VV ECMO (3)
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Some of the manoeuvres undertaken in this tutorial generate extreme circuit pressures and are only intended to demonstrate the performance characteristics of the system. They should not be employed under clinical conditions.
In this simulation, we’ll continue to explore the characteristics of a Veno-Venous Extra-Corporeal Membrane Oxygenation (VV ECMO) system.
The learning objectives of the session are to investigate:
We will be simulating the use of a system in a patient with essentially normal cardiovascular function, but with severe respiratory failure. The patient is a 24 year-old man, weighing 75 kgs, who has been transferred to your Intensive Care Unit for Veno-Venous ECMO.He had been admitted to the Intensive Care Unit of a large regional hospital 3 days before transfer to your institution. At that time, he presented a four day history of increasing respiratory distress, fever and a productive cough. The patient had been sedated, intubated and ventilated shortly after admission to the regional ICU. Blood and sputum cultures grew a methicillin-sensitive staphylococcus aureus. Despite treatment with appropriate antibiotics, the use of prone ventilation, permissive hypercarbia and inhaled nitric oxide, he continued to deteriorate. Following discussions with the clinicians at the regional hospital, he has been transferred to your institution for initiation of veno-venous ECMO. On arrival in your ICU the patient is being given assisted ventilation (via an ‘Ambu’ © bag) on 100% oxygen.
Blood gas analysis is reported as:
The patient has an Acute Lung Injury Score (‘Murray Score’) of 3.5 but has no other significant co-morbidities. He has a Haemoglobin of ~ 12 gm/dL and you estimate his ‘shunt fraction’ (Qs/Qt) to be about 65%. The results of some ancillary investigations (Echo, CXR and ECG) are also available.
We’ll assume that you have already worked your way through VV ECMO tutorials (1) and (2), so we’ll quite quickly set up our monitoring:
Ventilate the patient:
Paralyse the patient:
And perform a baseline blood gas analysis:
You elect to cannulate the patient percutaneously via the right and left internal jugular and right femoral veins using the Seldinger technique and ultrasonic guidance.
Imagine now that you have:
You are using the same ECMO system that is similar to that described in VV ECMO Tutorials (1) and (2) – except that we are using two drainage cannulae. Active venous drainage from the inferior vena cava (IVC) is via a 24F femoral cannula, drainage of the SVC is via a right internal jugular cannula and ‘arterialised’ return to the right atrium via a 21F left internal jugular cannula (Figure 1.).
In this exercise, we’re only interested in the effect of changing the size of the return cannula, so we’ll make sure that the venous drainage is absolutely optimal. To do this:
Advance the SVC drainage cannula:
Advance the IVC drainage cannula:
Note the pre-membrane pressure (~130 mm Hg), and the rpm (~2375).
We’re now going to reduce the return cannula size from 21F to 15F. To do this:
Note that the blood flow has now fallen from ~ 4.0 lpm to ~ 2.9 lpm and the pre-membrane pressure has risen to (~150 mm Hg). To return the flow to 4.0 lpm you will need to increase the rpm to ~3100 (and the pre-membrane pressure will rise to about 250 mm Hg as a result). Thus, we can see that the resistance of the 15F cannula is much higher than that of the 21F cannula, and the ECMO pump has to work much harder to achieve a flow rate of 4.0 lpm.
Now let’s examine the effect of increasing return cannula size. To do this:
Note that the blood flow has now increased to ~ 4.4 lpm and the pre-membrane pressure has fallen to (~140 mm Hg). Return the flow to 4.0 lpm by decreasing the rpm to ~2500 and note that the pre-membrane pressure falls further to 115 mm Hg.
The pre-membrane pressure measurements for the complete range of cannulae at a flow rate of 4.0 lpm are summarized in Figure 2. Note how the pre-membrane pressure increases quite markedly once the return cannula size is reduced below 21F. Although there are no absolute rules for determining the maximum pressure which can be accepted within the ECMO circuit, it is believed that, ‘ceteris paribus’, higher pre-membrane pressures are more harmful than lower pressures.
There are several reasons for this belief.
Thus, the take home message (in adults) is to use a 21F return cannula if possible.
This concludes the exercise.