Several cannula configurations may be used for VV ECMO (Fig. 16.2). Our preferred configuration is femeroatrial (Fig. 16.2a). For drainage, a long 25–29 French (Fr) multiport cannula is placed in the femoral vein and advanced so the tip lies in the inferior vena cava (IVC) just below the hepatic vein, 5–10 cm from the junction of the IVC and right atrium (RA) (Fig. 16.3). For return, a short 19 Fr cannula is introduced into the right internal jugular vein and advanced so the tip lies just proximal to the junction of the RA and superior vena cava (SVC). This arrangement typically allows a circuit flow in excess of 6 L/min. A commonly used alternative to femeroatrial cannulation is to use a purpose-designed double-lumen ECMO cannula (Avalon Elite Bi-Caval Dual Lumen Catheter, MAQUET, Rastatt, Germany) inserted into the right internal jugular vein (Fig. 16.2c). The tip of the drainage lumen sits in the IVC at about the level of the hepatic vein, while the return lumen opens into the RA. For adults a 27 or 31 Fr cannula typically allows flows up to 5 L/min.

EMCO cannulae may be inserted by a surgical cutdown or a Seldinger technique. A particular advantage of the Seldinger technique is avoidance of cannula-site bleeding that often occurs with surgical cutdown. Peripheral cannulation through the jugular and femoral veins is preferred to central cannulation through the wound, as this approach allows closure of the chest wound, minimizing surgical-site bleeding and reducing the risk of mediastinal infection. The position of guide wires and cannulae should be confirmed with TEE. Once ECMO has commenced, flow in the return cannula should be interrogated with color Doppler imaging to ensure it is directed through the tricuspid valve (see below) (Fig. 16.4).

Modern ECMO circuits are comprised of a polymethylpentene (PMP) oxygenator, a centrifugal pump, heparin coated tubing, a pump controller, a heater unit, a gas blender, and sampling ports (Fig. 16.5). Compared to older-style circuits comprising roller pumps and hollow fiber or silicone oxygenators, modern circuits are more durable, cause less damage to blood components, and provide more efficient gas exchange [19, 20].

During VV ECMO, deoxygenated blood is drained from the IVC and oxygenated blood is returned to the RA. Ideally, all the blood from the return cannula passes through the tricuspid valve into the pulmonary circulation. Because oxygenated blood from the return cannula mixes with deoxygenated blood from the patient’s systemic venous return, it is not possible to achieve a normal S_aO₂ with VV ECMO. However, if ECMO circuit flow is at least 70 % of cardiac output, and the majority of the ECMO return blood passes into the pulmonary circulation, a S_aO₂ in the range 88–92 % can be achieved even if the lungs are not contributing to gas exchange [20].

The main determinant of S_aO₂ during VV ECMO is circuit flow. Low S_aO₂ (<88 %) despite adequate circuit flow (4–6 L/min.) may be caused by recirculation, abnormally high cardiac output (e.g., due to sepsis), or oxygenator failure. Recirculation occurs when oxygenated blood from the return cannula passes directly to the drainage cannula. Measuring the pre-oxygenator SO₂ in the circuit helps distinguish between recirculation and high cardiac output as the cause of hypoxemia. A high pre-oxygenator SO₂ (>75 %) suggests recirculation, whereas a low pre-oxygenator SO₂ (<60 %) suggests high cardiac output.

The severity of recirculation is influenced by intravascular volume, ECMO flow, and, most importantly, by the relative positions of the drainage and return cannulae. If recirculation is suspected, a TEE examination should be performed to assess the position and flow patterns of the ECMO cannula (Fig. 16.6). If recirculation is confirmed the cannulae positions should be adjusted under TEE guidance.

P_aCO₂ is not influenced by circuit flow but by the balance between the sweep gas flow and the patient’s metabolic state (i.e., CO₂ production). Normocarbia can usually be achieved with a sweep gas flow of 1–2 times the circuit blood flow.

Common problems and their potential solutions during ECMO support are listed in Table 16.2.

Once EMCO has commenced the ventilator should be set to rest settings to minimize further ventilator-induced lung injury. Typical rest settings are a F_iO₂ 0.4, PIP 20 cm H₂O, PEEP 10 cm H₂O, and a RR of 10/min.

Systemic anticoagulation with unfractionated heparin is routine during ECMO to prevent thrombus forming in the circuit. However, in the early postoperative period, when surgical-site bleeding is a concern, is may be appropriate to run the circuit heparin-free. ECMO can usually be managed heparin-free for several days without adverse consequences. Once postoperative bleeding has settled the patient should be fully heparinized. Traditionally heparin anticoagulation during ECMO has been monitored by measuring the activated partial thromboplastin time (APTT) . However, because APTT readings are highly variable between different laboratories and because APTT levels are influenced by factors other than heparin anticoagulation, many centers have moved to titrating heparin dose to the antifactor Xa level, aiming for a value of 0.3–0.7 U/mL [22]. Marked heparin resistance (heparin dose requirement >20 IU/kg/h) is suggestive of low antithrombin III (ATIII) levels. ATIII levels below 50 % of normal in the presence of high heparin requirements should be treated with fresh frozen plasma or recombinant ATIII. Bleeding is an ever-present risk while patients are anticoagulated for ECMO. If possible, invasive procedures (e.g., tracheostomy, intercostal drainage of simple pneumothoraces) should be deferred until ECMO has been discontinued.

To promote graft recovery a restrictive approach to fluid therapy and regular diuretic therapy is appropriate. Early institution of renal replacement therapy for control of metabolic acidosis and acute kidney injury is indicated. Strict attention to asepsis is essential as the combination of immunosupression, critical illness, and invasive cannulae all increase the risk of nosocomial infection. Two strategies of proven benefit that are particularly applicable to ECMO-supported patients are daily bathing with chlorhexidine-impregnated washcloths [23] and the use of chlorhexidine dressings for covering vascular (including ECMO) catheters [24]. Careful attention to hand hygiene, early institution of enteral nutrition, and daily circuit cultures are also important.

Signs of pulmonary recovery are indicted by reduced ECMO flow to maintain S_aO₂, an improving chest radiograph, and increased tidal ventilation. When the required circuit flows to maintain a S_aO₂ above 92 % has reduced to less than 3–4 L/min and tidal volume on rest ventilator settings have increased to more than 200 mL, a trial on standard ventilation (F_iO₂ 0.4, PIP 25–30 cm H₂O, PEEP 5–10, RR 15 breaths/min.) off extracorporeal support is indicated. Leaving the circuit flow at 3–4 L/min but turning the sweep gas off keeps the circuit patent but provides a trial off ECMO support. If, after 2–4 h, tidal volume and gas exchange are adequate (i.e., VT > 4–5 mL/kg, P_aO₂ > 80 mmHg, P_aCO₂ < 60 mmHg) ECMO may be discontinued and the patient decannulated. If an unmodified Seldinger technique has been used for insertion, the cannulae can be removed and manual pressure applied to the insertion site for 10–15 min. If the cannulae were inserted by a cut-down technique, surgical removal and repair of the veins is indicated.