Now that we have a little more foundation, let’s return to our circuit, and dive a little deeper into the circuit components that you may come across and how you may interact with them ( Fig. 9.1 ).
Tubing
The tubing is the conduit by which the blood flows through to the various components, namely the pump and oxygenator. Tubing is made of a flexible, clear, plastic that allows visualization of the color of blood as well as any areas of potential turbulence or clot.
Circuits can be built to different specifications, but the standard for adults usually includes tubing with a diameter of 3/8″. This width is wide enough to minimize resistance to flow while not requiring an excessive amount of extracorporeal blood. Some pediatric circuits and low-flow extracorporeal membrane oxygenation (ECMO) circuits may take advantage of smaller tubing such as 1/4″, which allows for less circulating blood but may have some impedance of flow.
The length of the tubing can vary – usually the standard is around 15 feet. You may encounter circuits with longer or shorter tubing. The main advantage of longer tubing is less constraint of having to be close to the circuit, but this should be compared to the disadvantages to include more length to monitor, more possibilities for occlusion, and higher priming volume.
3/8” tubing carries about 20 mL per foot, meaning that a circuit with 15 feet of tubing has a priming volume of 300 mL plus the priming volume of the oxygenator (usually around 250–300 mL). The priming volume is important for several reasons:
- 1.
It represents the dilutional volume when going on ECMO.
- 2.
It represents the amount of blood lost when a circuit is lost or changed.
- 3.
It represents the increase in volume of distribution to be considered for pharmacokinetics and effect of medications.
Pump
We were introduced to the blood pump in Chapter 6 , now let’s go a little deeper into what it involves. Blood pumps were traditionally roller pumps, which milked the blood forward; however, these have been largely replaced by centrifugal pumps in adults.
Although there are many different models of centrifugal pump, the central mechanism is a cone or a fin that spins around in a centrifugal manner. This pump head is connected to a motor that drives this rotation ( Fig. 9.2 ).
The result is a creation of a vortex of blood that is pulled in through the top and pushed out through the bottom as illustrated in Fig. 9.3 .
This pull/push mechanism by which blood enters/exits the pump is an essential part to understanding the flow dynamics related to ECMO, so let’s reflect on it.
Because of the rotational mechanism of the centrifugal pump, any blood going into the pump will be under negative pressure, while any blood leaving the pump will be under positive pressure.
We will go into how this affects the flow of blood in much greater detail in Chapter 10 , when we cover flow dynamics. For now, we can start to appreciate how this phenomenon will affect the flow of blood and how we can approach the circuit.
Essence of Centrifugal Pump: Preload Dependence and Afterload Sensitivity
Because the centrifugal pump generates a negative pressure on the inlet and positive pressure on the outlet, you have preload dependence and afterload sensitivity. This means that the availability of blood going to the pump (preload) is requisite for the pump to work, while impedance of flow (afterload) will result in a decrease in the forward flow of blood.
To better visualize afterload sensitivity, think of a boat that is parked in front of a dock. The propeller can spin as fast as you would like; however, if there is a barrier in front of the boat (the dock in this case), there will be no forward movement. The same is true with the centrifugal pump. If you kink or clamp the outflow line, the pump will keep spinning at the same number of revolutions per minute (RPMs), but the flow will drop off.
This represents a major differentiator from roller pumps, where an occlusion downstream from the pump will only cause pressures to increase, as the forward flow of blood will be relatively fixed.
There are two major implications of this afterload sensitivity, the first being that flow will be impacted by the pressure that exists downstream of the pump. This could be anything that impedes flow such as a clot in the oxygenator, a kink in the return line, a bend in the return cannula, or an elevation in the systemic blood pressure if on VA ECMO. The remedy for this would be to fix any of these impediments to flow or to increase the RPMs in an attempt to overcome this afterload and optimize flow ( Fig. 9.4 ).
The second implication of the afterload sensitivity of the centrifugal pump is that the pump can be subjected to retrograde flow at lower RPMs while on VA ECMO. Why is this? Let’s consider an example to illustrate.
Say you are decreasing the RPMs down to lower the blood flow on a patient who is slowly recovering with improving blood pressure. As you continue to decrease the RPMs, eventually, the afterload of the patient’s mean arterial pressure (MAP) drops the flow to 0. At this point, there exists a higher pressure on the return side of the pump than on the drainage side (as the MAP is higher than the venous pressure), and you will get retrograde flow from the arterial system to the venous system, as illustrated in Fig. 9.5 .
The clinical implication of this is that if your RPMs drop below a critical threshold, you can induce a left to right shunt, which can cause a rapid decompensation.
Mechanical backup
If our discussion of the pump does anything, it should highlight how bad things can get if the circuit stops flowing. Let’s review why:
- 1.
Drop in the flow of oxygenated blood in a patient who may be dependent on the oxygenated blood to maintain delivery of oxygen
- 2.
Lack of blood flow in a patient who may be dependent on that flow to maintain cardiac output
- 3.
Retrograde flow with worsening of the right to left shunt and infusion of venous blood into arterial circulation (on VA ECMO)
- 4.
Stagnation of blood with thrombus formation
Taken together, these may lead to cardiac arrest and rapid deterioration.
Having a strategy to mitigate pump failure is paramount. Ensure there is a backup for power failure, whether it is a hand crank or a backup console next to the pump at all times. This includes during procedures, transport to anywhere in the hospital, physical therapy, etc. That is one thing about emergencies – they always seem to happen when we are least prepared!
Console
We discovered during our initial introduction to the console that in its most basic form, it is a mechanism for manipulating RPMs to the pump via connection with the electric motor and a display of flow and RPMs. The mechanism for increasing/decreasing RPMs can be a push button or a dial, with the convention being clockwise rotation to increase RPMs and counterclockwise rotation to decrease RPMs.
You are probably arriving to the conclusion that there are many determinants of blood flow (we will continue to develop these concepts in Chapter 10 ). Thus, RPMs are set by the console and transmitted to the motor/pump. The flow that is obtained by these RPMs has to be measured directly.
Sensors: measuring ECMO support
The way that blood flow is measured during ECMO support is by direct measurement through sensor probes. This is one advantage of having a support device with blood that is circulated outside the body – by contrast, ventricular support devices determine flow by a calculated algorithm as there is nothing to measure directly.
Flow sensors are circumferential sensors that surround the tubing and are connected to the console. They are placed on the return tubing and must be oriented in the same direction as the blood is flowing. If the sensor probe is on backwards, the console will read a negative flow rate ( Fig. 9.6 ).
