Physiology of VV-ECMO

The effect of VV-ECMO on oxygenation and CO2 removal is a function of a complex interplay between several factors. During VV-ECMO the ML and the native lung (NL) are in series (Fig. 7.1). We can understand the physiology of VV-ECMO by following the changes in blood gas content from the blood leaving the organs to the arterial blood. Delivery of oxygen and removal of CO2 during VV-ECMO involve different mechanisms and different efficiencies that ultimately result in different ECMO indication possibilities. For this reason, it is useful to discuss separately the effects of VV-ECMO on oxygenation and on CO₂ removal.

Oxygen Delivery During VV-ECMO

During VV-ECMO, the arterial oxygen content (CaO₂) will depend on the O₂ delivering contributions of both the ML and the NL (Fig. 7.1).

A schematic of a model of O delivery and consumption during VV-ECMO

Fig. 7.1 A schematic of a model of O₂ delivery and consumption during VV-ECMO. Part of the blood returning from the body with a low oxygen content (Cv) is diverted to the membrane lung (ML). In the ML lung the blood is fully saturated (Cout) and returned to the rest of the venous return. Cin and Cout are the oxygen content of the blood entering and exiting the ML, respectively. In the presence of recirculation, Cin is higher than Cv. The blood entering the lung (mixed venous blood (Cmv)) is determined by ECMO blood flow (ECBF) relative to patients’ cardiac output (CO). The arterial blood (Ca) is determined by the intrapulmonary shunt (NL shunt) relative to CO. An example is reported with relevant gas analytic values and oxygen content computation. The total oxygen delivery corresponds to total patient’s oxygen consumption (VO₂) and is the sum of the oxygen contribution of the NL (VO_2NL) and of ML (VO_2NL). A more detailed description is provided in the text

Venous blood returns from the peripheral tissues with low oxygen content (CvO₂). The blood pump generates the extracorporeal blood flow (ECBF) diverting part of the venous return, i.e. cardiac output (CO), towards the ML. The ML loads the ECBF with oxygen, raising the oxygen content from the inlet (CinO2) to the outlet (CoutO2) of an amount corresponding to the oxygen delivery of the ML (VO2ml = ECBF * (CoutO2 - CinO2)).

The VO2ML and then the contribution of ML to total oxygen delivery depend on three main factors:

1. The ECBF is the most important factor: VO_2ML will increase directly with the increase in ECBF. However, depending on intrinsic characteristics of the membrane lung, there is a limit to the oxygenator flow (“rated flow”) above which no more O2 can be added to the blood.
2. The O₂ pressure gradient between the extracorporeal sweep gas flow (ECGF) and the inlet blood: on the gas side, the O2 partial pressure depends on the FiO2 of the ECGF. On the blood side, the higher the CinO₂, the lower the quantity of

O₂ that can be added to the blood. If the drainage and the returning cannula are too close or in specific cannula configuration (e.g. drainage from the jugular vein and return in the femoral vein), part of the already oxygenated ECBF is sucked back into the ECMO circuit. This phenomenon is called recirculation. The main effect of recirculation is to increase CinO2 and decrease VO2ML and then the oxygenation efficiency of the system.

3. The intrinsic oxygenation capability of the ML which depends on the membrane diffusion characteristics (thickness, material) and the membrane surface area.
4. The O₂-bonding capability of the blood: It is important to remember that the main determinants of blood O2 content are haemoglobin saturation and haemoglobin concentration. The higher the haemoglobin concentration, the higher the amount of O₂ that can be bounded and then transferred from the ML to the blood.

The oxygenated ECBF is then returned to the venous blood and directed towards the right heart. The oxygen content of the blood returning to the lung (the mixed venous blood, CmvO₂) is the flow-weighted average of CoutO₂ and CvO₂: CmvO₂= [CoutO2*ECBF+ CvO2*(CO - ECBF)]/CO. In practice, the resulting effect of the membrane lung is to increase the O2 content of the blood returning to the lung from CvO2 to CmvO2. With the given CvO2, CmvO2 and then the mixed O2 saturation are directly proportional to ratio between ECBF and CO.

The oxygen contribution of the native lung (VO_2NL = (CaO₂ - CmvO₂)*CO) depends on its residual gas exchange capability which depends on both the severity of the lung disease (i.e. the intrapulmonary shunt fraction) and the ventilator management.

The sum of VO_2NL and VO_2ML, i.e. the total oxygen delivered, corresponds to the total oxygen consumption of the patient (VO₂Tot = VO_2NL + VO_2ML). Consequently, the lower the NL contribution, the higher must be the contribution of the ML and then the oxygen delivery efficiency of the ECMO system. A highly efficient VV-ECMO system, able to deliver a high VO_2ML, requires high ECBF, minimal recirculation and low CinO2.