ฝึกพูดภาษาอังกฤษด้วยเทคนิค Shadowing จากวิดีโอ: Introduction to ECMO Part 3 - V-A ECMO

Veno-arterial ECMO is a bit like Veno-Venous ECMO, except you return the non-pulsatile blood flow into a high pressure system.

Oh, and the blood is travelling in the wrong direction.

My name is Ken Hoffman.

I'm an intensivist at the Alfred Hospital in Melbourne.

This is the third video in our introduction to ECMO series, aimed at new staff joining our team.

It will cover the principles of Veno-arterial or VA ECMO,

the indications, complications and considerations for liberation from VA ECMO support.

To start with, the circuit itself for VA ECMO is very similar to VV ECMO.

The major difference is that blood is returned to an artery under high pressure.

In this way, we can consider the ECMO pump being an artificial heart pumping non-pulsatile blood in parallel around the body.

The circuit starts with an access cannula in the right atrium or vena cava which removes venous blood from the body.

As this blood will return to the arterial circulation without going through the lungs, it must pass through a membrane oxygenator for gas exchange.

It will then be returned back under high pressure to the arterial circulation, usually via the common femoral artery.

The returning blood travels in the opposite direction through the arterial system, back towards the heart.

If the heart is still able to contract and eject some blood, this blood will mix with the ECMO blood somewhere in the proximal aorta.

This combination of a native pulsatile circulation and an artificial retrograde non-pulsatile circulation will be important when considering some specific complications later.

One specific difference with a VA ECMO circuit is the requirement for a backflow cannula.

As the large return cannula will sit in the common femoral artery, this can occlude blood flow to the leg.

To prevent leg ischemia, a smaller cannula is placed in the superficial femoral artery

which is connected into the ECMO circuit and allows downstream perfusion of the leg with oxygenated ECMO blood.

To titrate the amount of VA ECMO flow, we rely on clinical parameters such as mean arterial pressure,

lactate, urine output and peripheral perfusion.

Turning up the fresh gas flow doesn't really change the oxygen delivery.

So if the patient needs more support, the ECMO circuit flow should be increased.

It is important to remember that with VA ECMO, the fresh gas flow should never be turned to zero,

as this would result in deoxygenated blood being returned into the arterial circulation.

To summarise, VA ECMO can provide cardiac support by pumping oxygenated blood into the arterial circulation.

It can also be used for respiratory support as the returned blood is fully oxygenated.

In terms of the indications for VA ECMO, it is used as support for severe heart failure.

It is important to remember that VA ECMO doesn't actually fix anything.

Instead, it provides time so the patient can be bridged to recovery, bridged to a durable left ventricular assist device, or transplantation.

This is why VA ECMO is still a niche support modality.

A lot of the complexity comes from decision-making about how and when to liberate the patient from VA ECMO support.

The next thing to talk about are complications.

These can be divided into the common ECMO complications of bleeding,

thrombosis, infection and hemolysis and specific VA ECMO complications.

The specific VA ECMO complications to discuss are differential hypoxia,

left ventricular distension, leg ischemia and cardiac arrest on VA ECMO support.

Differential hypoxia occurs as a result of mixing between the forward flowing native cardiac output and retrograde flow from the ECMO circuit.

If the native lungs are poorly oxygenating the native cardiac output, then hypoxic blood will be pumped into the aorta.

The relative proportions of native cardiac output to ECMO flow will determine where the mixing point will be in the aortic arch.

If native heart function improves but the lungs still have respiratory failure, increasing amounts of hypoxic blood will enter the arterial circulation,

which will disproportionately affect the upper parts of the body.

In order to detect this, the arterial line is routinely inserted in the right radial artery

and the pulse oximeter should be placed on the right hand or right ear lobe.

This will confirm if the blood in the first branch of the aortic root, which is the brachiocephalic trunk,

is delivering adequately oxygenated blood to the right subclavian artery and more importantly, the right common carotid artery.

When differential hypoxia occurs the focus should be on trying to

improve the native lung function to increase oxygenation of the native cardiac output.

Other options include reducing inotropy to decrease the native cardiac output,

increasing ECMO flow to move the mixing point proximally or reconfiguring to return sites other than the femoral vessels.

Rarely fancy ECMO modes are required such as VAV or even separate VA and VV ECMO circuits.

Next complication to talk about is left ventricular distension.

This occurs

when the left ventricle is unable to generate enough tension for

the aortic valve to open during systole due to the pathology affecting the ventricle plus the increased afterload from retrograde ecmine flow.

This is exacerbated by any aortic valve regurgitation.

When left ventricular distension occurs,

the forward flow through the lungs is impaired and can result in pulmonary edema and pulmonary hemorrhage.

Rarely, stagnant blood in the left ventricle can clot.

Signs of left ventricular distension include a loss of arterial line pulsatility, a chest x-ray with pulmonary edema

or an echo with a dilated poorly functioning left ventricle with an aortic valve that is not opening.

Management of left ventricular distension involves increasing left ventricular ejection with inotropes,

reducing peripheral resistance and lowering mean arterial pressure targets and fluid removal to reduce the main systemic filling pressure.

If these interventions don't work, the left ventricle needs to be decompressed.

Usually, this is with a left ventricular vent cannula, which is joined into the access part of the ECMO circuit and drains the left ventricle with negative pressure.

The complication of leg ischemia has already been discussed.

It is prevented with insertion of a backflow cannula in the superficial femoral artery.

The leg still requires regular vascular observation by monitoring non-pulsatile Doppler flow as the backflow cannula can develop thrombosis.

The last complication to discuss is cardiac arrest on VA ECMO.

This is detected by a loss of pulsatility on the arterial line and an ECG that is not in a perfusing rhythm.

When this occurs, chest compressions are not required.

Instead, attempt to increase the VA ECMO flow to provide support whilst trying to restore a perfusing rhythm.

The reason restoring a perfusing rhythm is important is that stagnant blood in the left ventricle may clot.

Large clots in the left ventricle are unsurvivable.

and even small clots may result in embolic complications with restoration of cardiac output.

The final thing to discuss is liberation from VA ECMO support.

As already mentioned, if there is no cardiac recovery, then ECMO is a bridge to another treatment,

such as a durable left ventricular assist device or transplantation.

If there is cardiac recovery, then an echo-guided weaning study is performed.

This examines cardiac function with progressively reducing VA ECMO flow to determine

if the heart is able to cope without VA ECMO support.

If the weaning study is passed,

the VA ECMO is removed in the operating theatre as the large arterial cannulation site requires some stitches to control bleeding.

In summary, VA ECMO is used for cardiac support when the heart is unable to pump sufficient blood to maintain organ function.

Complications occur related to the ECMO circuit including the VA specific complications of differential hypoxia and left ventricular distension.

Liberation from VA ECMO is complicated as it is used either

as a bridge to recovery guided by an echo weaning study

or as a bridge to a durable left ventricular assist device or transplantation.

If you would like to read more about our ECMO protocols, they are freely available at ecmo.icu.

Thank you for listening.

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ฝึกพูดภาษาอังกฤษด้วยเทคนิค Shadowing จากวิดีโอ: Introduction to ECMO Part 3 - V-A ECMO

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