Veno-venous ECMO circuit, with drainage from a cannula inserted in the femoral vein (tip in the inferior vena cava) and the return cannula inserted in the internal jugular vein (tip in the superior vena cava, next to the right atrium).
A pump is required to move the blood through the circuit and across the membrane. The returned blood is mixed with the venous blood and then continues as normal (i.e. from vein to right heart to lungs to left heart to systemic circulation). If the proportion of blood going through the ECMO circuit is increased while the patient’s cardiac output remains the same, a greater proportion of ECMO blood will bring a higher concentration of oxygen (O2) to the right side of the heart. If the cardiac output increases while the ECMO blood flow remains the same, the proportion of oxygenated blood that arrives in the right side of the heart will be decreased. Analysis of blood gases in an arterial sample (obtained from the patient) will give the end result of the ECMO blood mixed with the patient’s blood, which has passed through the patient’s own lungs.
When the blood is taken from a vein and returned via an artery, the system is known as veno-arterial ECMO. The return cannula can be inserted in a peripheral artery (Figures 3.2 and 3.3) or the aorta (Figure 3.4). Alternatively, drainage can be from a cannula inserted in a peripheral vein with the return cannula inserted through the chest into the aorta (Figure 3.5).
Veno-arterial ECMO circuit, with drainage from a cannula inserted in the femoral vein (tip in the inferior vena cava) and the return cannula inserted in the femoral artery.
Veno-arterial ECMO circuit, with drainage from a cannula inserted in the femoral vein (tip in the inferior vena cava) and the return cannula inserted in the subclavian artery.
Veno-arterial ECMO circuit, with drainage from a cannula inserted in the right atrium and the return cannula inserted into the aorta, through an open chest.
Veno-arterial ECMO circuit, with drainage from a cannula inserted in the femoral vein (tip in the inferior vena cava) and the return cannula inserted into the aorta, through an open chest.
In the absence of a pump, the blood would flow in the opposite direction, driven by the patient’s own blood pressure. If the pump generates a higher pressure than the patient’s own, the blood will go from vein to artery and bypass the function of the heart. This would introduce a shunt with the injection of venous blood (non-oxygenated) straight into the arterial system. An oxygenator is indispensable to add O2 into the returned blood. This system can then support a failed heart (by providing the pump support) or a failed lung (by providing the required gas exchange), or both a failed heart and lungs. Of note, the system can pump in line with the normal circulation (such as when a return cannula is inserted in the ascending aorta; Figure 3.4) or pump against the normal circulation (such as when a return cannula is inserted in the femoral artery; Figure 3.2).
Analysis of blood gases in an arterial sample (obtained from the patient) may lead to misinterpretation, as the sampled blood could be coming from the ECMO circuit only, the patient’s own circulation only, or both.
From these two basic approaches, a combination of drainage and access can be configured, including return in both an artery and vein. The chosen configuration will determine what support is provided, hence the mixed (and confusing) terminology of cardiac ECMO, respiratory ECMO, veno-venous ECMO, veno-arterial ECMO, veno-veno-arterial ECMO, etc. We prefer to refer only to the support being provided, rather than the type of ECMO, using cardiac ECMO when supporting the heart and lungs, and respiratory ECMO when supporting gas exchange.
Examples of the various configurations are shown in Figures 3.6, 3.7, 3.8 and 3.9.
Veno-venous ECMO circuit, with drainage from a cannula inserted in the jugular vein and the return cannula inserted in the femoral vein (tip in the superior vena cava, next to the right atrium).
Veno-venous ECMO circuit, with drainage from two cannulas, each inserted in one of the femoral veins (one long cannula with the tip in the inferior vena cava and one short cannula with the tip in the iliac vein) and the return cannula inserted in the internal jugular vein (tip in the superior vena cava, next to the right atrium).
In the absence of a pump, a veno-arterial approach will become arterio-venous with the patient’s own blood pressure driving the blood through the oxygenator. This equates to introducing a new vascular bed through which part of the blood is diverted (blood will be pumped through the liver, kidneys, gut, skin and the ECMO circuit). Gas exchange will happen in the oxygenator.
