Thoracic Imaging 2003 - Society of Thoracic Radiology

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ventricular apex to access blood and with cannulation into the
ascending aorta to return blood to the circulation (4). The
HeartMate was the first implantable heart pump to gain Federal and
Drug Administration approval to bridge patients to transplantation.
The CardioVad
In the early 1970’s, Adrian Kantrowitz implanted a pumping
chamber similar to the intraaortic balloon into the wall of the
descending aorta in three patients. Pneumatically driven by an
external pump, the prosthesis was called the “dynamic aortic
patch.” The clinical trial was terminated because of the problem
of infection. Redesign and further animal studies have apparently
solved the problem of sepsis caused by migration of bacteria
along external tube tracts. A modified version of the Kantrowitz
implantable pumping chamber, the CardioVad (LVAD
Technology, Detroit, Mich.), is under development and clinical
assessment has begun.
Jarvik 2000
The Jarvik 2000 is a relatively new small axial continuous
flow pump that is inserted into the left ventricle. A conduit carries
the pumped blood to the descending thoracic aorta. Early
trials show promise for long term assist with a low complication
rate. The small size of the pump will be very helpful in children
and smaller adults.
Cardiomyoplasty
The long-standing idea of using skeletal muscle to augment
cardiac function has been tested clinically. Dynamic cardiomyoplasty
is a surgical technique involving wrapping of the latissimus
dorsi muscle around the heart to augment contractility.
Electrodes powered by an implantable pulse generator induce
cyclical contraction of the pedicled muscle (5). Alternatively,
the muscle can be sewn into the left ventricular wall as a
replacement for extensively damaged muscle. Dynamic aortoplasty,
wrapping latissimus dorsi around the ascending aorta,
has also shown promise in animals. An additional innovative
experimental technique utilizes skeletal muscle wrapped around
a pumping chamber implanted adjacent to the thoracic aorta.
Muscle contraction is induced during cardiac diastole, the chamber
is compressed, and diastolic pressure and flow are augmented
as blood is squeezed out of the device.
Biventricular Pacing
Patients with moderate to severe heart failure with prolonged
intraventricular conduction delay may have asynchronous ventricular
contraction and a reduction in cardiac output. Prolonged PR
intervals can also contribute to significant atrial-ventricular valve
regurgitation. Pacing both right and left ventricles, biventricular
pacing can significantly improve forward output, and atrioventricular
pacing can reduce tricuspid and mitral valve regurgitation.
Left ventricular pacing can be accomplished by mini-thoracotomy
or thorascopic placement of left ventricular electrodes. A recent
technologic development using a retrograde approach through the
coronary sinus facilitates placement of electrodes in epicardial
veins on the surface of the left ventricle (6).
Total Artificial Heart (TAH)
As of 1992, 116 TAH implants had been reported using at
least eight different models, most commonly using the Symbion
Jarvik-7 device (3). The Jarvik-7 incorporates two artificial ventricles
that are attached to the native atria, the pulmonary artery
and the aorta. Four disk-type artificial valves control blood flow
direction. Each ventricle consists of a ridged housing separated
into blood and air containing spaces by a polyurethane membrane.
An external pneumatic pump connects to the airspace of
each chamber by transthoracic polyurethane tubes. Pressurized
air, cyclically driven into the pumping chamber, pumps the
blood with sufficient cardiac output to totally support circulatory
requirements. Radiographs show a radiolucent crescent inside
each chamber during diastolic filling, whereas during systole,
each air-containing chamber is completely filled producing two
lucent globe-like structures (7). Total permanent replacement
has not been successful because of complications including
mechanical malfunction, thromboembolism, bleeding secondary
to anticoagulation, infection from bacterial seeding along external
tube tracts, and physical compression of adjacent anatomic
structures, particularly pulmonary veins and inferior vena cava.
Successful implantation of a permanent TAH remains a future
goal. Recently, clinical trial of a new total artificial heart built
by Abiomed was begun, although the results are too early to
reach any conclusions.
Respiratory Support Technology
The need for technology to assist pulmonary gas exchange is
another issue, particularly for patients with the adult respiratory
distress syndrome (ARDS). ARDS has a reported mortality rate
of 50 - 90% depending on group selection. Despite vigorous
intensive care, the chances of survival have not significantly
improved since the syndrome was first described. Patients with
severe ARDS can be treated with artificial gas exchange devices
while damaged lung recovers. Extracorporeal membrane oxygenation
(ECMO) technology uses a synthetic semi-permeable
membrane as a blood/gas interface for exchange of oxygen and
carbon dioxide. Investigation of an implantable artificial lung for
long term or even permanent respiratory support is underway.
Extracorporeal Membrane Oxygenation (ECMO)
Neonates with respiratory failure have been successfully
managed with ECMO since 1975, reducing mortality rates from
85% to approximately 15%. This procedure has been modified
and used for the treatment of adults (8). Using selection criteria
which would predict a 90% risk of death, early trials report a
reduction in mortality of approximately 50%. ECMO removes
blood, perfuses it with oxygen and eliminates carbon dioxide,
and returns blood to the patient. Two types of ECMO are used:
veno-venous (VV) and veno-arterial (VA) (26). VV is used for
pure respiratory support. Venous blood is drained from the right
atrium by a cannula introduced into the right internal jugular
vein or occasionally other large systemic veins. Oxygenated
blood is reintroduced into a large peripheral vein, usually a
femoral vein, or directly back into the right atrium by a double
channel catheter. In VA bypass, venous blood is removed as in
VV methods. Blood is returned to the arterial system by a cannula
placed in the right internal carotid artery with the tip in the
aortic arch. Sometimes the blood is returned to the femoral
artery. VA ECMO is physiologically similar to cardiopulmonary
bypass, providing both cardiovascular and pulmonary assist.
Intravenous Oxygenator (IVOX)
The IVOX device is a membrane oxygenator system configured
as a complex of thin hollow fibers mounted on a catheter.
The device is placed in the vena cava and venous blood circulates
through the porous oxygenator while pure oxygen is sup-