Exploring the Impact of Blood Components on Myocardial and Vascular Health During Normothermic Ex-situ Heart Perfusion
Creators: Wagner, Mitchell J.
Publisher: name: University of Alberta Library
Year: 2024

<p>Each year, roughly 100,000 Canadians are diagnosed with heart failure, a condition that can only be cured by heart transplantation. In recent years, there has been a plateau of heart transplantation volume, thought to be the result of declining donor sources and the bottleneck of short preservation time of procured organs. Cold storage (CS) is the standard method of preservation for solid organ transplant, but has a limited out of body time to a maximum of 4-6 hours for heart transplants. Normothermic ex-situ heart perfusion (ESHP) preserves the heart in a semi-physiologic state by enabling out-of-body perfusion of the coronary arteries with a blood-based perfusion solution (perfusate) at physiologic temperatures.<br>
While clinical ESHP platforms have shown promise in increasing preservation times, our group and others have documented persistent decline of myocardial function whilst on the apparatus. Our group has established that perfusion of the heart on the apparatus begets an inflammatory and pro-oxidative environment. One source of inflammation could be the breakdown of red blood cells (hemolysis) over time within the blood-based perfusate. Furthermore, the field is yet to investigate optimal levels of hemoglobin, which has significance for the development of optimal perfusion solutions. A current point of contention is whether dilution of whole blood used as a perfusate in clinical scenarios with buffers to fulfill volume requirements leads to inadequate oxygen delivery, and therefore functional decline.<br>
As presented in chapter 2, we tested the hypothesis that greater hemoglobin (Hb) concentrations could mediate increased functional preservation. We subjected hearts of Yorkshire pigs to normothermic ESHP in the working mode for a duration of 12 hours in groups varying in perfusate composition of hemoglobin: control perfusate (low hemoglobin due to dilution of whole blood with buffer, ~4 g/dL Hb), whole blood (no dilution, ~8.5 g/dL Hb), concentrated blood (whole blood concentrated by packed red blood cells obtained through cell saver, ~12 g/dL Hb). We also tested a low hemoglobin group whereby whole blood was cut with donor plasma (‘plasma’ group), rather than manufactured buffer. Overall, we found that functional preservation was worse when hemoglobin concentration was high. Markers of cardiac function including cardiac index, stroke work, left ventricular dP/dT maximum and minimum were significantly reduced in the high-Hb group compared to other groups by late perfusion. This coincided with evidence of increased hemolytic burden during these perfusions, denoted by elevated concentrations of free hemoglobin, perfusate potassium, and heightened transferrin saturation.<br>
Throughout the study conducted in chapter 2, we noticed that coronary flow rates were much lower in the new controls and in the ‘plasma’ group than in runs conducted historically by our group under similar conditions during ESHP. Such supraphysiologic coronary flow rates are recognized as damage endured to the endothelium. In order to avoid hemodilution via fluids provision during heart procurement, the new controls were given minimal fluids whereas historical controls were given a high degree of fluids. This led us to hypothesize that plasma proteins may play an important role in preventing supraphysiologic coronary flow rates during ESHP.<br>
In chapter 3, we explored this hypothesis, comparing historical control ESHP runs given high amounts of crystalloid fluid to newer runs given low amounts of crystalloid fluids. We also compared the runs to the plasma group (provided a low amount of fluids during procurement). We also trialled supplementation of the Krebs-Heinseleit buffer with high amounts of albumin with low fluids provision during procurement. Overall, we found that in groups provided low fluids, coronary flow rates were significantly lower than in the group where high fluids were provided. This was associated with increased damage to the endothelial glycocalyx, which is an endothelial structure implicated in vascular mechanosensation and regulation of vessel vasomotor tone, preceding perfusion.<br>
The work presented in this thesis provides a variety of valuable insights into how the perfusate can be optimized during ESHP to mitigate inflammatory and oxidative stressors, such as free heme. It also preliminarily emphasizes the importance of plasma proteins as a modality to effect improved coronary vascular preservation. These may be important modalities to improve the preservation of hearts subject to prolonged ESHP and mitigate debilitating post-transplant pathology such as cardiac allograft vasculopathy.</p>