/ExVivoLungPerfusion
Lung transplantation is considered an effective treatment for many types of end-stage pulmonary disease, and in some cases, the only treatment option. The main indication for lung transplantation is COPD, accounting for 53% of all single lung transplant cases or 39% of all lung transplants. Donor pools are limited resulting in an average wait time of 18 months. While criteria exist for lung transplant, the evaluation of donor lungs is subjective, based on very little prospective data, and is made more difficult because decisions must be made quickly. The result is 4 out of 5 potential lung donors are rejected. Most transplant surgeons reject lungs with even minor flaws because of the fragile nature of the organs. The main limitations of its application include the limited donor pool, and sub-optimal preservation techniques which have led to various degrees of ischemia-reperfusion injury.
The goal of this project was to develop an ex vivo lung perfusion system, that maintains stable physiologic function, and at physiological flow rates, with has the ability to control ventilation and perfusion. For the development of our system, we adopted methods from lung transplantation and coupled the system techniques with Dual-Source CT (DSCT) imaging and a continuous physiological monitoring system to functionally assess the ex vivo lungs over time.
Breathing lung
I won’t go too deeply into all the methods, since our protocol was a number of pages long. However, the heart-lung block resection was performed with cardiothoracic surgical techniques that are commonly used in lung transplantation. The main difference is the pulmonary vasculature flush used:
Instead of Dextran, the lungs were flushed clear of blood with 2 liters of cold Perfadex (Vitrolife, Gothenburg, Sweden) at a height of 12-20 inches above the lungs to provide continuous flow through the lungs. Perfadex is an extracellular low potassium electrolyte solution developed for the rapid cooling, perfusion and storage of organs in connection with lung transplantation. Each 1,000 ml of Perfadex contains 5% dextran 40, Na+ 138 mmol, K+ 6 mmol, Mg2+ 0.8 mmol, Cl- 142 mmol, SO42- 0.8 mmol, H2PO+ plus HPO42- 0.8 mmol and glucose 5 mmol.
During the flushing, crushed iced saline is placed around the lungs in the pleural cavity to further cool the lung tissue to minimize ischemia injury. The heart-lung block was isolated from the thoracic cavity, the lungs were carefully dissected free from the heart, removed from the thorax, inflated to 25 cm H2O pressure, and the endotracheal tube clamped to hold the airways open.
After lung resection, the left atrium (LA) appendage is cannulated using a cup-shaped cannula with a built-in pressure catheter (Vitrolife, Gothenburg, Sweden) which is sutured to the LA to create a closed circuit. The cupped design of the LA cannula was specially designed to keep the appendage open to create reliable and consistent outflow drainage. The pulmonary artery (PA) was cannulated using a straight cannula, from the same cannula set, with which the anastamosis was secured using 0 silk ties around the main PA. A retrograde flush was then performed with 500 ml of Perfadex. During this flush, perfusate leaks from the LA and PA cannula are checked for and secured as needed. The lungs were cooled and stored until ready for use.
Our Ex Vivo lung system layout can be summed up with the following images:
This is what a "clean" setup looks like in practice:
Donor organ evaluation with Ex Vivo lung perfusion:
Strictly speaking, our lungs must quantitatively demonstrate stability/improvement in function over time.
We are using improvements based on 0 and 4 hour time points, but we
would like to one day be able to keep the sytem at homeostasis for
24 hours.
Our primary measured criteria for the demonstration of
ventilation stability is tidal volume over peak airway pressure (Vt/PawP), as well as dynamic lung compliance (Cdyn) decreasing less than 10% from initial values.
We also look for less than a 10% increase in hemodynamics - as measued by pulmonary vascular resistance (PVR). For our experiments we used tissue volume fraction from quantitative CT imaging as an additional marker for physiological change in the lungs.
Perhaps the most important indicators we look at to maintain is good oxygen exchange (PaO2/FiO2 > 400 mm Hg) and pH (7.3-7.45).
We are able to show that pulmonary blood flow patterns and lung heterogeneity on the ex vivo circuit are comparable to in vivo lung perfusion.
Our experiments have demonstrated that the physiologic function of the ex vivo lungs can be maintained, for at least four hours. Some of our longer experiments demonstrated viability for up to 12 hours, and we would not be surprised if future work demonstrates ex vivo lung sustainability for 24 hours.
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