Veno-Arterial ECMO

Temporary mechanical support for partial or complete circulatory failure, used as:

Bridge to recovery
For short-term, reversible forms of cardiac failure. Include:
- Acute fulminant myocarditis
- Cardiogenic shock secondary to ACS
  Prior to onset of MOF.
- Drug overdose
- Pulmonary embolism
- Primary graft failure
  - Heart transplant
  - Lung transplant
Bridge to treatment
Provide time for definitive management of the underlying condition
Bridge to transplant
Maintain organ perfusion whilst awaiting organ availability for heart transplant. More commonly used as a ‘bridge to bridge’, where VA ECMO is used to temporise prior to durable mechanical support (e.g. LVAD), with the intention for heart transplant in the future.

Indications for bridge to recovery with significantly poor (<25%) chance of survival include:

Sepsis with MOF
Cardiogenic shock with established MOF

Indications

In suitable patients, consider initiating support when:

Pharmacological haemodynamic support likely to worsen underlying disease
e.g. Arrhythmia burden.
CO and pressure inadequate despite significant support
Evidence of organ ischaemia
- AKI
- ALF
- Skin mottling
- Lactate >5 and ↑

Contraindications

Physiological contraindications to VA ECMO include:

Aortic dissection
Severe AR

Concepts

Include:

Native cardiac function
Afterload dependence
Differential Hypoxia
Weaning

Native Cardiac Function

VA ECMO adds a second paralell circuit to the circulation, which bypasses the lungs. Native CO varies on ECMO support but will contribute variably to the total CO:

Ideally, the aortic valve will open each beat to promote some native forward flow
Absence leads to LV stasis and significantly ↑ thromboembolic risk.
Native CO should be periodically assessed at the bedside
- Pulsatile arterial trace
- End-tidal CO₂
- Echocardiographcally
Inotropic supports should be weaned to the minimum required for AV opening to minimise myocardial work and promote myocardial recovery
:::column-margin Note that thermodilution is inaccurate on VA ECMO support due to loss of indicator into the ECMO circuit. :::

Afterload Dependence

The pressure gradient across the circuit is significantly greater in VA ECMO compared with VV ECMO, due to the downstream systemic arterial pressure. Consequently:

RPM settings are higher for any given flow, as compared to VV ECMO
ECMO flow is affected by systemic pressure
↓ Flow with an ↑ in afterload.

Differential Hypoxia

Differential hypoxia is also known as Harlequin syndrome.

Differential hypoxia occurs when hypoxaemic blood is ejected from the LV whilst oxygenated blood enters the circulation via the ECMO circuit. Differential hypoxia:

Gas tensions in blood from the ECMO circuit will reflect the function of the oxygenator, and therefore usually be supranormal.

Gas tensions in blood from the native CO reflect the function of the respiratory system.

Implies hypoxaemia of blood (predominantly) in the arch vessels
- Key tissue beds affected are the heart (via the coronaries), the brain, and the right arm
- Left SCA and ICA are relatively less affected
Occurs when the:
- Function of the native respiratory system is poor
  Such that hypoxic blood is return to the LA.
- Native cardiac function is recovering
  Such that it makes up a reasonable proportion of total cardiac output.
Is affected by the position of the return cannula
Significantly ↓ with a central aortic return, as blood should be mixed. Highest risk with a femoral return, as circuit flow favours the caudal regions.
Management involves:
- ↓ native CO
  Wean excessive inotropes.
- Correcting intrapulmonary shunt
  - PEEP and recruitment
  - Bronchoscopy
  - iNO
- ↑ ECMO flows
- Changing circuit configuration May be indicated if SpO₂ <85% despite above therapy.
  - VV ECMO if cardiac recovery has occurred
  - VAV if some ongoing cardiac support required

Detection of differential hypoxia is why SpO₂ of VA ECMO patients should always be measured on the right arm or ear.

Weaning

Weaning of VA ECMO is more complex than VV ECMO. The process involves:

Underlying cardiac recovery
Indicated by ↑ pulsatility with low or no inotropic support.
Conduct of a weaning study:
- Monitoring of haemodynamics whilst progressively ↓ circuit flows to 0.5-1L/min
- Includes echocardiographic assessment
- Requires adequate anticoagulation to prevent sudden oxygenator or pump thrombus due to ↑ stasis
- Weaning studies are limited by the fact that ECMO support is:
  - Not stopped
    The last 0.5-1L/min of flow often provides disproportionately high support.
  - Weaned only briefly
    Unlike VV ECMO where ECMO support is ceased for hours, ECMO support during a VA wean is reduced only for a short time.
Surgical decannulation
Following a successful weaning study, full ECMO flows are restored. Decannulation and actual cessation of support occurs in the OR, as the arterial cannulation sites require formal surgical repair.

Configurations