Biomedical Engineering Reference
In-Depth Information
of which was his wife, prepared a blood filter and an artificial inner layer of silicon
under the trade name UHB 300. This was applied to all parts of the perfusion ma-
chine, particularly the rough red rubber tubes, to delay clotting and save platelets.
Bjork took the technology to the human testing phase.
Gibbon also performed the first successful open heart operations using heart-
lung machines on human patients in 1953. A 45-minute operation was performed
to correct a defect; the patient made a complete recovery and was alive and well
at least five years later. Initially, Gibbon's machine was massive, complicated, and
difficult to manage. The blood elements were damaged by the machines actions,
causing bleeding problems and severe consumption of red blood cells. However, in
view of its ability to permit corrective operations to be performed inside of the hu-
man heart for the first time, these side effects of the heart-lung bypass were accept-
able. In 1955, British physician Dennis G. Melrose and his colleagues developed a
technique to safely stop the heart and reduce the leakage of blood using potassium
citrate and then potassium chloride. Subsequent experiments by Melrose and Brit-
ish biochemist David J. Hearse, using the isolated hearts of rabbits and rats, estab-
lished optimum concentrations of potassium chloride to stop the heart, and ways
of preserving the heart while starved of blood. It is now commonplace for surgeons
to stop the heartbeat even for several hours while the circulation is maintained by
modern, commercially available heart-lung support equipment.
The safety and ease-of-use of heart-lung equipment has improved since its first
usage. With better understanding of the flow properties of blood (discussed in
Chapter 4) and impact of stress on the components of blood, the major cause of
blood damage in the oxygenator was determined to be the direct exposure of blood
to gases. Interposing a gas exchange membrane of plastic or cellulose between the
flowing blood and the gas solved much of the blood-damage problems. However,
the devices required very large surface areas due to limited gas permeability. In the
1960s, availability of gas permeable thin sheets of silicone rubber (dimethyl polysi-
loxane polymer) changed the design of artificial lungs with potential clinical ap-
plication. By eliminating the gas interface it was possible to use the heart-lung ma-
chine for days at a time. In the early 1980s, an integrated reservoir was introduced
to accomplish oxygenation by more sophisticated methods. The modern heart-lung
machine is actually more sophisticated and versatile, and can perform a number of
other tasks necessary for safe completion of an open-heart operation. First, blood
loss is minimized by scavenging all the blood spilled around the heart using suction
pressure and returning the blood to the pump. Returning shed blood into the heart-
lung machine greatly preserves the patient's own blood throughout the operation.
Nevertheless, risks associated with blood clotting within some internal organs still
persist. Second, the patient's body temperature can be controlled by selectively
cooling or heating the blood as it moves through the heart-lung machine. Thus, the
surgeon can use low body temperatures as a tool to preserve the function of the
heart and other vital organs during the period of artificial circulation. However,
heart-lung machines need a large priming volume (i.e., the amount of blood or
blood substitute required to fill the device prior to use). With the advances in com-
putational fluid dynamics (CFD), a full map of the velocity and pressure changes
in a conduit of complex geometry such as a blood heat exchanger and oxygenator
can be generated. Heat and gas transfer could then be calculated from the CFD
