ECMO physiology

Temporary mechanical cardiopulmonary support may be used intraoperatively to facilitate cardiac surgery. Over the past decade, this intervention has been extended for prolonged use in the intensive care unit (ICU) as extracorporeal membrane oxygenation (ECMO). 1 Two types of ECMO exist in clinical practice: veno-arterial (VA) and veno-venous (VV). VV ECMO provides support to the pulmonary system by extracting blood from the right atrium, oxygenating it and returning it to the right atrium, where the patient remains dependent on their own circulation. In VA ECMO, blood is extracted from the right atrium and returned to the arterial system, bypassing the heart and lungs, which provides both respiratory and hemodynamic support. 2 Patients who require ECMO support receive anticoagulation with heparin 3 before the cannulas are inserted, either in the right jugular area or in the left/right femoral area(s). In VV ECMO, blood is drained from the venous cannula placed typically in the inferior vena cava (IVC), and returned through a cannula with its tip at close proximity of the right atrium, either through the femoral vein or through the right internal jugular vein. In VA ECMO, blood is drained from the IVC and returned through right femoral artery or the aorta to the systemic cannulation. 2 ECMO support is initiated when the patient is connected to the ECMO circuit. The deoxygenated blood will be withdrawn from the patient through the access line under negative pressure (usually created by a centrifugal pump) entering the oxygenator (artificial lung membrane), where gas exchange occurs. Blood is then returned back to the patient through the return line. 4 Oxygenated Flow on ECMO machine is controlled across the hollow fiber membrane. The blood flows outside of the fibers and the gas flows through the fibers in a counter current flow pattern. The non-direct contact between blood and gas reduces blood trauma. The gas exchange occurs through diffusion due to the concentration gradient between O 2 and CO 2 . Oxygenated blood is then returned back to the patient through the return line. 4 Pressures within an ECMO circuit are highly important and need to be understood (access, pre-membrane, and post-membrane pressures) to quickly identify potential issues. The transmembrane pressure (TMP) is the difference between the pre- and post-membrane pressures. The TMP baseline should be monitored when ECMO is initiated and be compared with the TMP trend during the ECMO run, as it reflects the function of the circuit membrane. The membrane acts as the blood oxygenator and should be regularly checked for clots. The patient oxygen saturation, temperature, hemoglobin, sedation, preload, and afterload will be continuously monitored. FiO 2 and sweep gas flow is adjusted according to pre- and post-membrane blood gases. Anticoagulation is sustained while the patient is on ECMO and it is monitored through the activated clotting time (ACT). ACT of 180–210 seconds or APTT of 35–45 seconds are accepted. 3 The use of Bioline coating on the internal circuit surfaces improves biocompatibility of extracorporeal circulation system devices: oxygenators, cannulas, connectors, centrifugal pumps, and tubes by mimicking human tissue. It is highly important to understand the possible complications that may occur while patients are on ECMO, whether it is patient related (bleeding, thromboembolism, etc.) or ECMO circuit related. 1 Therefore, all safety devices within an ECMO machine and circuit should be connected properly, functioning well, and checked continuously until the patient is weaned from ECMO. 4