Vol. 15 •Issue 4 • Page 20
To ECMO Or Not to ECMO That Is the Question
Extracorporeal Membrane Oxygenation (ECMO) is a modified cardiopulmonary bypass technique used for the treatment of life-threatening cardiac situations and respiratory failure. It is generally applied for periods of less than eight hours outside the operating room environment.
Respiratory care practitioner education provides extensive training in the maintenance of normal base balance, oxygenation and oxygen delivery, ventilation and cardiorespiratory anatomy, physiology and pathophysiology. As a result, the fundamentals of respiratory care education make the RCP uniquely qualified to undertake further education as an ECMO specialist. 1
Infants with respiratory failure due to meconium aspiration syndrome (MAS) are generally treated with standard pressure-limited ventilation as the first step in patient management.2,3 Those who fail to adequately respond to this therapy and continue along a path of deterioration may become candidates for ECMO. 4,5
Other candidates are infants who are more than 35 weeks of gestation with Persistent Pulmonary Hypertension of the Newborn (PPHN) and fail to respond to conventional therapy. ECMO may be used for congenital diaphragmatic hernia for those infants who have undergone repair but must continue with supported ventilation. Failure to respond to therapy for sepsis may also be an indication for the therapy.
ECMO is appropriate when optimal ventilatory management fails to work for Infant Respiratory Distress Syndrome (IRDS), but it is not appropriate for infants under two kilograms and 46 weeks of gestation.
Although normally associated with neonatal care, ECMO is currently being used as “rescue therapy” for severe respiratory failure in pediatric patients (nine to 19 years). However, mortality and morbidity rates must be evaluated prior to initiation. Some centers use ECMO as an adjunct to cardiac surgery, but it must be kept in mind that ECMO is a temporary bridge and not a corrective intervention.
In cases of MAS, a substantial number of babies referred for ECMO do not receive the treatment. Some physicians prefer over-ventilation ventilatory support as tolerated. It is always important to keep in mind though that high levels of oxygen can make significant lung trauma. Neurological status is an important and critical measure of assessing the long-term functional results of therapy. Both cellular and plasma blood components may be damaged by the ECMO device.
Alternatives to ECMO therapy include the use of surfactant, high-frequency ventilation, negative pressure ventilation, liquid ventilation, and the use of INO (inhaled nitric oxide).
The pediatric population considered for adjunct circulatory support present with: severe pneumonia or pneumonitis, bronchiolitis, septic shock, RDS, aspiration pneumonia, trauma, burn, poisoning, cardiogenic shock, post-cardiac surgery myocarditis.
Some babies will require ECMO because they cannot make the placenta to air transition for oxygenation as a result of severe hyaline membrane disease. In this instance, their pulmonary blood vessels frequently have a constricting muscle layer that prevents blood flow into the lungs, and the myocardium tries to compensate by beating abnormally fast.
FORMULA FOR INCLUSION
Inclusion criteria for ECMO includes the following:
Oxygenation Index (OI) must be >30-40. This is determined by multiplying the mean airway pressure in cmH2O times the FiO2, multiplying this number by 100, then dividing the resulting number by the post-ductal PaO2 times 100.
To determine this, the FiO2 must be room air and measured by means of three arterial blood gases drawn at least 30 minutes apart.
1. The A-a DO2 > 600 for 8 hrs
2. Acute deterioration with the PaO2 < 35-mmHg
3. Shock acidosis with a pH < 7.25 units
4. Predicted mortality of at least 80 percent
1. OI > 40
2. Gestational age > 34-weeks
3. Weight > 2-kg
4. Reversible lung disease
5. No major (greater than grade 1) intracranial hemorrhage
6. No lethal congenital abnormalities
7. No ongoing or uncorrectable coagulaopathy
8. No mechanical ventilation for > 14-days
9. No severe neurological insult or asphyxia
10. No irreversible CNS injury
11. No major immunodeficieny
FOLLOWING THE PROCEDURE
To treat an infant using ECMO, the surgeon must place a catheter into the right internal jugular and to the level of the right atrium via the superior vena cava and place an arterial catheter into the left common carotid down into the aortic arch. The ECMO circuit is basically similar to the adult bypass circuit and is designed to operate for days rather than for hours.
Basically the operation of the ECMO circuit is quite simple. Blood is removed from the right atrium through the venous catheter by means of a pump, passed through a membrane type oxygenator for oxygenation and gas exchange, a heat exchanger for warming to body temperature, and then returned through the arterial catheter to the aortic arch (see Fig. 1).
The older, more traditional ECMO circuit employed a roller pump, membrane oxygenator, heat exchanger and a servo-controller for regulating the blood flow and venous and arterial catheter.
Along the circuit, there are ports for arterial blood sampling and venous blood sampling, and a port reserved for administering necessary medication directly into the bloodstream (see Fig. 2).
