Lung Transplantation: The
Operation
The Lung Donor
Donor selection
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.
Lung Transplantation
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.
Cardiopulmonary bypass
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.
Heart–lung transplantation
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.
