Biomedical Engineering Reference
In-Depth Information
ventilation gases. Oxygen diffuses from the gas mixture into the blood, and carbon dioxide
diffuses the other way.
When blood is ready to be returned from the heart-lung machine to the patient, it
must first pass through an arterial line filter. This device is used to filter small air bubbles
that may have entered or been generated by the heart-lung machine. Finally, the blood is
returned to the arterial cannula and back into circulation.
In addition to the previously described flow process, fluid being returned from the left
ventricle or via surgical suction requires filtration before being reintroduced into the heart-
lung machine. Blood enters a filtered reservoir, called a cardiotomy, which is connected
with tubing to the venous reservoir. Other fluids such as blood products and medications
are also added into the cardiotomy for filtration of particulates.
A heat exchanger allows body and organ temperatures to be controlled. The simplest
heat exchange design is a coiled tube immersed in a water bath. As the blood passes through
the coil, the blood temperature will shift toward that of the water. A more sophisticated
system separates the blood and water with a metallic barrier. Once the blood is returned
to the body, it will alter the temperature of the surrounding tissue. A closed-loop control
system adjusts the water temperature to maintain body temperature.
Because respiration is being controlled and a machine is meeting metabolic demand,
it is necessary to monitor the patient's blood chemical makeup. Chemical sensors placed
in the blood path detect the amount of oxygen bound to hemoglobin, while other, more
elaborate sensors constantly measure and plot the blood pH, partial pressure of oxygen
and carbon dioxide, and electrolyte levels. This constant trending can quickly analyze
the metabolic demands of the body to allow the appropriate adjustments to be made
(Encyclopedia of Surgery, 2007).
From this description and the photograph shown in Figure 8-10, it is obvious that
heart-lung machines have progressed significantly since their debut in 1952 and are now
extremely sophisticated pieces of equipment.
FIGURE 8-10
Modern heart-lung
machine. (Courtesy
of Jörg Schulze with
permission.)
