Circulation Membrane Oxygenation Therapy for Acute Respiratory Diseases
Survival rate of newborns. Survival rate of 34,650 newborns after reported ECMO to the Extracorporeal Life Support Organization (ELSO), grouped by cause of admission to ECMO (ERCP Extracorporeal cardiopulmonary resuscitation)
For pediatric patients with respiratory conditions, the indications that lead to using ECMO are more diverse and harder to define than during the newborn stage, but during the past few years there has been an increase in the number of reported cases to ELSO, with close to 350 cases a year displaying a global survival rate of 57% at the discharge (Figs. 73.3 and 73.4).
It is worth noting that survival rate in pediatric patients varies according to the disease that determined the connection, with survival rates reported as high as 83% for severe asthma attack. Acute hypoxic respiratory failure is the most frequent pathophysiological mechanism leading to respiratory ECMO, of which viral pneumonia takes first place (22%), followed by respiratory failure (18%), bacterial pneumonia (10%), acute respiratory distress, and aspiration pneumonia. Acute viral pneumonia shows a survival rate of up to 70% as described for VRS, with an average of 64% for viral pneumonia. Survival rates are also high in the pediatric group for aspiration pneumonia and post-traumatic ARDS (Fig. 73.5). Patients are oftentimes admitted because of immunosuppression and suspected sepsis. These patients usually display multi-organ failure. Pediatric patients with the worst prognosis are those who have received bone marrow transplants, those who have suffered from Bordetella pertussis induced pneumonia and pulmonary hypertension, and those admitted to ECMO with multi-organ failure, as opposed to the positive prognosis for those who only show isolated lung involvement.
In 1972, Bartlett reported the first case of successful prolonged post-operative cardiac support in a 2-year-old patient who suffered from post-surgery heart failure after a Mustard procedure because of transposition of great arteries. At present, more than half of patients requiring perioperative heart ECMO are those suffering from complex cyanotic congenital heart diseases. The largest group of patients requiring ECMO support is those who, after cardiotomy, present complete AV canal (20%), complex single ventricle anomaly (17%), and tetralogy of Fallot (14%). Among the chief causes requiring perioperatory heart ECMO are hypoxia (36%), cardiac arrest (24%), and failure after leaving extracorporeal circulation support (14%).
During extracorporeal circulation support, blood is drained from the patient to an outside pump (either a roller or a centrifugal pump), which is pushed through an exchange membrane (a silicone or a polymethylpentene oxygenator) for oxygenation and CO2 removal. Then it passes through a heat exchanger and finally returns the blood to the patient’s blood flow (Fig. 73.6). This therapy requires anticoagulation of the circuit and the patient through heparin administered to the ECMO circuit so as to avoid the activation of the coagulation cascade in the system. In addition to that, a variety of pressure, flow, bubble, and temperature monitors are used during therapy. It is of vital importance to continually monitor coagulation through the hourly measurement of activated clotting time (ACT) and the measurement of anti-factor Xa, fibrinogen, platelet count, PT, APTT and, in some patients, anti-thrombin III levels, and a thromboelastography.
There are in principle two different forms of ECMO:
Veno-arterial (VA): Blood is drained from the right atrium through a cannula inserted into the right internal jugular vein, the femoral vein, or directly to the right atrium; and it is picked back up at the thoracic aorta through a right carotid cannula, a femoral cannula or an aortic cannula. VA ECMO provides heart-lung support. It is common to use a transthoracic cannula (right ventricle and aortic cannula) in patients who have endured heart operations.
Veno-venous (VV): Blood is drained from the right atrium through the posterior and inferior orifices of a double-lumen cannula inserted into the right jugular vein and returned to the same right ventricle through the anterior orifices of the cannula, which are pointed toward the tricuspid valve. One of the limits of this method lies in the recirculation of already oxygenized blood through the double lumen cannula. This has been corrected with the new design for VV cannulas. VV ECMO may also be performed in older children through the use of two cannulas, draining blood from the jugular vein and returning it through the femoral vein. VV ECMO requires a well-functioning heart. This modality of ECMO prevents cannulation in the carotid or femoral arteries, thus reducing complications arising from cannulation or ligation of these arteries, as well as those arising from air entering the ECMO circuit. This method has seen an increase in usage during recent years, covering 40% and 50% of respiratory cases in newborn and pediatric patients, respectively. Oxygen is delivered during ECMO by the combination of blood oxygenation through the membrane, blood flowing through the extracorporeal circuit, native lung oxygenation, and native heart output. In turn, oxygenation at the ECMO membrane is a function of its geometry, material composition and thickness, blood and FiO2 laminar thickness, time of permanence of red blood cells in the exchange area, hemoglobin concentration, and O2 saturation. On the other hand, CO2 removal during ECMO is a function of the geometry, materials, and surface area of the membrane, blood PCO2 and, to a lesser extent, it depends on blood and gas flows through the membrane.
In a VA ECMO, the bypass generates an essentially non-pulsatile blood flow. In this way, as the blood flow to the extracorporeal circuit increases, the pulse wave decreases, completely ceasing when it reaches a 100% bypass, except for occasional waves. However, it is normal for VA ECMO to only involve about an 80% bypass, allowing a blood flow of 20% or more through the left heart and lungs, resulting in a reduced but visible pulse wave. The kidney is without doubt the most affected organ by the absence of pulsatility, producing an antidiuretic effect because of juxtaglomerular stimulation. In addition, non-pulsatile flow has been linked to stimulation of the pressure receptors in the carotid sinus, causing a large release of catecholamines, with damaging effects to microcirculation.
Selection Criteria for Applying ECMO
Selection criteria differ for newborn (Table 73.1) or pediatric patients (Table 73.2), depending on whether the primary cause of admission is cardiac or respiratory. These are general criteria and must be individualized for each patient, assessing the risks and benefits of applying ECMO. The basic selection criteria for pediatric patients with respiratory failure are similar to those for newborns, with particular emphasis on whether the patient faces a serious pulmonary condition with a high risk of death, or whether it is a process that can be reversed through respiratory, gasometrical, and hemodynamic rest. On the other hand, there are general exclusion criteria (Table 73.3), though none of them are absolute and must be discussed by the ECMO team. In summary, selection criteria have evolved and continue to do so as a result of discussion, debate, experience, and the emergence of new treatments and techniques. Currently, there are no unique or exclusive criteria, and not withstanding general inclusion criteria, the decision to exclude a patient must be discussed by the team, taking into account all possible points of view, without excluding any party from this discussion.
Selection criteria for newborns
Gestational age ≥34 weeks
Weight at birth ≥2000 grams
Unresponsive to maximum medical care (including HFOV, iNO, surfactant)
Reversible cardiopulmonary condition
Mechanical ventilation ≤14 days
High pulmonary mortality (50–100%), considering:
Oxygenation index (OI) >35–40 for 4–6 hours (iNO, HFOV)
PaO2 < 40 mmHg for 4 h (100% O2)
OI ≥ 25 after 72 h with HFOV-iNO
Unmanageable metabolic acidosis (ph < 7.15 for 2 h)
Reduced cardiac output with reversible etiology
Impossibility to wean from cardiopulmonary bypass
As a bridge for heart transplant
No untreatable congenital cardiopathy or injuries after heart surgery
Absence of major intracranial hemorrhage (≥ III degree)
Absence of uncontrollable hemorrhage
No evidence of irreversible brain damage
No malformations or genetic syndromes with fatal prognosis