In terms of circuitry, veno-venous and veno-arterial ECMO are identical and this will be discussed further in this chapter. Arterio-venous circuits are discussed in Chapter 13.
The principle components of the ECMO circuit include the cannula (discussed in Chapter 6), tubing, blood pump, oxygenator and heater/cooler (all discussed in this chapter). A diagram illustrating the circuit is shown in Figure 3.10.
Components of the ECMO circuit (except the cannula)
Tubing
The tubing comprises the pipes connecting the various elements of the ECMO circuit. Blood flows through them and they are joined to other components, such as the cannula and oxygenator. ECMO centres may have slightly differing configurations of tubing, but the basic principle is similar.
The tubing is transparent to allow clinicians to observe the blood colour and detect the accumulation of a thrombus. The tubing should be as short as possible but long enough not to impede the patient’s movement. Shorter tubing allows for less priming volume and decreases exposure of the blood to foreign surfaces and heat loss. Patient movements include passive mobilization (e.g. transport to the CT scanner) or active exercise (e.g. a patient on a fixed bike). Modifying the length of the circuit is possible but dangerous, as cutting the tubing can lead to air entrainment, blood loss, thrombosis or infection, as well as subsequent circuit rupture or disconnection.
The majority of tubing is made of polyvinyl chloride. The tubing is often heparin coated to improve biocompatibility, reducing the risk of thrombosis and a systemic inflammatory response as the blood is exposed to foreign material. The search for a more biocompatible material is ongoing, and different types of coating are being tested.
By convention, and to allow sufficient blood flow, adult tubing has an internal diameter of 3/8 inch.
The tubes can have side ports (Figure 3.11) to allow access for blood sampling or connection of other circuits, such as continuous renal replacement therapy. The side ports can also be used to give drugs or fluids.
Illustration of the side ports that can be added to an ECMO circuit.
Connections and divisions are areas of increased blood turbulence, increasing haemolysis and thrombus formation.
The high pressures on the arterial (return or after the pump) side of the ECMO circuit limit the use of access ports on this side. Rapid blood loss will occur if inadvertently opened.
The negative pressures generated on the venous side (drainage from the patient with negative pressure generated by the pump) means that air can be entrained into the circuit when accessing a port on this side. Air can cause pump malfunction or even return to the patient. Clear guidelines and careful handling of these ports are required. Protocols should be in place to manage inadvertent air entrainment, and qualified staff should be trained to deal with these situations (practising it repeatedly to be ready for the rare times it happens).
A bridge between the venous and the arterial lines (Figure 3.12) allows recirculation of blood within the ECMO circuit (note: many clinicians wrongly assume initially that the recirculation of blood is on the patient side).
The bridge could, in theory, be used during weaning of a patient from ECMO, maintaining high blood flow through the oxygenator. This is a dangerous manoeuvre, as the blood flow will be lower in some components of the circuit, such as the cannula, leading to the formation of a thrombus. A bridge can be used to recirculate blood while removing air, a technique that can prove life-saving when air has suddenly been entrained into the system. When not in use, the bridge is filled with priming fluid. Bridge tubing not in use should not be left full of stagnant blood.
Blood pump
Blood flow through the ECMO circuit is driven by a pump. Centrifugal pumps are now preferred to roller pumps as they cause less haemolysis and require less anticoagulation. This book assumes you will only use centrifugal pumps.
Centrifugal pumps operate by creating a fluid vortex formed by a rapidly spinning impeller. The impeller is magnetically levitated or spins on a small bearing (Figure 3.13). Magnetically levitated centrifugal pumps require no direct contact between the impeller and the pump housing, eliminating particle or heat generation. This reduces haemolysis and thrombus formation, and has a lower risk of mechanical failure.
Centrifugal pumps operate by creating a fluid vortex formed by a rapidly spinning impeller. The impeller is magnetically levitated or spins on a small bearing.
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