VA ECMO (3)

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.

Scenario:

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.

History:

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.

Initial Examination:

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.

Subsequent Management:

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 VA ECMO tutorials (1) and (2), 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.).

Figure 1.

We’re now nearly ready to commence VA ECMO.  To complete our preparations we need to:

To initiate VA ECMO:

Because the mean arterial pressure is high, commence an infusion of SNP by:

And starting it at 20 ml/hr:

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 425 mm Hg and the PCO2 about 40 mm Hg.

Now, ask the Supervisor to disconnect the gas supply to the oxygenator and

After five minutes note that:

Now, repeat the <Arterial> and <Oxygenator Blood Gas Analysis>.

Note that the:

These are the hallmarks of disconnection of the gas supply to an oxygenator.

In real life, how should we respond to suspected disconnection of the gas supply to an oxygenator?

Our response can be divided into four separate stages.

To check the integrity of the oxygen supply chain, the gas supply should be systematically checked from the wall to the oxygenator.

At the wall (Figure 2.), confirm that:

Figure 2.

If the system includes a blender, confirm that it is set to deliver 100% oxygen (Figure 3.).

Figure 3.

Next, make sure that there is no obstruction anywhere along the course of the supply line (Figure 4.).

Figure 4.

And, finally, confirm that the supply line is connected to the oxygenator (Figure 5.).

Figure 5.

If the oxygenator supply chain appears intact, you should now perform a ‘Bobbin Drop’ test.

To perform this test, transiently clamp the oxygen supply line at the oxygenator whilst observing the bobbin. During clamping, the bobbin should transiently fall. If the bobbin does not fall, it suggests that there is a leak somewhere in the supply chain and this check should be repeated.

If the supply chain check and the ‘Bobbin Drop’ tests fail to reveal any form of disconnection, exhaust gas analysis should be performed.

To do this, place the tip of a side-stream gas analyser catheter into the exhaust port of the oxygenator. If pure oxygen is being supplied to the oxygenator, the effluent gas should contain about 95% oxygen and 5% carbon dioxide. The absence of carbon dioxide implies that the gas supply to the oxygenator has been disconnected.

Finally, repeat the oxygenator blood gas analysis. If hypoxia and hypercarbia persist, urgently consider the possibility of oxygenator failure. We will deal with this in the next tutorial.

However, before we conclude this exercise, we’ll use the simulator to explore the effect of inadvertently administering air (rather than 100% oxygen).

To do this:

After five minutes,

Note that the effluent blood from the oxygenator is relatively hypoxic (PaO2 ~ 75 mm Hg), but hypo- rather than hyper-carbic (PCO2 ~ 34 mm Hg). This picture (relative hypoxia in association with normo- or hypo-carbia) is characteristic of an oxygenator which is being supplied with air rather than oxygen.