A 39-year-old female is admitted to a tertiary centre and intubated and ventilated for severe Legionella pneumonia. Two days after admission to ICU she remains profoundly hypoxaemic (PaO2/FiO2 = 55), despite optimising ventilatory support and appropriate antimicrobial therapy.
a) Outline the factors that would influence your decision whether or not to institute extra-corporeal membrane oxygenation (ECMO) in this patient.
b) Outline the relative merits of veno-venous (V-V) and veno-arterial (V-A) ECMO for this patient.
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.
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 following situations call for ECMO:
Contraindications for ECMO
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:
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.
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.
UpToDate has a nice summary chapter about ECMO.
The world is sustained by the guidelines published by ELSO (the Extracorporeal Life Support Organisation)
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