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Management of Persistent Severe Hypoxaemia During VV-ECMO

Severe hypoxaemia can persist despite total ECMO support or ensue during the ECMO run due to complete loss of gas exchange capabilities of the natural lung (i.e. intrapulmonary shunt around 100%) or important increase of total O2 requirements.

To overcome this event, different strategies are proposed [100-102]:

  • 1. Optimize the ECMO blood flow, eventually inserting a second drainage cannula if necessary, or move to hybrid configurations like veno-venous-arterial ECMO.
  • 2. Identify and correct recirculation through repositioning of the cannulas.
  • 3. Optimize residual native lung function with recruitment manoeuvres and prone position.
  • 4. Identify oxygenator failure and replace it.
  • 5. Optimize the effective ECMO blood flow (i.e. the ratio ECBF/CO) through a reduction of cardiac output (avoid fever, active cooling, beta-blockers).

6. Liberal blood transfusions to increase the arterial oxygen content and therefore O2 delivery.

Complications and Limits of Total Extracorporeal Support

The major limit of extracorporeal support still lies in the need of anticoagulation to prevent thrombus formation and the related risk of bleeding.

Murphy and coworkers recently reviewed the haemostatic complications related to ECMO [103]. The contact of blood with the non-endothelial surfaces of the circuit leads to activation of the coagulation and fibrinolytic pathways and excites a complement-mediated inflammatory response. Anticoagulants, mainly unfractionated heparin, are used trying to inhibit this prothrombotic state. “Bleeding is the Achilles heel of ECMO support” [103] and adversely affects the outcome of the patients. Many factors play a role in the increased risk of bleeding during ECMO; some are related to the necessary anticoagulation, some are patient related and some are related to the ECMO circuit per se. Common alterations that can lead to bleeding complications are thrombocytopenia, hyperfibrinolysis, disseminated intravascular coagulation and acquired von Willebrand syndrome. Surgical site bleeding is the most common complication reported; therefore any invasive procedure undertaken during ECMO must be weighted against the potential for its subsequent bleeding.

The costs of ECMO support, not only the equipment-related ones but also the manpower associated with the overall care of these severe patients, are another reported limitation of the technique. Despite cost-effectiveness in terms of quality of life has been proved by the Cesar trial [55] and despite high survival rates with no long-term sequelae reported in the major patients’ series, the widespread and increasing use of the technique [104] arises questions about patient selection and ECMO centres’ capabilities. Some outcome prediction scores [68, 81, 82, 84, 86, 105-107] have recently been proposed to help answering the question “which ARDS patient deserves ECMO support?”. Major factors contributing to outcome are patient age, comorbidities, other organ failures (SOFA score), immunosuppression and days of mechanical ventilation before ECMO institution. A score will never give the final answer about the single patient we are caring of. As Parhar and Vuylsteke conclude, “There is at present no definite tool to tell the clinician when ECMO should or should not be used. We can only, at best, list a few reasons why it should not” [107].

Another important point that showed an impact on outcome of ECMO-supported ARDS patients is the early retrieval to a high-volume referral centre [108], not only able to conduit “good ECMO” but also with full knowledge of the best care of ARDS per se and with the multidisciplinary skills to face all the possible complications ensuing during the ECMO run. This idea was proposed already in the late 1990s [87] and has recently stated by the position paper of the International ECMO Network (ECMOnet) [109, 110] and the Consensus Conference on ECMO held by the Societe de Reanimation de Langue Frangaise (SRLF) [111]. In both documents the requirements of an ECMO referral centre are clearly stated: “possess all human and material means essential to the care of ARDS patients and to setting up and use of extracorporeal life support techniques”; an ECMO support mobile team should be available 24 h a day, 7 days a week; the centre must perform continuing medical education and training in ECMO and manage at least 20 ECMO cases/year with 12 being respiratory ones.

ECMO is used in ARDS as a bridge to recovery. Sometimes the time required to reach the respiratory autonomy of the patient from ECMO can be very long. A lot of ECMO centres report long (>1 month) ECMO runs with a good outcome, rarely the ECMO-supported ARDS patients can be considered candidates for lung transplantation [112-115]. Therefore, in the longest runs, ethical issues regarding the continuation or withdrawal of this life-sustaining procedure can arise [116].

 
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