Newer circuits have replaced the roller type pump with a centrifugal non-pulsatile pump. The roller pump rotates in a confined track, propelling the blood through the tubing by alternately compressing and releasing the tubing. This method did cause a great deal of hemolysis. The centrifugal non-pulsatile pump eliminated this hemolysis and was considerably gentler on the cells. Since long-term circulatory support is not available in children without high risk, immediate goals remain organ perfusion while awaiting transplant or cardiac recovery.
It has been possible to provide excellent results within 48 to 72 hours after initialization. If significant recovery has not been demonstrated after 72 hours, then transplant must be considered.
It becomes an interesting point to consider whether ECMO is the only viable alternative. As discussed previously, there are medication regimens and non-invasive ancillary techniques that can be utilized prior to any major invasive modality. Adults frequently require additional circulatory support as a result of coronary artery disease after procedures such as coronary artery bypass grafting (CABG) or events such as a myocardial infarction (MI).
When left ventricular dysfunction becomes the major abnormality, Intraortic Balloon Pump counterpulsation (IABP) or a ventricular assist device (VAD) is often the support measures of choice. Children, however, often have bi-ventricular dysfunction, pulmonary hypertension and hypoxia, so they require bi-ventricular assist devices (BVAD) or ECMO.
This is not to say that children with left ventricular dysfunction resulting in anomalous left coronary artery and cardiomyopathy could not be adequately supported with an LVAD. Both ECMO and VAD can be used to support the smallest neonate.
MAS is characterized by the presence of meconium below the vocal cords. Clinical definition includes the following triad:
.1. the presence of amniotic fluid tainted with meconium;
2. aspiration of meconium through the tracheobroncial tree; and
3. a compatible radiographic examination.
It is manifested by respiratory distress, hypoxemia of varying grades, respiratory acidosis and pulmonary hypertension. MAS patients are generally full term but show significant loss of weight, nails and skin. If meconium is observed in the younger fetus (<=34-weeks), it is necessary to investigate whether the liquid is or is not purulent which would indicate the presence of listeria or pseudomonas.
Typical radiographic findings include infiltrates, areas of consolidation, atalectasis, poor inflation, pneumothorax and pneuomediastimun with an enlarged cardiac silhouette. The use of antibiotics for treating MAS is based on the idea meconium produces segmental atalectasis that can foster bacterial pneumonia. This bacterial infection can cause stress that instigates the passage of meconium and, consequently, MAS.
INO therapy can be used as a primary approach to solve the dilemma of those infants who are unable to breathe because of dangerously high levels of pulmonary tension, reducing the need for more invasive surgical interventions. INO has the ability to create preferential pulmonary vessel dilation increasing both blood flow and oxygenation. This reduces both treatment time and costs.
INO should be considered for hypoxic respiratory failure when it is impossible to maintain a PaO2 of >80-mmHg or evidence of poor cardiac output of <150 ml/kg/min. Tolalazine and Prostacycline have both been used for PPHN.
However, they share equal vasodilating effects on both the systemic and pulmonary systems, so profound hypotension is likely. While these drugs may provide dramatic increases in oxygenation, their general vasodilating properties may actually increase the intrapulmonary right to left shunt. INO is more specific than these drugs.
PPHN affects upwards of 7,000 infants yearly and is a major contributing factor in respiratory distress syndrome (RDS). Surfactant does not help here since surfactant aids the microscopic lung units to inflate but does not cause vasodilatation to increase perfusion.
Primary PPHN is the form that most closely fits the major clinical picture presenting soon after birth and is primarily due to dysfunction in the pulmonary endothelial vasodilating mechanism. Secondary PPHN develops as a result of lung parenchymal disease. Only the sickest of the full term newborns, with an OI of >25 or >15 in pre-term infants, will require additional support. It may occur as a result of MAS, pneumonia, severe hyaline membrane disease, diaphragmatic hernia, and pulmonary hypoplasia.
Oxygen can act as a potent vasodilator, however, optimal pulmonary vasodilatation occurs at a PaO2 of 120-mmHg. No additional benefit is derived from higher levels of oxygen, and the likelihood of oxygen injury is increased. Therefore, the goal is to maintain a PAO2 of 100-120mmHg, with low normal PaCO2 of 35-40mmHg. Lower rates than this will increase cerebral vasoconstriction.
It is clear that after all efforts to support with non-invasive techniques have failed, invasive techniques such as VAD and ECMO become necessary for the survival of those infants with PPHN, MAS or post-surgical complication.
However, it is still incumbent upon the provider to make every effort to find a less invasive approach to circulatory support prior to considering extreme measures. Nonetheless, ECMO still remains an approach with good outcomes and success rates.
• For a list of references, please call Vern Enge at (800) 355-5627 ext. 1119.
Paul B. Olkin, MS, RRT, CCP, CCPT, is the director of Clinical Education for the Respiratory Therapy Program at Shenandoah University, Winchester, Va.