Biomedical Engineering Reference
In-Depth Information
6.4
Blood pumps and circuitry
The complete heart-lung machine includes many additional components.
Most manufacturers consolidate a membrane oxygenator, venous reservoir,
and heat exchanger into one unit. Sites for obtaining blood samples and
sensors for monitoring pressures, temperatures, oxygen saturation, blood
gases, and pH are included, as are various safety devices. A separate circuit
for administering cardioplegic solutions at controlled composition, rate, and
temperature is usually included in the system. Less often a hemoconcentra-
tor (for removal of water and small molecules) is added to the primary
circuit.
6
Most heart-lung machines use two types of pumps, although roller
pumps can be used exclusively. Centrifugal pumps are usually used for the
primary perfusion circuit for safety reasons and a possible reduction in
injury to blood elements. They consist of a vaned impeller or nested,
smooth plastic cones, which, when rotated rapidly, propel blood by cen-
trifugal force. An arterial fl owmeter is required to determine forward
blood fl ow, which varies with the speed of rotation and the afterload of the
arterial line. Roller pumps consist of a length of 1/4 to 5/8 inch (6.3-
15.9 mm) (internal diameter) polyvinyl, silicone, or latex tubing, which are
compressed by two rollers 180° apart, inside a curved raceway. Forward
fl ow is generated by roller compression and fl ow rate depends upon the
diameter of the tubing, rate of rotation, the length of the compression
raceway, and completeness of compression. Compression is adjusted before
use to be barely non-occlusive against a standing column of fl uid that pro-
duces 45-75 mmHg pressure.
7
The various components of the heart-lung
machine are connected by polyvinyl tubing and fl uted polycarbonate con-
nectors. Medical grade polyvinyl chloride tubing is universally used because
it is fl exible, compatible with blood, inert, non-toxic, smooth, non-wettable,
tough, transparent, resistant to kinking and collapse, and can be heat steri-
lized (Fig. 6.2).
6.5
Technologies for moderating infl ammatory
response to cardiopulmonary bypass
The effort to improve biomaterials and to control, limit or eliminate the
adverse effects of blood-biomaterial interaction remains one of the most
active areas of extracorporeal circulation research. Strategies to modulate
the infl ammatory response to CPB may limit morbidity and mortality, and
improve early organ function and clinical outcomes. Past efforts to reduce
the infl ammatory response have consisted of the use of pharmacological
regimens, bioactive coated circuits, elimination of the air-blood interface,
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