+++
Extracorporeal Membrane Oxygenation
(ECMO)
++
Advances in the biological inertness of the materials used to
build membrane oxygenators and advances in the reliability and efficiency
of the blood pumps made it possible in the 1970s to begin expanding
the benefits of cardiopulmonary bypass from the operating room to
the intensive care unit. Through the tenacity of a few pioneers
like Robert Bartlett,13 ECMO became a viable method
for respiratory and circulatory support in pediatric patients even
after initial controlled studies failed to demonstrate differences
in outcome compared to conventional mechanical ventilation in adults.14
++
Almost 30 years later, ECMO offers a potentially salvaging therapy
to infants and children with hypoxemic respiratory failure and,
perhaps as a more natural extension of the origins of the therapy,
to infants and children with cardiogenic shock after surgical correction
of complex congenital heart lesions or cardiomyopathy. Paradoxically,
the rates of ECMO referral have decreased throughout the world,
in great part because of new therapeutic options and generally improved
care by more conventional or less invasive means. ECMO is now used
much more rarely in newborns with respiratory failure, the primary
beneficiaries of the early efforts, and is used more selectively
as a means of support in infants and children with severe circulatory
impairment. The reversibility of lung and myocardial injury is the
most significant determinant of survival and therefore should be
the most important consideration in establishing indications. Yet,
refinements in technique and the ability to quickly institute support
have led to new practical applications of ECMO, such as in patients
suffering from unexpected circulatory arrest while in the hospital.15
++
ECMO is usually provided using two alternative circuit designs:
venoarterial ECMO and venovenous ECMO (Fig. 109-5).
At least in the United States, venoarterial ECMO is used most frequently
in pediatric patients, because it is familiar and because it can
be used simultaneously to support gas exchange and cardiac output.
In the typical arrangement, a portion of the patient’s
venous return is redirected via a cannula placed in the right atrium
(usually through the internal jugular vein) to a venous reservoir
in the ECMO circuit, where a rotary or centrifugal pump forces it
sequentially through a membrane oxygenator and a countercurrent
heat exchanger before returning it to the arterial circulation via
another cannula positioned in the ascending aorta (usually through
the carotid artery). In the oxygenator, blood flows in contact with
a permeable membrane that separates it from a flow of oxygen and
carbon dioxide. The composition and gas-flow rate are adjusted to
optimize the oxygen and carbon dioxide contents of the blood exiting
the oxygenator.
++
++
The ECMO pump provides the driving force for the return of blood
to the aorta. Because blood is removed from the right atrium by
gravity, it is the flow of venous blood that limits ECMO flow. Lowering
the venous reservoir relative to the right atrium increases the flow
of blood into the circuit. However, the increase becomes limited
by the collapse of the right atrium or the systemic veins. Increases
in intrathoracic pressure (eg, PEEP), decreases in the circulating
blood volume, or venous dilation reduce the maximal flow that can
be achieved for a given right-atrial pressure.
++
In venovenous ECMO, blood is removed and returned to the venous
system. Consequently, this method supports only gas exchange and
is not suitable for patients requiring cardiac support. Unlike venoarterial
ECMO, systemic and pulmonary blood flow are the same. Extracorporeal
carbon dioxide removal has been used successfully in adults
with hypercapnic respiratory failure. It capitalizes on the extreme
diffusibility of carbon dioxide across synthetic membranes to remove
large volumes of this gas without the need for a substantial extracorporeal
diversion of venous blood. Extracorporeal carbon dioxide removal
has no advantages in patients with refractory hypoxemia and is rarely
applied in pediatrics patients.
+++
Ventricular
Assist Devices
++
Infants and children with uni- or biventricular failure often
have no intrinsic abnormalities in lung function, and therefore
they might not benefit from an external oxygenator and carbon dioxide
exchanger. In recent years, the medical industry has produced several
increasingly smaller ventricular assist devices that can be used
to support the function of one or both ventricles. Many of these
devices are now practical for use in small children and infants. Potential
indications include cardiogenic shock after bypass surgery and cardiomyopathies.
The availability of implantable devices has considerably improved
the autonomy and quality of life of many pediatric heart transplantation
candidates, often allowing them to be discharged from the hospital
while they await their new organs.