Thoracic Imaging 2003 - Society of Thoracic Radiology

Imaging of Cardiopulmonary Support Technology
Philip N. Cascade, M.D.
Department of Radiology, University of Michigan
Objectives and Need for Presentation:
To inform radiologists of the clinical use and imaging characteristics
of cardiopulmonary support technology.
Presentation:
Circulatory Support Technology
The first open-heart surgical procedure was performed in
1953 using an extracorporeal pump system that had taken 20
years to develop. During the 1950’s this technology gained
widespread acceptance and open-heart surgery became a routine
procedure. This was the beginning of artificial cardiovascular
support.
Other clinical needs for temporary and even permanent circulatory
assist have since become evident. Up to 4% of patients
undergoing open-heart surgery cannot be weaned from cardiopulmonary
bypass. While the majority of these patients can
be salvaged with modern drug therapy, others require mechanical
cardiopulmonary assist. Cardiogenic shock following acute
myocardial infarction is a more common problem occurring in
approximately 7.5% of patients admitted to hospitals with acute
myocardial infarction. This condition has an 85% or greater
mortality rate. Some of these patients survive with combined
pharmacologic therapy and short-term cardiopulmonary assist;
others require circulatory assist during cardiac catheterization
and subsequent coronary artery angioplasty or coronary bypass
surgery. Assist devices can also be used as a temporary bridge to
transplant in patients with overwhelming acute and irreversible
cardiac failure following myocardial infarction or acute
myocarditis. More than 16,000 patients in the United States
become candidates for heart transplant each year. When patients
55-65 years of age are included the figure rises to >40,000.
Many die before donor hearts become available (5,200 donor
hearts available) (1).
There have been many configurations of circulatory support
devices. Some remain in clinical use, while others have been
abandoned because of unacceptable complication rates. New
technologies are in various stages of development. Following is
a discussion of a variety of cardiopulmonary assist technologies,
their modes of action and their radiographic appearance.
Short Term Circulatory Assist
Intraaortic Balloon Pump (IABP)
The IABP provides partial cardiovascular assist, supplementing
cardiac output by 20 - 30%. This is currently the primary
tool for partial, temporary cardiovascular assist. The IABP is
often in place for days, occasionally for a few weeks and in
experimental situations it has been used for months at a time.
This sausage shaped flexible balloon pumping chamber works
by counterpulsation, inflating and then decompressing out of
phase with the cardiac cycle. By increasing blood pressure during
diastole, the balloon pump increases blood flow to the coronary
arteries and other core organs. By deflating when the aortic
valve opens in systole, there is a significant decrease in oxygen
consumption as myocardial work diminishes with the decrease
in afterload. Placed percutaneously by femoral artery approach,
the IABP is ideally positioned with its tip overlying the aortic
knob on frontal chest radiographs (2). If placed too proximal, the
tip will extend into the left subclavian, or even left vertebral
artery. If placed too distal, the balloon will lie adjacent to the orifices
of the visceral arteries with the potential for microembolism.
Roller Pumps and Centrifugal Pumps
Cardiovascular pumps are simple; relatively inexpensive left
ventricular assist devices. These systems consist of a drainage
cannula, usually placed in the left atrium, the pump itself and a
catheter to return blood to the arterial system either to the aorta
or via a femoral artery (3). The roller pump repeatedly compresses
a narrow tubing filled with blood, creating the force to
propel blood into the arterial system. The action is relatively
traumatic to the blood resulting in a high incidence of hemolysis.
Centrifugal pumps spin blood to create flow by a forced
vortex pumping action. These pumps are less traumatic to blood
and have had greater clinical success than the roller type. Fibrin
deposits and thrombus formation in the pumping chamber limit
duration of use to approximately 5 to 7 days. Centrifugal pumps
generate flow rates of 5 - 6 liters per minute and can thus completely
support the circulation. Both roller and centrifugal
pumps yield continuous, non-pulsatile flow. Most experts
believe that pulsatile flow is clinically superior and for this reason
intraaortic balloon pumps are often placed simultaneously.
The Thoratec device is an example of a pulsatile pumping apparatus
that can be used for short-term assist for one or both ventricular
chambers simultaneously.
Hemopump
The Hemopump is a left ventricular assist device that works
on the Archimedean principle (3). This concept has been known
for centuries, that a rapidly rotating screw confined within a
tube will propel fluid. With rotational speeds of 25,000 per
minute, the pump withdraws a stream of oxygenated blood from
the left ventricle and pumps it into the aorta. Flow rates can
supplement up to 80% of resting cardiac output.
Thrombocytopenia, footdrop and peripheral embolism are the
most frequent complications. Hemolysis occurs but has not been
reported to be clinically significant.
Permanent/Long term Circulatory Assist
There are two basic functional types of pulsatile ventricular
assist devices (VAD). Those driven pneumatically and those that
use electromagnetic power. Air powered assist devices work by
pulsing bursts of compressed air into a ridged chamber, inflating
and deflating a polyurethane sack to produce pulsatile blood
flow. Electromechanically driven assist devices use opposing
pusher plates to compress a seamless polyurethane sack, propelling
blood in a pulsatile fashion.
The HeartMate Device
The Heart-Mate device is a pump that can be implanted in
the left upper quadrant of a patient with cannulation of the left
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