Temperature and flow rate limit the optimal ex-vivo perfusion of the heart - an experimental study

BACKGROUND: Ex-vivo heart perfusion can be utilized to study a variety of physiologic and molecular pathways in a controlled system outside of the body. It can also be used in clinical settings such as for organ preservation before transplantation. Myocardial oxygen consumption (MVO(2)) correlates with energy production in the myocardium and can also be used to determine the balance between the oxygen supply and demand of the perfused heart. This study sought to determine an ex-vivo perfusion rate that matches the metabolic demands of the heart according to different temperatures and solution compositions (with and without the addition of erythrocytes), a flow below which the supply of oxygen is not sufficient to maintain an aerobic state of the perfused heart (“D(CRIT)”). METHODS: Under general anesthesia, rat hearts were procured and preserved by perfusing with the University of Wisconsin Belzer machine perfusion system (UW Belzer MPS) solution saturated with 100% O(2). The key elements of this solution include supraphysiological potassium (to stop the heartbeat and reduce the cellular metabolic demand), starch, gluconate and mannitol (to maintain cell wall integrity), glucose (to sustain basal metabolism), and glutathione (to scavenge free radicals). Three groups of rat hearts (n = 7) were randomly allocated to be perfused at 15 °C, 22 °C or 37 °C, at a varying flow index (FI) starting from a minimum of 380 mL/min/100 g to less than 50 mL/min/100 g, decreasing by 50 mL/min/100 g at 10 min intervals while measuring the MVO(2) at each FI. Lactate was measured from coronary sinus samples to determine the onset of tissue hypoxia/anaerobic state. RESULTS: The D(CRIT) at 15 °C was 99.9 ± 4.9 mL/min/100 g; however, at 22 °C and 37 °C we could not reach a D(CRIT). The myocardial oxygen demand could not be met at 22 °C and 37 °C with the maximum FI above 380 mL/min/100 g even when erythrocytes (10% V/V) were added to the solution. At 15 °C, the production of lactate was evident only below the D(CRIT), while at 22 °C lactate production was present at all flow indices. CONCLUSIONS: Determining the D(CRIT) for optimal ex-vivo perfusion of the heart is necessary to ensure adequate tissue oxygenation and limit anaerobic state. Temperatures employed above 15 °C limit the efficient ex-vivo perfusion preservation of heart with the UW Belzer MPS solution.