Technical Requirements

To achieve the aim of viable oxygen delivery in severe ARDS patients, a high-flow, low-recirculation ECMO system is required. Both VA and VV-ECMO configurations can be used.

In the early 1970s, at the dawn of ECMO application in severe ARDS [28], the VA approach was used to achieve maximum oxygenation. VA-ECMO is characterized by high blood flow and null recirculation and therefore is an efficient system to oxygenate the blood, but in the setting of severe lung injury, its use has been abandoned due to the high rate of complications related to the arterial can- nulation (bleeding, compartment syndrome, embolization) and the development of differential hypoxia (Harlequin syndrome) if the arterial cannula is not positioned centrally at the level of the aortic arch. Nowadays, the percutaneous VV configuration is used in more than 70% of severe refractory hypoxaemic patients, as reported by data from the Extracorporeal Life Support Organization (ELSO) registry [66, 67].

ECBF as stated before is the main determinant of oxygenation during total respiratory ECMO support. Reported ECBF in the refractory hypoxaemic patients ranges from greater than 3-6 l/min. The amount of cardiac output (CO) oxygenated by the artificial lung, i.e. the ECBF indexed to CO (ECBF/CO), must be greater than 60% to obtain an arterial O₂ saturation >90% or a PaO₂ >60 mmHg [68].

Large-bore drainage cannulas are a prerequisite for achieving the goal of a high ECBF in the setting of refractory ARDS. The other important factor is their location in relation to the position of the return cannula (Fig. 7.2). If the drainage and return cannula are in the inferior vena cava, through a femoro-femoral approach, the distance between the tip of the two cannulas is critical to minimize recirculation. The best VV configuration in terms of effective ECBF (total ECBF-recirculated ECBF) is the femoro-jugular approach. In contrast, the jugulo-femoral approach is the least efficient in terms of recirculation, followed by the femoro-femoral one. In the recent years the use of a single-vessel approach gathered the favours of the ECMO community; a double-lumen large-bore cannula (AVALON ELITE® Bi-Caval Dual Lumen Catheter) is inserted percutaneously in the internal jugular vein and drains blood from both inferior and superior vena cava if correctly positioned. The inner return cannula has a hole that is directed towards the tricuspid valves, minimizing recirculation of oxygenated blood [69].