyond this branch is also isolated and divided with an articulating endoscopic stapler. This provides more options for constructing the PA anastomosis and reduces size mismatches when recipient pulmonary hypertension leads to PA enlargement. Care is taken to ensure that the PA catheter is not within the PA before stapling. Similarly, the pulmonary veins are divided separately with the endoscopic stapler (Fig. 2.2). With retraction on the pulmonary vein stumps, the pericardium is opened widely and circumferentially, exposing the left atrium.
Figure 2.1 Division of right pulmonary artery. The truncus artery and the ongoing pulmonary artery are divided with an endoscopic stapler. Seen are the divided right superior pulmonary vein and the distal right mainstem bronchus.
Figure 2.2 Preparation of the recipient left atrium. The superior and inferior pulmonary veins are stapled and divided with an endoscopic stapler. A clamp occludes the recipient left atrium beneath the origin of the pulmonary veins. The stapled ends are removed and the orifice of the two veins are connected, creating a cuff of left atrium suitable for anastomosis.
The mainstem bronchus is dissected at its distal aspect. The bronchus is generally divided just proximal to the origin of the upper lobe bronchus. Division requires diligent control of peribronchial lymphatic and bronchial artery branches with suture ligatures or electrocautery. The recipient mainstem bronchus may be trimmed back slightly at this point, taking care not to devascularize the terminal end. The exposure and anatomy is slightly different after right or left pneumonectomy (Figs. 2.3 and 2.4).
Figure 2.3 The right hilum after recipient pneumonectomy. Seen are the divided bronchus, pulmonary artery, superior and inferior pulmonary veins, phrenic nerve, and vagus nerve.
Figure 2.4 The left hlium after recipient pneumonectomy. Seen are the divided bronchus, pulmonary artery, superior and inferior pulmonary veins, phrenic nerve, and aorta.
The donor lung is placed within the chest wrapped in gauze containing crushed ice. There are a number of common anastomotic techniques and sequences, and familiarity and reproducibility should dictate how the implantation is performed. Trimming of the donor bronchus to within one ring of the origin of the upper lobe is recommended to avoid a long ischemic segment of donor mainstem bronchus. First, the bronchial anastomosis is completed with an interrupted anastomotic technique using absorbable 4-0 Vicryl sutures (Fig. 2.5). When possible, the donor bronchus is intussuscepted into the recipient bronchus. Others have recommended performing the bronchial anastomosis with a running absorbable monofilament suture and avoid routine intussusception.
A vascular clamp is placed on the left atrium and the pulmonary vein stumps are opened to create a cuff of recipient atrium. The donor left atrium is anastomosed to the recipient atrium with a running 4-0 polypropylene suture using an everting technique to ensure endothelial apposition (Fig. 2.6). The final stitch is left loose to permit flushing of the donor lung and de-airing prior to release of the left atrial clamp.
Figure 2.5 Bronchial anastomosis. The anastomosis can be performed with an interrupted suture technique, using 4-0 Vicryl, similar to that used for tracheal reconstruction. Alternatively, a running PDS suture technique is used.
Figure 2.6 Left atrium to donor atrial cuff anastomosis. The recipient left atrium is anastomosed to the donor atrial cuff with a running 5-0 Prolene suture. An everting suture technique is used to ensure endothelium to endothelium apposition.
The PA anastomosis is constructed after placing a vascular clamp to control the proximal recipient PA and opening the artery in a location that matches the donor PA best (Fig. 2.7). Care is taken to appropriately trim the donor PA to avoid excessive length that may predispose to kinking when the graft is revascularized. The anastomosis is constructed with a running 5-0 polypropylene suture. Other centers routinely perform bronchial and PA anastomoses before completing the atrial anastomosis.
Ventilation and reperfusion of the implanted lung is performed in a careful and deliberate fashion. Gentle recruitment of the lung is performed before reperfusion. Appropriate immunosuppression should be administered prior to reperfusion. The PA clamp is slowly opened and a controlled and deliberate reperfusion and de-airing is performed. When the graft is sufficiently de-aired, the left atrial clamp is released and the atrial anastomotic suture is tightened. Hemostasis of the anastomoses is ensured. Once appropriate graft function is confirmed, the chest is closed after positioning a 28-French chest tube anteriorly and a 19-French soft flexible catheter posteriorly.
