We will now continue our exploration of VA ECMO physiology by considering the physiology of centrally cannulated VA ECMO. The proportion of central cannulation is largely facility dependent. While some programs mainly rely on peripherally placed ECMO cannulas, others have a significant percentage of central cannulation. Regardless, it is important to understand the physiologic subtleties unique to this configuration.
Primary central cannulation configuration
As we learned in our configurations discussion in Chapter 8 , central cannulation involves cannulation directly to the heart and great vessels. The usual configuration is a dual-stage cannula implanted directly to the right atrium with a cannula implanted directly onto the aorta with an additional vent of the left ventricle either by a drain of the pulmonary veins or directly from the left ventricular apex ( Fig. 15.1 ).
Antegrade flow and perfusion of coronary arteries
The result is the ability to provide antegrade flow that bypasses both the heart and lungs. The physiologic advantages over retrograde support should be immediately apparent. Retrograde flow is no longer required, meaning that everything that is flowing from the extracorporeal membrane oxygenation (ECMO) circuit is what the body is receiving. The adverse effects of retrograde flow that we discussed in Chapter 14 (left ventricular afterload, competing flows, upper body hypoxia, etc.) are significantly mitigated in this configuration. Specifically, perfusion of the cerebral vessels is much more reliable, as the aortic cannula will be providing antegrade flow prior to the takeoff of the innominate and left common carotid artery ( Fig. 15.2 ).
Perfusion of the coronary arteries on the other hand is more dependent on retrograde flow from the cannula. While the majority of flow is directed in an antegrade direction, there is a small proportion of blood that flows back towards the heart which is needed for provision of oxygenated blood to the coronary arteries supplying the heart.
The second major advantage of this form of support is the amount of flow that can be provided. Remember that in peripherally inserted VA ECMO, we have a distinct limitation of flow that exists due to the retrograde nature of support. We can increase retrograde blood flow, but at a certain point, the advantages of higher ECMO blood flow (better perfusion/support) are outweighed by the disadvantages of retrograde support (left ventricular afterload, aortic valve closure, pulmonary edema).
The limits of retrograde flow to the overall amount of flow provided do not exist in central cannulation; therefore, central cannulation can be an option for a higher degree of support that can be considered on initiation of ECMO or as an upgrade of support. While retrograde limits to flow do not exist, limits to flow on central cannulation do exist.
Limits to blood flow on central cannulation
Let’s envision a patient on central cannulation. As we review the clinical data, we note that the lactate is rising slightly, urine output is dropping off, mean arterial pressure is low, and venous oxygen saturation is low. We conclude that we are not supporting her optimally and want to increase our ECMO blood flow. What are the limits to flow in this patient?
Flow Limit #1: Limit of the ECMO Circuit/Pump
All of the considerations for flow limitation that we considered in Chapter 10 still apply. There is still a maximal flow that the oxygenator can accommodate as well as the preload dependence/afterload sensitivity of the pump.
Preload dependence can still be a major limit to flow. Even though the cannula is placed directly into the right atrium and the collapsibility of the inferior vena cava (IVC) is not a limit to flow, a lack of venous return in the form of hypovolemia/bleeding can represent a major limit to flow.
Flow Limit #2: Mean Arterial Pressure
Afterload to the pump includes anything post pump to include tubing, oxygenator, cannulas, and ultimately mean arterial pressure. As flow increases, this can increase mean arterial pressure and increase the pressure differential that the pump will have to overcome. As this pressure differential increases, the pump may have to run at a higher rate of revolutions per minute (RPMs), which increases the risk of hemolysis.
Flow Limit #3: Pressures of the System
As flows across the pump increase, the post-pump pressure will increase as well. Higher pressures are sensed across the entire post-pump circuit, from the oxygenator all the way down to the cannula. The difference in pressure differential at normal ( Fig. 15.3 ) and higher blood flows is illustrated in Fig. 15.4 . The higher the blood flow, the higher the post-pump pressures will be (points 2 and 3).
