Blood Flow Titration

Imagine that you are initiating a patient on extracorporeal membrane oxygenation (ECMO). The cannulas are in excellent position and the circuit is connected and ready to go. You release the clamp and dark blood flows out smoothly.

“Time on ECMO, 7:38 a.m.” you say.

Within a few seconds, bright red blood is infusing into the return cannula. You give a reassuring glance at the console – circuit pressures look good, flows are steady, everything is running well. Next, your eyes move to the monitor. The oxygen saturation begins to slowly improve, blood pressure is coming up, and the heart rate comes down.

As the cannulas are being secured and dressed, someone asks, “what settings should we start?”

Now comes the real challenge…

We will now get into the nuts and bolts of the daily management of ECMO. We have said again and again that ECMO really only involves the manipulation of a few variables, namely, sweep gas and blood flow. Now that you are armed with the physiologic principles at play, you can start to develop some management principles for how to titrate flow and sweep for your patients on ECMO.

The ins and outs of flow titration

Blood flow determines the amount of oxygen delivered by increasing the relative amount of oxygenated blood from the circuit relative to the overall output of the native heart and lungs. The higher the flow, the more oxygenated blood is being delivered to the body. Flow titration requires identification of the ideal amount of blood flow through the ECMO circuit required to meet the desired physiologic end points.

How much flow is enough? How much is too much?

The ideal blood flow can be hard to define. While the individual physiologic needs and limits of the patient are the primary consideration, other considerations may come down to the style of the individual practitioners/programs. Needless to say, let’s explore some principles that can be applied when trying to reach this flow.

As you increase flow, you will likely run into the following limits:

1.

Minimum flow needed to maintain the pump
2.

Achievement of a physiologic end point
3.

You have reached a physiologic limit
4.

You have reached a limit of the pump

Once you define these limits for each individual patient, you can better understand the ideal amount of flow to apply. Just because you can flow more doesn’t mean you should flow more.

Limit #1: Minimum Flow Needed to Maintain the Pump

When you are first initiating ECMO, before the last clamp is released, there will be a minimum revolutions per minute (RPM) that is maintained, usually around 1500 RPM. Why is this? Why not just start at 0 and work your way up?

Remember when we were talking about the physiology of the pump, that forward flow is generated by the pressure differential from the pump. This pressure differential has to be enough to overcome the afterload of the pump, which comes in the form of the resistance of the circuit and either the mean arterial pressure (MAP) (in VA ECMO) or the central venous pressure (CVP) (in VV ECMO) ( Fig. 17.1 ).

At very low flows, the afterload may overtake the pressure generated by the pump at which you can have either no flow or retrograde flow. Additionally, the lower the flow, the greater the risk of stasis of blood and thrombosis in both the pump and in the oxygenator.

Put together, there is a minimum amount of blood flow that is maintained, usually no less than around 1.5 to 2 L in adults. Maintaining less flow may require a connection in the circuit separating the flow generated from the pump from the flow delivered to the patient (referred to as a bridge).

Limit #2: Achievement of a Physiologic Endpoint

Good critical care involves identifying a deficiency, applying an intervention, assessing the effect, and adjusting course. It is one thing to place a patient on fluids. It is much more effective to notice that a patient’s labs, examination findings, and urine output are likely consistent with hypovolemia, give a bolus of fluid, and monitor the patient clinically to observe the clinical effect.

The same is true for flow, which is a form of support that can be dosed just like fluids, pressors, or mechanical ventilation. We spent a good deal of time in the ECMO Physiology section discussing and unpacking the physiologic rationale for ECMO, all of which are directly related to flow. Let’s review:

Physiologic rational for ECMO
VV ECMO:
Clear CO ₂
Mitigate shunt physiology
Optimize right heart function
VA ECMO:
Optimize perfusion
Support MAP
Perfuse visceral organs
Support right heart
Provide oxygenation

VV ECMO: Recall that VV ECMO improves oxygen by mitigating shunt physiology. In respiratory failure, as shunt increases, more blood passes from the right side of the heart to the left side without participating in gas exchange. As ECMO blood flow increases, it takes on a higher percentage of this total blood flow that is crossing the pulmonary circulation, and oxygenation improves. In a similar way, while sweep gas flow is the primary determinant of CO ₂ clearance, recall from Chapter 11 that the higher the blood flow, the more CO ₂ is cleared for every increase in sweep gas flow.

What does this mean for titration? The main take-home is that flow is related to DO ₂ . If there is evidence of inadequate DO ₂ (low venous oxygen saturation, tachycardia, hypoxia), then this is where we have to evaluate for higher flow. The decision to consider at this point is increasing flow, tolerating lower DO ₂ , or giving blood.

VA ECMO: In Chapter 13 , we talked about the vicious cycle of cardiogenic shock and how VA ECMO flow can help to mitigate these factors. Now is the time to titrate these effects. MAP is low? Lactate is not clearing fast enough? Urine output is borderline? CVP and PA pressures are increasing showing evidence that the right heart is straining? These are the clinical indicators that more blood flow may be needed.

Remember, flow is like a pressor or any other drug that must be titrated. Notice a deficiency, titrate the flow up or down, and then observe if this change achieved the target effect. Now we are getting somewhere!

Limit #3: You Have Reached a Physiologic/Anatomic Limit

At a certain point, flow starts to adversely impact the physiology of the patient. This can be subtle or very obvious and can overlap with the beneficial effects of increased flow as described earlier. As such, it may be difficult to compile a comprehensive list, but for the meantime, let’s explore some adverse physiologic implications of too much flow.

Excessive negative venous pressure: Remember that to generate flow, the centrifugal pump generates a negative venous pressure, sucking blood out of the venous system through the pump. As flow increases this negative pressure grows, pulling the vena cava and the venous structures towards the cannula ( Fig. 17.2 ).

The effect of this can range anywhere from some mild kicking of the lines to dropping the flows for the circuit.

Biotrauma: Biotrauma refers to the inflammatory effect of support to tissues on a microscopic level. It is often described for mechanical ventilation, quantifying the damaging effects of mechanical ventilation that may be not accounted for by barotrauma and volutrauma effects alone. It is possible that higher blood flow through an artificial membrane oxygenator may lead to biotrauma as well, which can increase with increasing blood flow.

Recirculation (VV ECMO): This is a concept that we are now very familiar with but one that should be put in the context of flow titration. As flow increases on VV ECMO, there is more negative pressure generated by the drainage cannula, meaning there is a higher likelihood of pulling blood that is returning from the circuit back into the drainage cannula, as illustrated in Fig. 17.3 . This has the dual effect of decreasing the effective ECMO blood flow and decreasing the efficiency of the membrane, by raising the saturation of blood entering the membrane.