Question 23

College Answer

a)
ECMO is indicated for potentially reversible life-threatening cardiac and/or respiratory failure unresponsive to conventional support, buying time for recovery from the underlying condition and specific treatment to take effect.
This patient meets criteria for ECMO with a potentially reversible condition (Legionella pneumonia) and P/F < 60 and age < 65 years.

Alternative treatment strategies
 Ensure all other strategies have been tried – (e.g. - recruitment manoeuvres, prone positioning, NO/inhaled prostacyclin, diuresis, etc.)
 Exclude easily treated reversible problem e.g. pneumothorax, mucous plugging
 Ensure optimisation of haemodynamics, consider measurement of adequacy of DO2
Exclude contra-indications / relative contra-indications – severe pre-existing organ dysfunction, presence of other severe co-morbidities e.g. advanced malignancy, co-existing irreversible lung/cardiac pathology, and presence of bleeding disorder.
Available resources – appropriate level of expertise with trained staff to insert catheters, set up, monitor and troubleshoot ECMO circuit, and adequate equipment.
(Reference to meeting unit/regional criteria for institution of ECMO, or similar, are an acceptable answer.)

b)
Choice of V-V or V-A ECMO will depend on co-existing cardiogenic shock. V-A ECMO provides complete cardio-respiratory support, whereas V-V ECMO only provides respiratory support. If profound septic shock with myocardial depression and EF<25%, V-A ECMO indicated. If adequate cardiac function then V-V ECMO indicated otherwise significant native blood pulmonary blood flow and cardiac output results in relatively hypoxic perfusion of upper body compared with lower half.
V-V ECMO also avoids risks of serious arterial injury, has less severe consequences in case or air or clot embolization, and as a low-pressure system may prolong circuit life.
Animal studies suggest preservation of pulmonary blood flow with V-V ECMO may improve recovery from lung sepsis compared with V-A ECMO.

Discussion

For a 10 mark question, this college model answer seems somewhat barren. One might expect at least a bit of a digression into ECMO. Applications of ECMO and literature regarding the use of ECMO are discussed elsewhere. In brief:

Indications for ECMO

In order to qualify for this level of critical care, one must be special in the following ways:

• The condition must be reversible; OR the patient qualifies for a heart/lung transplant
• The conventional management strategies have failed.

The following situations call for ECMO:

• Cardiac arrest (in certain settings)
• Failure to wean from cardiopulmonary bypass
• Cardiogenic shock
• Hypoxic respiratory failure
• Hypercapneic respiratory failure

Contraindications for ECMO

• Contraindications to anticoagulation: recent surgery, uncontrolled bleeding, intracranial haemorrhage
• Irreversible condition
• Contraindications for heart/lung transplant

Caveats to ECMO

Before subjecting a patient to such a perversely unnatural therapy, one ought to satisfy onself that every "conventional" strategy has failed. These include:

• Recruitment manoeuvres
• prone positioning
• NO/inhaled prostacyclin
• diuresis
• Fluid resuscitation and optimised PEEP to improve V/Q matching

The latter point needs to be expanded. Often people with severe hypoxic respiratory failure are on a high PEEP, with the aim of recruiting more alveoli. Unfortunately this pressure is often transmitted to healthy lung regions which results in overdistension, and basically creates a large Zone 1 (of Wests' Zones). At the same time the blood flow which would have gone to these well-aerated regions will be distributed to collapsed lung regions, where the high PEEP does not reach -i.e. increasing shunt. The effect is of worsening hypoxia and hypercapnia with increasing PEEP ( a larger Zone 1 and a larger physiological dead space).. The solution is to reduce the PEEP and give some fluid boluses, so that the right heart can deliver blood into these previously poorly perfused lung regions.

Veno-venous vs veno-arterial ECMO

Each has advantages and disadvantages.

• VA ECMO has the advantage of providing complete cardiorespiratory support, and is therefore applicable in patients with very poor cardiac function (LVEF less than 25%)
• VA ECMO has the disadvantage of large-bore arterial puncture, which is a major problem. VV ECMO has less vascular access issues, but is only indicated for patients with good myocardial function.

In summary, the evidence:
 

These abovementioned rules are fairly elastic. Specifically, where it comes to concerns regarding vascular access complications, it is now unclear whether VA ECMO is truly more dangerous. Similarly, where it comes to severe haemodynamic compromise, it is unclear whether VV ECMO is truly pointless.

• In neonatal respiratory failure, back in 1996, Knight et al found that the frequency of intravascular thrombosis was significantly lower in patients receiving venovenous ECMO, and that otherwise things which were felt to be contraindications (eg. severe cardiac failure) weren't real barriers to successful VV ECMO.
• A 2000 review agreed with this in principle, but failed to discern any difference in the rate of complications in the neonatal/paediatric population.
• A more recent 2015 conference abstract also failed to find any difference in complications among adults.