VA ECMO (4)
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Some of the manoeuvres performed in this simulation must never be performed on a real patient.
The learning objectives of the session are:
We will be simulating the use of a system in a patient with severe left ventricular failure – which is associated with significant respiratory impairment.
You are called urgently to see a 24 year-old man in the Emergency Department of your hospital. The patient had been brought in by ambulance complaining of chest pain and dyspnoea. You arrive in the department just after the patient has been intubated by the resident. The resident tells you that the patient deteriorated shortly after having a portable chest x-ray. He performed the intubation because the patient became profoundly hypotensive, unresponsive to commands and started to have frequent ventricular ectopy.
According to the family, the young man had suffered a flu-like illness some weeks before and had been complaining of increasing shortness of breath and lethargy ever since. His family practitioner had prescribed two courses of broad-spectrum antibiotics and some bronchodilator therapy.
The endotracheal tube appears to be correctly positioned and breath sounds are symmetrical. The patient is hypotensive, tachycardic and has an irregular pulse. He is afebrile. His ventilation is being assisted using 100% oxygen and an Ambu © bag. Pulse oximetry indicates a saturation of ~80%.
Previous Medical History:
The family tells you that the patient has previously been in good health, that he takes no regular medications, and that he has no drug allergies. He has no relevant previous medical history.
The patient is transferred to the Intensive Care Unit, further investigations are performed, a tentative diagnosis of viral myocarditis is made, and the decision is taken to support the patient with VA ECMO. The results of some of these investigations (Echo, ECG, CXR, and ABG) can be seen by clicking <Investigations> on the main menu.
We’ll assume that you have already worked your way through the previous VA ECMO tutorials, so quite quickly we’ll:
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 femoral artery and vein using the Seldinger technique and ultrasonic guidance.
Imagine now that you have:
The cannulae have been connected to the ECMO system. The entire system is heparin-coated. The system’s centrifugal pump is responsible both for generating the negative pressure which is required to facilitate drainage and the positive pressure which is required to pump blood through the artificial lung and back into the patient. The pump head and oxygenator are integrated into a single, disposable unit which is mounted on the system console (Figure 1.).
We’re now nearly ready to commence VA ECMO. To complete our preparations we need to:
To initiate VA ECMO:
The mean arterial pressure is very high, but for the purposes of this exercise, we’ll just accept it.
After five minutes:
As there is no ventricular ejection, there is little difference between the arterial and oxygenator samples. In both cases, the PO2 should be about 400 mm Hg and the PCO2 about 35 mm Hg. Note, also that the pre-membrane pressure is ~ 305 mm Hg and that the rpm are about 3500.
Now, ask the Supervisor to cause the oxygenator to fail **.
Over the next few minutes, observe that:
Start the <Timer> and after five minutes::
Note that the Arterial and Oxygenator PO2 has fallen precipitously to about 45 mm Hg and that the PCO2 has risen by about 10 mm Hg to ~ 47.
How can we interpret these results?
We suspect that the oxygenator is becoming partially occluded by the deposition of fibrin and/or thrombus. As a result, the pre-membrane pressure is rising and the delivered flow falling. (If we were to measure the pressure drop across the oxygenator, we would see that it had increased.) The gas transfer function of the oxygenator is also failing. (The supervisor has ‘failed’ about 50% of the membrane). As a result, the PO2 is falling and the PCO2 rising.
These results are typical of one of the more common forms of oxygenator failure and can be summarized as:
In real life, how can we think about oxygenator failure?
One method is to classify it according to the mechanism of failure of the oxygenator. Using this approach, we can identify three categories:
In the example given above, the oxygenator failure is manifest as both a failure of gas transfer function and as a failure of structural integrity (the membrane resistance has increased).
Fibrin or thrombus deposition is one of the most common causes of this type of oxygenator failure in ECMO systems which use non-porous, polymethylpentene, hollow-fibre membrane systems (Figure 2.).
In contrast, the older, micro-porous, polypropylene, hollow-fibre systems were prone to plasma leakage which occurred much earlier in the life-cycle of the device. An example of this is shown in figure 2.
In this case, plasma leakage was associated with a significant degree of gas transfer failure which necessitated an urgent oxygenator change out.
This concludes VA ECMO Tutorial 4.
** Supervisor’s note:
over the course of a few minutes.