Purpose

An increasing number of patients worldwide are awaiting organ transplantation, which is directly leading to an increased organ shortage¹. This warrants new strategies to improve the outcome and donor pool size. Transplantation using organs donated after circulatory death often increase the risk of delayed functional restoration or even lost function of the graft. Especially hearts are often discharged due to a restricted transplantation safety level². Highly sensitive measures for estimation of organ viability is therefore needed to improve the handling procedures of the organs from extraction to insertion in the recipient. This study introduces a novel MRI compatible porcine organ perfusion device for ultra-sensitive quantification, using hyperpolarized and conventional MRI, of the most important metabolic pathways to monitor graft viability of the heart.

Materials and methods

A MRI compatible perfusion chamber holds the organ which is supplied with oxygen and nutrients using approximately 1.2 L heparinized whole blood. A BioMedicus Medtronic Bio Console 540 centrifugal pump (Medtronic, Minneapolis, MN, US) maintains physiological perfusion pressure at 85 ± 5 mmHg. The perfusate is heated and oxygenated using a Medos Hilite 1000 neonatal oxygenator (Xenios, Heilbronn, GE) and a water heater/pump, see Figure 1. Temperature, flow and pressure sensors mounted on the perfusion lines allow for continuous monitoring. A liquid filled latex balloon inserted in the left ventricle allows the heart work and enables monitoring of the heart rate via a pressure transducer, which can be used for cardiac gating during the acquisition of MRI data. Prior to the ex-vivo perfusion MRI experiment, the heart and perfusate is retrieved from fully anesthetized female pigs (40 kg body weight). The heart is arrested using a cold 1:4 mixture of Harefield cardiolpegia solution (Terumo BCT Ltd., Larne, UK) and Ringer Lactate (Fresenius Kabi, Bad Homburg, GE) just prior to the removal of the organ, and subsequently kept cool at 5° C until the start of perfusion. A cannula is applied to the aorta³ and the balloon is inserted into the left ventricle. The heart is perfused at 38° C in reverse, shutting the aortic valve and forcing the perfusate into the coronary vessels, see Figure 2. Glucose (Fresenius Kabi, Bad Homburg, GE) and insulin (Humulin, Eli Lilly Demark A/S, Herlev, DK) is infused continually. The perfusion chamber was placed in a 3.0 T Signa HDx MRI scanner (GE Healthcare) equipped with proton and carbon-13 imaging capability, see Figure 2. Cardiac anatomy was assessed using T₁ weighted FLAIR and T₂ weighted PROPELLER imaging sequences with the following parameters. T₁ FLAIR: TR/TE/TI = 4.4 s/24.2 ms/1.6 s, flip angle = 111°, FOV/matrix = 200×200 mm²/256×256. T₂ PROPELLER: TR/TE = 7.4 s/94.1 ms, flip angle = 142°, FOV/matrix = 200×200 mm²/256×256. Both sequences were used to acquire 15 slices of 4 mm slice thickness in 2 and 4 averages respectively, see Figure 3. CINE imaging was performed to visualize cardiac function: TR/TE = 5.45 ms/2.59 ms, flip angle = 55°, FOV/matrix = 120×120 mm²/512×512 in 30 images throughout the cardiac cycle. ¹³C imaging was performed, following a 9 mL injection of hyperpolarized [1-¹³C]pyruvate, using a spectral spatial (SPSP) imaging sequence with parameters: TR/TE = 0.5 s/1 ms, FOV/matrix = 120×120 mm²/128×128, one long axis slice with 40 mm slice thickness and a 90° or 8° flip angle on lactate/bicarbonate/alanine or pyruvate, respectively. The effective TR for pyruvate and its metabolites was 1 and 3 seconds respectively. Imaging was performed in either long or short axis orientation.

Results

Three porcine hearts (238 ± 43 g) were investigated ex-vivo, using the developed perfusion system. In cases where the heart did not defibrillate spontaneously upon reperfusion, electrical defibrillation was performed. The heart rate was in the range 68-75 BPM. Ex-vivo cardiac perfusion with accurate control of physiological parameters was verified with ¹H MRI and [1-¹³C]pyruvate MRI. Preliminary results from the hyperpolarized experiments are shown in Figure 3 and 4. A predominant lactate production was observed in all the pilot experiments.

Discussion and conclusion

Increased understanding of the role of deranged cardiac metabolism in donor grafts during storage pending transplantation has the potential to increase utilization of marginal organs, thereby combatting organ shortage. This is due to the heightened potential of enhanced therapeutic strategies in the form of novel storage solutions and pharmaceutical interventions, with the effect of preventing the degradation of the and modulation of the graft viability. With further work and analysis, we hope to investigate the metabolism of the ex-vivo porcine heart as well as the effect of pre-conditioning strategies on graft viability. The preliminary results shown here demonstrates the ability to monitor ex-vivo cardiac metabolism and function in a large animal model, resembling the case in humans.

Acknowledgements

No acknowledgement found.