**6.4 Donor heart reanimation and** *ex-vivo* **preservation**

During donor heart instrumentation, the OCS circuit is primed by mixing 1200–1400 ml of donor blood and TransMedics priming solution containing Mannitol and electrolytes. Antibiotics, steroids, albumin and supplements are added to the circuit. Epinephrine, Levothyroxine and the TransMedics maintenance solution containing multivitamins, electrolytes, dextrose, insulin and adenosine are also infused into the system. The aortic cannula is deaired and attached to the circuit with the left atrium and ventricle facing anteriorly. Thirty-four degree-Celsius of oxygenated blood enters and perfuses coronary arteries, returns to coronary sinuses at the right atrium, enters the right ventricle through the tricuspid valve and drains across the pulmonic valve to the pulmonary artery which connected to the pulmonary cannula [35].

After the aortic cannula is attached to the circuit, gentle cardiac massage is usually performed to decompress the heart. At this point, the heart usually starts to beat. If ventricular fibrillation occurs, electrical defibrillation is performed with small defibrillator pads located behind the heart. Biventricular pacing wires are placed, and the ventricular pacing rate is set for VVI at 80 beats per minute. The LV vent is opened and drains into the OCS box connected to the reservoir. When sinus rhythm is achieved, the pulmonary artery cannula is attached to its port [35].

Serial arterial and venous blood gases and lactate are sampled to determine myocardial lactate extraction. A lower venous lactate concentration indicates lactate absorption and reflects adequate myocardial perfusion. Maintenance solution infusion, epinephrine and pump flow are adjusted to keep aortic pressure 65–90 mmHg to maintain coronary flow 650–900 ml/min during transportation. Lactate levels should be absorbing, declining and less than 5 mmol/L before implantation [36, 37].

Once the recipient is ready to implant, the OCS pump is turned off, and cold cardioplegia is delivered to the aortic cannula to achieve rapid electromechanical arrest. The pulmonary artery cannula is disconnected and IVC is opened for drainage to avoid heart distension during the second cardioplegia dosing. After completion of cardioplegia, the aortic cannula is disconnected from the OCS box, and the heart is placed on ice and transferred to the recipient operative field [37].

In March 2023, the Stanford team initiated a new strategy of OCS beating heart transplant without a second cardioplegic arrest to avoid additional ischemic time after heart procurement and to minimize potential ischemic reperfusion injury. This technique can also be applied with DBD heart procurement utilizing OCS. The procedure is performed by perfusing the donor heart with warm blood from the cardiopulmonary bypass circuit via antegrade cardioplegic cannula while the donor heart is still connected to the OCS circuit, then the aortic cross clamp is placed above the antegrade cardioplegia cannula. The donor heart is disconnected from the OCS circuit without interrupting coronary perfusion [36]. The recipient recovered uneventfully with a full return to normal activities at 90 days of follow-up [36].
