Normothermic Machine Perfusion of Kidneys

Kidneys were retrieved with the intention of clinical transplant during standard multiorgan recovery in donor hospitals in the United Kingdom. Organs were preserved with either Marshall hypertonic citrate solution (Soltran Baxter Healthcare, Thetford, UK) or UW solution (Belzer UW CSS, Bridge to Life, Columbia, SC), packed, and shipped according to standard clinical practice. On arrival at the research laboratory, the kidneys were immediately assessed and prepared for connection to the NMP circuit. Back table preparation and priming of the perfusion machine were carried out in parallel. If necessary, accessory arteries were reconstructed during the back table preparation to provide a single inflow. Cannulation of the renal artery and the renal vein was carried out with a 20‐Fr and a 10‐Fr cannula. The ureter was cannulated with a 6‐Fr or an 8‐Fr tube to recirculate the urine. Urine production was continuously monitored by using an inline flow sensor (LD20 Liquid Flow Sensor; Sensirion AG, Stäfa, Switzerland). All cannulas were secured with ligatures; 30‐40 minutes were needed for graft preparation. Back table preparation was performed with kidneys still immersed in ice‐cold preservation solution. Grafts were then flushed with approximately 200 mL of crystalloid Ringer’s lactate at room temperature, before connection to the NMP device.
A diagram and the perfusion device are shown in Figure 1A,B. The system was designed to support kidneys ex vivo for a prolonged preservation period by using perfusion with an oxygenated suspension of packed red blood cells (RBCs) in a colloid, supplemented by nutrients, at normal body temperature. The components of the circuit were sourced from cardiopulmonary bypass suppliers and consisted of a blood reservoir (Capiox venous reservoir, Terumo Medical Corporation, Somerset, NJ), centrifugal blood pump (AFFINITY CP centrifugal blood pump; Medtronic, Minneapolis, MN), membrane oxygenator/heat exchanger Sorin (Lilliput 2 extracorporeal membrane oxygenator, LivaNova PLC, London, UK) and medical‐grade silicone tubing with internal diameters of 1/4 and 3/8 inch. A custom‐built thermoelectric heater and cooler were used to maintain the blood temperature at 37°C. Pressure in the renal artery was measured in‐line with single‐use pressure sensors (PendoTECH sensors; PendoTECH, Princeton, NJ). Hemodynamic control was based on arterial pressure, without any direct adjustment of arterial flow and automatically maintained in the range of 70‐100 mm Hg via continuous adjustment of the centrifugal pump speed between 1300 and 1500 rpm. The flow through the kidney was measured with an external ultrasonic flow sensor (Sonoflow CO.55/080; SonoTec, Halle, Germany). Blood gases were automatically maintained within physiological limits by using a closed‐loop controller, consisting of an oxygen concentrator and air compressor that delivered oxygen flow rates on the order of 10 mL/min and air flow rates on the order of 100 mL/min. Proportional control valves were used to automatically regulate gas flow rates to maintain po₂ between 10 and 26 kPa and pco₂ between 2 and 6 kPa.