**4. ECMO cannulation and management**

The procedure for the institution and management of VA-ECMO support for bridge to transplant is a sterile surgical procedure and requires intravascular access with drainage and inflow cannula. This intravascular access may be achieved via the Seldinger technique or via a cut down to the target vessels. Before the commencement of ECMO, ACT should be confirmed to be greater than 200 seconds. The oxygen line is connected to the oxygenator and the gas flow set at about 5–6 L/ min. The sterile loop is handed to the cannulating physician; this is cut between clamps, and the connections of the tubing to the canulae are made to the exclusion of air. Flows are adjusted to maintain mean arterial pressure and adequate arterial oxygenation. Baseline ventilator requirements and anticoagulation sampling times are determined. The lines are secured as required using anchoring sutures and Opsite® (Smith and Nephew). Typical ventilatory setting may include FiO2 less than 70%, positive inspiratory pressure less than 35cmH2O, PEEP of <10, and respiratory rate < 10. Typical flow rates for a VA-ECMO would be in the range of 2.1 to 2.4 L/min/M2. Prolonged flow rates less than 2 liters should be avoided as this predisposes to thrombosis. Pre-membrane pressures should be kept below 300 mmHg to avoid catastrophic line accidents. Target transmembrane gradient should be less than 50 mmHg. The circuit should be changed if transmembrane gradient is above 150 mmHg as this may be due to clot formation and/or fibrin accumulation in the oxygenator. The ECMO pumps are almost exclusively centrifugal pumps and consist of a rotor that drives blood on rotation. Contemporary oxygenators are commonly Quadrox® (Getinge) or Nautilus® (Medtronic) oxygenators. The Quadrox® (Getinge) oxygenator uses hollow fiber membranes for gas exchange and have a compact design that integrates the heat exchanger with the oxygenator. The Nautilus® (Medtronic) oxygenator uses hollow fiber or flat sheet membranes for gas exchange and has a modular design with separate

heat exchanger and oxygenator. Understanding these designs will assist healthcare providers customize the set up to individual patient needs.

Signs of limb mal perfusion as well as differential perfusion as seen in Harlequin syndrome should be monitored for and documented. Harlequin syndrome results from blood flow through the heart that is not oxygenated, because of lung pathology, being ejected to upper body. The oxygenated blood which is delivered back to body through a groin cannula meets the unoxygenated blood near the distal arch. Coronary and brain perfusion may be compromised. It is imperative to monitor PaO2/ SaO2 in right upper extremity. For ECMO-Impella combination therapy, the cannulation strategy involves a standard femoral or central cannulation for ECMO while the Impella device may be deployed through the femoral or subclavian artery. Typically, the Impella is deployed on the contralateral side of the ECMO cannulation site.

Other parameters monitored include temperature with a goal of maintenance of normothermia using the heater - cooler. ACT/ aPTT guided titration of heparin dosing should be done 6-hourly for the first 24 hours. Following the exclusion of reactionary hemorrhage, ACT may be lowered from commencement levels of above 200 to about 150 if platelet counts are stable. Measurement every six hours aPTT with a target of 40 to 80 seconds should be used to monitor patient subsequently. Direct thrombin inhibitors such as Bivalirudin may be used in cases of heparin resistance requiring frequent antithrombin III replacement or heparin-induced thrombocytopenia. Serum lactate, renal function hepatic function and daily wake up assessment should be monitored according to institutional protocols.

While VA ECMO improves systemic flow and unloads the right ventricle it does increase LV afterload. The LV is subjected to significant increases in afterload from blood being pumped backwards toward the aortic valve. This may lead to a loss of pulsatility, aortic valve closure, and if aortic insufficiency is present LV distension and transmural ischemia and increasing LVEDP and eventually pulmonary edema. Decreased aortic valve opening, LV distention and stasis can cause LV cavity thrombosis and subsequent strokes and peripheral artery embolic events. Careful monitoring for left-sided distension is required using chest x-rays, attention to ventilator plateau pressures and oxygenation needs, as well as assessment of pulmonary artery pressures using a pulmonary catheter, and frequent echocardiographic assessment. Early treatment with inotropes may be sufficient, however with continued LV distension, further interventions may be necessary. Intra-aortic balloon counter pulsation reduces after load and may be easily inserted in ICU. The placement of a LV axial flow pump, (Impella ® (Abiomed)) is an excellent way to achieve decompression. This may be done in the Cath lab or the OR under fluoroscopic guidance. If a patient has prosthetic aortic valve, percutaneous pulmonary artery vents can be placed or surgical vents which are less desirable because of the incision with bleeding consequences. Prevention, rather than treatment, of these complications should be the rule and routine use of LV unloading early during an ECMO run with an LV axial flow pump is strongly encouraged.
