Let’s now turn our attention to the membrane itself. We will explore the function of the membrane oxygenator, its limits, and how we can appraise and assess this function at the bedside. Understanding the unique characteristics of the membrane is going to be essential – the better we understand the strengths and limitations of the membrane, the better we will be able to leverage its capabilities when we are treating our patients.
Specifically, we are going to explore the concept of sweep gas flow and how adjustments to the rate of flow lead to changes in CO 2 . Recall the convention that we first introduced in the extracorporeal membrane oxygenation (ECMO) Fundamentals section on how to manipulate and adjust the ECMO circuit.
FiO 2 and ECMO blood flow mainly affect oxygenation/oxygen delivery while sweep gas flow affects ventilation/CO 2 removal
We spent the last chapter exploring the effect of blood flow on oxygen delivery. Now we will answer the following question:
Why does manipulation of sweep gas flow mainly affect CO 2 clearance?
Answering this question will require an exploration of the membrane itself and the ways that sweep gas flow interfaces with the membrane.
What is the basic function of the membrane oxygenator?
Let’s return to our illustration of the membrane at the most basic level. You will recall that blood flows into the membrane, diffusing around plastic tubes that carry gas, which establishes a semi-diffusible barrier between a sweep gas phase and a blood phase ( Fig. 11.1 ).
Much like the alveolar-blood interface, this blood/gas interface does not allow the diffusion of blood components but is diffusible enough to allow oxygen and CO 2 to travel down their concentration gradients. That means that if the FiO 2 is set to 100%, the PaO 2 of the sweep gas flow would be around 713 mmHg while the PaO 2 of the venous blood flowing into the oxygenator is much lower, say around 40 mmHg. Therefore, O 2 flows down its concentration gradient, from an area of high concentration (sweep gas) to an area of low concentration (blood). Even if the blood has a relatively high oxygen content, with a PaO 2 of 55 mmHg or even 60 mmHg, the gradient between sweep gas and blood is so high that oxygen will still diffuse, such that the PaO 2 of the blood will increase.
In a similar manner, CO 2 will move in the opposite direction out of the blood, since it exists at a higher concentration in the venous blood, say around 45 mmHg, versus 0 mmHg in the sweep gas.
With such large gradients existing between oxygen and CO 2 clearance, you can imagine that these gases are able to diffuse at high efficiencies.
Is ECMO more efficient at oxygenation or CO 2 clearance?
The answer here can be surprising. Many think oxygenation – it’s called extracorporeal membrane oxygenation after all. Moreover, the relative gradient between PaO 2 in the sweep gas and in the blood is much greater. But the answer is CO 2 clearance. It’s actually not even close – it is CO 2 clearance by a long shot. Why is that?
Efficiency of the Membrane: Effect of Relative Diffusibility
The main reason is diffusibility. CO 2 is more diffusible across the membrane. How much more diffusible? Six times more! This higher diffusibility allows for higher CO 2 clearance and overtakes the larger gradient that exists for oxygen between the sweep gas and blood.
Let’s reflect on why this occurs. Imagine two lines at a toll booth to a bridge. One line is for cars with an automated pass. A sensor detects their pass and they can drive right through. The second line is for cars that have to pay cash. They have to stop, pay the operator, and receive change before proceeding on. If that first line is 6 times faster, it would not matter how many cars were in the second line; the concentration of cars from the first line on the bridge would be higher.
This is why sweep gas flow is so important to CO 2 clearance. The faster it flows, the more it clears CO 2 since CO 2 diffuses so much more rapidly.
Efficiency of CO 2 Clearance: Effect of Hemoglobin
The other reason that ECMO is so much more efficient at CO 2 clearance than oxygenation comes back to the difference between how oxygen is carried throughout the body and how CO 2 is carried throughout the body. You will no doubt recall from Chapter 1 that the way that oxygen is carried throughout the body is through binding to hemoglobin. This is for good reason – if it wasn’t bound to hemoglobin, it would just diffuse to tissues based on proximity and there would be no efficient distribution throughout the body. Like we have established before – SaO 2 is more important to oxygen delivery than PaO 2 .
CO 2 is different. It does not need to be delivered and distributed evenly like oxygen does. Which means that CO 2 can simply remain dissolved in the blood, represented by PaCO 2 .
Now that we have established these premises, let’s examine the effect of the membrane on oxygenation and CO 2 clearance.
Let’s start with oxygenation. Say you have a normal venous saturation of 70% of the blood entering into the oxygenator. This may correspond to a PaO 2 of 40 mmHg, which is much lower than the PaO 2 of 713 mmHg in the sweep gas, allowing for excellent diffusion of O 2 into the blood, so much so that the PaO 2 of blood leaving the oxygenator is 400 mmHg with a saturation of 100% ( Fig. 11.2 ).
Now let’s compare this to CO 2 . In this case, the PaCO 2 of the blood entering into the oxygenator may be say 45 mmHg. This is significantly higher than the PaCO 2 of the sweep gas flow of 0 mmHg. Thus, the CO 2 molecules diffuse down their concentration gradient and the CO 2 is drawn down to <10 mmHg.
You can now see how CO 2 clearance can also be relatively more significant. Even though in this example, O 2 concentrations are increasing 10 fold and CO 2 concentrations are decreasing 10 fold, the parameter that matters to oxygen delivery of the body only increases by 30% (from 70% to 100%) ( Fig. 11.3 ).
For oxygenation, you are limited by the saturation capacity of hemoglobin, while for CO 2 clearance, no such limit exists limiting PaCO 2 .
Leveraging sweep gas flow rate
The sweep gas flow rate will allow for CO 2 to be swept away at a proportional rate. Therefore, the higher the rate of sweep gas flow, the higher the CO 2 clearance.
This works out in a favorable manner when it comes to how sweep is titrated. It means that the only limit to how high this gas rate can be increased is the capacity of the oxygenator/blender, which is usually 10–15 L/min.
Compare this to the limits of blood flow that we discussed in the prior chapter – everything from anatomic limitations to the limitations of the circuit/oxygenator to the preload dependence/afterload sensitivity of the pump to hemolysis to recirculation can limit blood flow. For sweep, it is a manner of turning a dial and increasing gas flow.
You can see that increasing sweep gas flow rates by 2–3× can easily be done, which cannot always be said about ECMO blood flow.
Does This Mean That Blood Flow Through the Membrane Has No Effect on CO 2 Clearance?
Blood flow actually can have a significant effect on CO 2 clearance. Higher blood flows translate into higher CO 2 clearance to a certain point, in a similar manner to how higher blood flow translates into higher oxygen delivery ( Fig. 11.4 ).
