Extracorporeal Membrane Oxygenation for Cardiac Support

Circuit Components

Technology of ECMO circuits (heparin-coated), pumps, and oxygenators have been improved considerably during recent years. The ECMO system consists of four main components: (1) tubing and cannulas; (2) pump; (3) gas exchanger (artificial lung); (4) heat exchanger. This main system commonly is completed by pressure monitoring sensors, accesses for therapy infusion, hemofiltration circuit, user interface displaying the main treatment parameters. Circuit setup is tailored to the patient depending on body weight, clinical purposes, and institutional protocols.

The venous cannula (drainage side) and blood tubing are the major flow-limiting components through the circuit. In line with the Poiseuille's equation, the shorter and larger (internal diameter) is the cannula placed in the right atrium, the higher is the blood flow into the circuit. In the adult setting, vascular cannulation usually is accomplished percutaneously using the Seldinger technique and ultrasound guidance, whereas the surgical vessel exposure is more common in the pediatric population. However, cannulation techniques vary depending on the type of support needed, patient's age and size, and clinical situation. In VA-ECMO for adults, the femoral vessels are usually the first choice; only in particular conditions (e.g., burns or significant peripheral vascular disease) could the axillary artery be used alternatively as an inlet line. Small shunting catheters generally are placed distally to the arterial femoral cannula to avoid limb ischemia (a not-uncommon complication before shunting technique application); alternatively, the femoral artery can be approached by anastomosing a PTFE vascular graft (end-to-side) to create a bidirectional flow. Additional inflow cannulation into the right internal jugular vein has been proposed to guarantee the adequate oxygenation of the upper body.

Roller or centrifugal pumps are the main systems currently used for ECMO circuits. The roller pumps have been replaced almost totally by the centrifugal ones for routine ECMO assistance because of the application of nonocclusive propeller that reduces the trauma on the tubing system and of smaller priming volumes. Centrifugal pumps consist of an impeller arranged with either vanes or a nest of smooth plastic cones inside a plastic holder. The impeller couples magnetically with a small and light motor. The impeller spins at 2000 to 5000 revolutions per minute (RPM), creating a constrained vortex that draws blood into the pumphead and drives it out toward the gas-exchanger. The magnet inside the disposable pump head creates a centrifugal force that contributes to blood circulation (i.e., a negative pressure at the inlet port of the pump pulling blood into the pump holder and a positive pressure at the outlet port). Blood stagnation and heating in the pump, thrombus formation, cavitation (i.e., air bubbles in the blood), and hemolysis are rare but potential complications that should be considered during the treatment with centrifugal pumps. Because centrifugal pumps are preload and afterload dependent, the generated blood flow decreases if blood volume coming from the draining vein is reduced and/or post-pump pressure increases. Pulsatile flow is recently available for VA assistance, potentially allowing a better perfusion than with continuous nonpulsatile flow. For similar reasons, the intraaortic balloon pump has been tested while VA-ECMO is applied.

Modern oxygenators are manufactured as hollow fiber microporous membrane lungs, significantly more efficient, easier to prime than the older silicon rubber membranes. Plasma leakage may be associated with the use of this oxygenator, and it has been addressed by coating the fibers with a very thin skin of gas permeable membrane. Less platelet and plasma protein consumption, more effective gas exchange, lower resistance to blood flow (facilitating the use of centrifugal pumps), and minimal priming volumes are advantages associated with hollow-fiber oxygenator. An air–oxygen mixture flow, delivered to the oxygenator, maintains the diffusion gradients for O ₂ supplementation and CO ₂ removal across the membrane. The gas flow rate must be set to obtain the desired pCO ₂ of the postoxygenator blood while the FIO ₂ determines its pO ₂ .

Patient's body temperature is maintained by hot water circulating in the heat exchanger of the ECMO circuit that puts the circuit and the water bath in contact. Thanks to the heat exchanger, therapeutic hypothermia and/or controlled temperature and rewarming are possible during VA-ECMO treatment.

During VA-ECMO, if left atrial pressure is elevated (e.g., in case of failing left ventricle), a transseptal catheter (vent) can be placed to drain the left atrium, or the left ventricle can be directly drained from the apex, via a left minithoracotomy.

During VA-ECMO, the patient's arterial O ₂ content depends on the blood coming from the oxygenator (highly oxygenated and well decarboxylated), the blood coming from the native lung (potentially badly oxygenated and decarboxylated), and the site of sampling. Oxygenation obtained by means of ECMO oxygenator mainly depends on (1) technical characteristics of the membrane (geometry, thickness of the blood film, materials, red blood cells transit time and FiO ₂ ), (2) oxygen saturation in the ECMO drainage cannula (coming from the patients), and (3) hemoglobin concentration and blood flow in the ECMO circuit. Depending on the organ's dysfunction, the ventilator usually is set in protective modality (low pressure, low volume, low FiO ₂ ) while sweep gas and oxygen supply to the ECMO oxygenator are targeted to the desired arterial CO ₂ and O ₂ .

An adequate anticoagulation is necessary during ECMO because the different biomaterials and plastics can induce thrombosis. However, as current generation circuits and oxygenators are heparin coated, or coated with a biocompatible materials, bleeding complications are more frequent and serious than thrombosis. Heparin should be titrated to obtain 40 to 55 seconds for activated partial thromboplastin or 1.2 to 1.8 times normal; alternatively 1.5 times normal activated clotting time can be considered.

The ECMO circuit should be monitored several times daily by the medical and nursing team and at least once a day by a perfusion technician or other ECMO specialists for fibrin deposits and clots. Moreover, the circuit should be evaluated periodically for appropriate functioning and cannulation site for signs of inflammation and infection. Similarly, particular attention should be paid for patients monitoring during ECMO. In particular, perfusion pressure, arterial pulsatility, lactate levels, urine output, and creatinine levels should be checked carefully. Oxygen saturation of the superior vena cava, together with common indicators of tissue perfusion and oxygenation (e.g., lactates, urine output), are indicators of oxygen delivery/oxygen consumption ratio adequacy during VA-ECMO.