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Principles
Extracorporeal CO2 removal (ECCO2R) provides support for patients with severe acute respiratory failure characterized by hypercapnia with subsequent respiratory acidosis. It can be used to support a low mechanical ventilation strategy by supporting CO2 clearance.
An ECCO2R circuit consists of tubing allowing the blood to flow through a plastic module containing a diffusion membrane (comprising built-in multiple hollow fibres to increase the surface area). The circuit is usually heparin coated to reduce thrombus formation and is resistant to plasma leakage. The blood flows on one side of the membrane and the sweep gas on the other. Increasing the rate of the sweep gas will increase the amount of CO2 eliminated.
The principle of ECCO2R is based on the fact that lower blood flows are required to exchange CO2 across a diffusion membrane (while maintaining sweep gas flow). The quantity of CO2 that can be exchanged is at least as high as that produced by the metabolism.
Using lower blood flows allows the use of smaller cannulas and decreases the overall risk.
During ECCO2R, O2 will be exchanged if O2 is added in the sweep gas. Oxygen will diffuse across the membrane, with the blood exiting the ECCO2R circuit being oxygenated. This will usually be insignificant to the patient, as this oxygenated blood will immediately be diluted with a higher volume of deoxygenated blood. If oxygenation is sought, higher blood flows are required and this is then ECMO.
Clearing CO2 reduces the ventilation required and therefore decreases ventilator-induced lung injury in the mechanically ventilated patient. The use of ECCO2R can prevent intubation or support liberation from a ventilator.
Using ECCO2R can be a bridge to lung transplantation and can support patients presenting with acute or chronic lung conditions (e.g. pneumonia in the patient with chronic obstructive pulmonary disease).
The low flow used during ECCO2R requires continuous anticoagulation. It is noteworthy that thrombi will develop more rapidly in these systems than in ECMO, due to the lower blood flow.
Devices used for ECCO2R
Arterio-venous ECCO2R
Arterio-venous ECCO2R utilizes the patient’s arterio-venous pressure gradient to pump blood through the membrane. Access is most commonly achieved with a percutaneous cannulation of the femoral artery and vein.
These systems are reliant on the patient’s blood pressure to pump the blood through the circuit, and a mean arterial blood pressure of greater than 60 mmHg is required to achieve flow rates between 0.5 and 1.2 L/min. The resistance to blood flow is low, but build-up from a thrombus in the membrane may progressively increase the resistance to flow, and higher pressures will be required to maintain an adequate flow.
The advantages of this system include the ease of cannula insertion, which can be done in the ICU using ultrasound guidance. In addition, due to the simplicity of the design allowing the membrane lung to be positioned on the bed between the patient’s legs, it is easily transportable.
The inability to mobilize the patient due to femoral arterial cannulation is one disadvantage of using arterio-venous ECCO2R.
The main risk is distal limb ischaemia, which can occur on the side of the arterial cannulation. The possible arterial obstruction by the cannula is compounded by the diversion of blood through the circuit.
Using a pumpless arterio-venous system introduces a new vascular bed to the patient. The heart has to pump blood through the brain, liver, kidneys and other organs and the membrane of the ECCO2R.
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