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Extracorporeal membrane oxygenation in adults: A practical guide for internists

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Contraindications to ECMO

Advanced age, comorbid conditions such as malignancy, nonpulmonary organ dysfunction (including complications of critical illness), and immunodeficiency or pharmacologic immune suppression have been associated with poor outcomes in ECMO patients.28 Severe aortic incompetence and aortic dissection are contraindications, since ventricular end-diastolic pressure can be increased with resultant ventricular distention, compromised myocardial oxygenation, and worsening of left heart failure.

ECMO is increasingly being used in situations in which it was previously considered contraindicated. Pregnant and postpartum patients with cardiorespiratory failure were previously not considered for ECMO because of a possible increased risk of coagulopathy and complications. However, a recent review showed that the outcomes of ECMO in pregnancy and postpartum were similar to those in nonpregnant patients, and the risk of catastrophic bleeding was minor.29

Similarly, ECMO is also being used increasingly in posttrauma patients and patients with other bleeding risks.30

Figure 3. Clinical decision-making in use of extracorporeal membrane oxygenation (ECMO) in respiratory failure.

Morbid obesity was once considered a contraindication because of difficulty in cannulation, but with newer types of cannulas, even patients with a body mass index greater than 60 kg/m2 are receiving ECMO.31

HOW DO YOU DO IT?

Figure 4. Clinical decision-making in utilization of extracorporeal membrane oxygenation (ECMO) in cardiogenic shock.

Figures 3 and 4 depict clinical decision-making in starting and weaning from ECMO in respiratory failure and cardiogenic shock, respectively.

Management of patients on ECMO

Appropriate patient selection and initiation of ECMO are only the beginning of a tough journey. Successful management requires minimizing lung injury from mechanical ventilation, careful monitoring of anticoagulation, and instituting adequate physical therapy, including ambulation when possible (Table 3).

Initial ECMO settings and monitoring

The cannulas for venovenous ECMO are frequently inserted under fluoroscopic or transesophageal echocardiographic guidance, whereas venoarterial ECMO cannulation does not require imaging and can be performed at the bedside in the intensive care unit or operating room.

The initial ECMO settings are titrated according to the patient’s hemodynamic and respiratory needs. There are three main variables: blood flow, fraction of oxygen in the sweep gas, and sweep gas flow rate. These are adjusted to achieve desirable levels of oxygen and carbon dioxide in the blood.

Blood flow is determined by the revolutions per minute of the pump, preload, and afterload of the circuit. Common patient conditions that may reduce flow are systemic hypertension, hypovolemia, cardiac tamponade, and tension pneumothorax, depending on the modality. In addition, mechanical factors such as clots in the oxygenator or kinks in the circuit can increase resistance and reduce flow. Resistance to flow is directly proportional to cannula lengths and inversely proportional to cannula radius to the fourth power. The greater the flow, the greater the oxygen delivery.

Fraction of oxygen in the sweep gas. The oxygenator has a gas blender that mixes air and oxygen and allows for a range of oxygen concentrations. Increases in fraction of oxygen increase the partial pressure of oxygen in the blood.

Sweep gas flow rate. Venous blood in the extracorporeal circuit is exposed to fresh gas (or sweep gas) that oxygenates the blood and removes carbon dioxide by diffusion. Increasing the sweep gas flow rate results in greater carbon dioxide elimination from the blood.

Laboratory monitoring. During ECMO, the following values are monitored frequently:

  • Arterial blood gases
  • Blood gases in the ECMO circuit before and after going through the oxygenator— to monitor the efficacy of the oxygenator membrane
  • Lactic acid—to monitor for tissue hypoxia
  • Plasma free hemoglobin (a marker of hemolysis)—to monitor for hemolysis.

Mechanical ventilation on ECMO

Low tidal volume ventilation greatly reduces the risk of death in patients on ECMO by reducing ventilator-induced lung injury. Proponents of ECMO believe that ECMO provides “lung rest,” and thus it is imperative that lung-protective ventilation strategies be followed in patients on ECMO.8 In most cases, after ECMO is started, low tidal volume ventilation (6 mL/kg) is possible and should be used—or even very low tidal volume ventilation (3–6 mL/kg).32,33 Many cases have also been described in which patients have been safely extubated while on ECMO to prevent ventilator-induced lung injury.34,35

If hypoxemia persists

Despite full support with venovenous ECMO, some patients remain hypoxemic due to inadequate blood flow to match metabolic demands, eg, patients with morbid obesity or severe sepsis and fever. The physician should ensure there is no recirculation, maximize blood flow, optimize the hematocrit to increase oxygen delivery, and consider ways to decrease oxygen consumption, including sedation, paralysis, and hypothermia.

Recirculation can be calculated by measuring the oxygen saturation of the blood in the ECMO machine before and after it goes through the oxygenator, and also in the central venous blood. Recirculation has been reduced by using double-lumen cannulas but can also be reduced by manipulation of the reinfusion cannula or increasing the distance between drainage and reinfusion ports in other configurations of venovenous ECMO.

Expert opinion suggests that oxygen saturation of 86% or more and Pao2 of 55 mm Hg or more in patients on venovenous ECMO are sufficient to prevent hypoxia-related end-organ injury.36 Venoarterial ECMO should be considered in patients on venovenous ECMO with refractory hypoxemia with the above measures.

Harlequin syndrome is characterized by upper body hypoxia resulting in cerebral hypoxemia due to poorly oxygenated blood in the coronary and cerebral circulations, especially in patients on peripheral venoarterial ECMO. It can be detected by sampling the blood in the arm (where the oxygen isn’t going) instead of the leg (where the oxygen is going), and it can be corrected by adjusting the Fio2, using positive end-expiratory pressure, or both to increase oxygenation. If ventilator settings do not improve this syndrome, the arterial cannulation site can be switched from the femoral artery to the axillary or carotid artery.

Alternatively, a mixed-configuration venoarterial-venous ECMO can also be created, in which a portion of arterialized blood from the arterial outflow cannula is diverted via the right internal jugular artery to the right heart. This enriches the blood traveling through the pulmonary circulation and to the left ventricle to provide better oxygen delivery to the coronary and cerebral circulations.

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