How does ECMO work? The essential components of ECMO include pump, to work as a heart, oxygenator, to work as lung, heater, to keep body warm, and there is some cannula, circuit, monitor, gas flow meter, and etc. ECMO usually nowadays we using the centrifugal pump as a heart. So the centrifugal pump will suck the dark blood into the pump and push the blood into the oxygenator. Before the pump, the pressure is negative so the dark blood from the body will suck into the pump, and after the pump, it will generate a pressure around 200 milli mercury. So the blood will pump into the oxygenator, and in the oxygenator, the blood will have a gas exchange to be oxygenated by oxygen. So the dark blood will become red blood in the oxygenator and perfuse back to the body to provide the body oxygenated blood. There are two types of ECMO mode. One is VA ECMO, VenoArterial ECMO. The other one is VV ECMO, VenoVenous ECMO. What is the VA ECMO mode? In VA ECMO mode, we drain dark blood from vein, usually from femoral vein or from superior vena cava. After oxygenation in the artificial lung, we perfuse the red blood into artery, usually using the common femoral artery in the leg. So it can work both lung and the heart. The figure, we can see that in the normal physiology of circulation, the systemic circulation, the dark blood comes from superior vena cava and inferior vena cava drains into right heart and then the right heart pumps the blood through pulmonary artery, goes into lung and have oxygenation, have gas exchange in the lung. After oxygenation, after gas exchange, the red blood will go into the left heart through the pulmonary vein, and the left heart will pump the red blood into the aorta and provide the whole body circulation, nutrition, and oxygenation. The VA ECMO drained the dark blood from superior vena cava or femur artery, the inferior vena cava. To let the dark blood goes into pump and oxygenation so the blood where it change in the artificial lung, instead of in the native lung. The blood in oxygenation will become red blood and using a cannula in common femur artery, perfuse the red blood back to the aorta, to support the body, to give the patient red blood, to give the patient oxygenation. The VA ECMO, the pump works as heart. It can support cardiac output and let the heart rest. The oxygenator works as lung. It can increase blood oxygenation saturation, and it can also wash out blood carbon dioxide. We can lower ventilator setting, prevent ventilator induced secondary lung injury by using the VA ECMO in the patients. The other type of ECMO mode is VV ECMO. We drain the dark blood from vein. After oxygenation, we perfuse the red blood into vein. We pump in the red blood to systemic circulation by native heart instead of the ECMO pump. We can see the speakers indicate the systemic circulations, the dark blood drains into the superior vena cava and the inferior vena cava goes into the right side of the heart, and we use the cannula in the vein to drain the blood out the body and goes into the VV ECMO oxygenation and the pump. In the oxygenation, the blood become the red blood, and we perfuse the red blood into the same side, the right heart. Now, the right heart will fill up with red blood instead of dark blood. The red blood in the right heart will pump to the pulmonary artery by the native right heart and goes into the lung, and usually lung is a diseased lung. The native lung already lost the oxygenation or loss the gas exchanged function. But because the blood already oxygenated in the ECMO, so we let the red blood goes through the lung and through the pulmonary vein and back to left side of the heart. In the left heart, the blood is still the red blood. Using the patient's native-heart function, the patient can pump the right heart through the aorta and to support the systemic circulation. This is VV ECMO. In VV ECMO, we usually tune three flows. The first flow is sweep flow, which means the flow rate of medical air, or sometimes we like to say medical gas into the oxygenator. The higher the sweep flow, the better the function of CO_2 washout. The second is a gas, FiO_2. The FiO_2 means the percentage of O_2 into the oxygenator. The higher the FiO_2, the better the O_2 saturation in oxygenator. The third flow is pump flow. The pump flow means the flow rate of blood through the oxygenator. Higher the pump flow, that means the more blood through the oxygenator. The indication of VV ECMO, we use the VV ECMO for lung disease only, because it need the native healthy heart to pump the blood to whole body. The purpose of VV ECMO is that we hope by using the VV ECMO, we can decrease the ventilator setting, we can lower the inspirator to lower the FiO_2 of mechanical ventilation, to lower the PEEP as possible. By decreasing the ventilator setting, we can prevent ventilator-induced lung injury, such as barotrauma. The barotrauma means the lung during the high pressure of mechanical ventilation, sometimes the mechanical ventilator will break down the lung. The function of the VV ECMO, we can provide gas exchange in the venous side in the right heart. The red blood goes into the right heart and pass through the lung and back into the left heart. By using the native heart, we can pump the red blood to body. It is function like CVVH, the continuous venovenous hemofiltration. The VV ECMO pump the blood by native heart only, but we can have more blood flow through the oxygenator in VV ECMO. When compared with VA ECMO, it is more efficient for CO_2 removal. Sometimes we're called the function the ECCO_2R, extracorporeal CO_2 removal and the more efficient for O_2 saturation. The VV ECMO is indicated for patients having bad lung, but good heart.