Figure 2.7 Pulmonary artery anastomosis. The donor pulmonary artery anastomosis to the recipient pulmonary artery is performed with a running 5-0 Prolene suture. A vascular clamp occludes the proximal recipient pulmonary artery.
CPB or extracorporeal support may be required at various points in single lung transplantation. Increasingly, patients present for transplantation after bridging with extracorporeal support and familiarity with management of this circuit and how it can be used during transplantation is necessary. CPB may also become necessary when single lung ventilation leads to significant hypoxemia or PA clamping severely compromises hemodynamics. It is preferable to centrally cannulate to avoid groin incisions. On the right, the ascending aorta and right atrium are easily cannulated. On the left, the aortic arch or descending aorta is cannulated and venous drainage is transfemoral or via the pulmonary trunk.
POSTOPERATIVE MANAGEMENT
Early postoperative management of single lung transplantation is focused on optimizing graft function and avoiding complications in the native lung. In patients with obstructive lung disease, positive end-expiratory pressure is minimized (≤5 mm Hg) or eliminated, to minimize risk of native lung hyperinflation. In patients with fibrotic disease and pretransplant pulmonary hypertension, particular attention is paid to the hemodynamics in the early postoperative period. Subtle changes in pulmonary vascular resistance of the lung allograft can have dramatic effects on right heart function and cardiac output. The single lung transplant may be more prone to primary graft dysfunction in fibrotic patients, making it prudent to gently wean from paralytics, sedation, and mechanical ventilator support.
COMPLICATIONS
Native lung hyperinflation is a complication of single lung transplantation, in patients with obstructive lung disease. Hyperinflation of the native lung does not always lead to allograft dysfunction and its effect may only be apparent when the allograft is compromised, as with acute or chronic rejection. Early postoperative difficulty with native lung hyperinflation occurs more often when there are large bullae in the remaining lung. This complication may be avoided by pre-emptively performing lung volume reduction in patients with very large bullae. Occasionally, patients require lung volume reduction of the native lung in the early postoperative period.
Lung allograft dysfunction, characterized by hypoxemia, increased pulmonary vascular resistance, and decreased compliance may be more common after single lung transplantation. It is more likely in patients transplanted for fibrotic lung disease and pre-existing pulmonary hypertension. Although there are no absolute criteria for initiation of extracorporeal support, requirement of high peak inspiratory pressure (>40 cm H2O) and 100% FiO2 are generally recognized indications. Early initiation of extracorporeal support may protect the lung allograft from barotrauma and oxidative damage, and protects the patient from multiorgan dysfunction due to shock. Extracorporeal support has its own set of complications, including increased bleeding associated with anticoagulation, cannula site complications, and stroke. In balance, though, extracorporeal support is protective against permanent graft injury and improves outcomes in patients with severe graft dysfunction.
Additional complications attributable to single lung transplantation include the development of neoplasm or infection in the native lung. The risk of lung cancer developing in the native lung is approximately 10% and is higher in the older recipient with a significant smoking history.
RESULTS
It is widely espoused that bilateral lung transplantation for both IPF and COPD leads to improved long-term survival compared with single lung transplantation. However, the benefit is diminished or absent in elderly patients, in whom bilateral transplantation is associated with increased early morbidity and mortality. When confounding variables are controlled during analysis of retrospective registry data, the benefit of bilateral transplantation disappears. Any potential benefit from bilateral transplantation is more likely in very specific populations, such as young patients with IPF.
CONCLUSIONS
Ongoing donor organ shortage and concern about the morbidity of bilateral transplantation in the elderly make single lung transplantation a relevant technique. Successful single lung transplantation requires expert donor and recipient selection. Although the majority of single lung transplants are performed without CPB, on occasion such support will be necessary for intraoperative management, and requires familiarity with cannulation techniques when operating in either chest. There are specific complications associated with single lung transplantation, including native lung hyperinflation and development of lung cancer in the native lung, which require vigilance and on occasion operative management.
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