The Heart-Lung Machine
Mark M. Levinson, MD
Hutchinson Hospital, Hutchinson Kansas
One of the major advances in medicine is the invention and refinement of artificial circulation, also known as heart-lung bypass. The efforts to bring this concept into reality began many decades ago. One of Americas most famous folk-heros, Charles Lindbergh, was partly responsible for the eventual development of artificial circulation. Years after his pioneering trans-atlantic flight, Lindbergh teamed up with Dr. Alexis Carrel, the foremost pioneer of vascular surgery. The two of them had a mutual interest in finding a way to put oxygen into the blood stream. A primitive oxygen exchange device was made, confirming that it was possible for machinery to perform the function of the human lung.
Another intrepid scientist, Dr. John H. Gibbons Jr. (image at right), pursued the goal of total artificial circulation for almost 3 decades in his laboratory at the Jefferson Medical College in Philadelphia.
Dr. Gibbon is credited with developing the first clinically successful heart-lung pump. He performed the first successful use of artificial circulation in humans on May 6th, 1953 as he closed a hole between the upper heart chambers in an 18 year old girl. Initially, the machine of Gibbon 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 human heart for the first time, these side effects of heart-lung bypass were acceptable. Gradually, the safety and ease-of-use of heart-lung equipment has improved. At the present state-of-the-art, minimal blood trauma occurs during conventional heart-lung support periods. This allows surgeons to apply this technology freely with excellent overall results. It is now commonplace for surgeons to stop the heart beat even for several hours while the circulation is maintained by modern, commercially available heart-lung support equipment.
The principle of the heart-lung machine (also known as pump-oxygenator or cardiopulmonary bypass) is actually quite simple. Blue blood withdrawn from the upper heart chambers is drained (by gravity siphon) into a resevoir. From there, the blood is pumped through an artificial lung. This component is designed to expose the blood to oxygen. As the blood passes through the artificial lung (also known as an oxygenator), the blood comes into intimate contact with the fine surfaces of the device itself. Oxygen gas is delivered to the interface between the blood and the device, permitting the blood cells to absorb oxygen molecules directly. Now the blood is red in color, indicating its rich content of oxygen destined to be delivered to the various tissues of the body. Finally, the heart-lung machine actively pumps the red blood back into the patient through a tube connected to the arterial circulation. The heart-lung circuit is a continous loop; as the red blood goes into the body, blue blood returns from the body and is drained into the pump completing the circuit.
The modern heart-lung machine is actually more sophisticated and versatile than the overview given above. In fact, the pump-oxygenator can do a number of other tasks necessary for safe completion of an open heart operation. Firstly, any blood which escapes the circulation and spills into the operating field around the heart can be suctioned and returned to the pump. This scavenging feature is made possible because the blood has been rendered incapable of clotting by large doses of heparin. Returning shed blood into the heart-lung machine greatly preserves the patients own blood stores throughout the operation. Secondly, the patients 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. And the bypass pump has connectors into which medications and anesthetic drugs can be given. In this way, medications arrive to the patient almost instantly by simply adding them to the blood within the heart-lung resevoir.
To initiate heart-lung bypass, the surgeon must first impair the bodies own clotting system. Otherwise, the patients blood would immediately clot upon exposure to the plastic tubing and artificial surfaces inside the heart-lung machine itself. Thinning of the blood (or anticoagulation) is done by first administering a powerful anticoagulant called heparin. Once clotting is impaired, a large drainage tube is placed in the upper chamber of the heart (called the right atrium). This tube drains the blue blood from the patient into the heart-lung machine. Then a smaller tube is placed into the arterial system so that red blood can be returned to the patients body where it is needed. The most common site for this tube is in the aorta (just above the place where the vein grafts will be connected). The photograph below illustrates the standard heart-lung tubing (also known as cannulas) after they were inserted into a patient undergoing CABG, but before artificial circulation has been established.
Once these "cannulae" are placed into the patient, and connections are made to the heart lung machine (via plastic extension tubing), the machine can now take over the pumping action of the heart, and the oxygen transfer action of the lungs, as described above. In the photograph below, the actual heart-lung machine and its attached equipment are easily seen.
During the period of artificial circulation, the machine is attended to at all times by a specialized technician, called the perfusionist. These individuals are highly trained in all aspects of artificial circulation and the equipment involved. Each heart operation requires a dedicated and trained perfusionist to manage all aspects of the heart-lung machine during the time period of artificial circulation. The perfusionist maintains the function of the pump and also monitors the delivery of blood back to the patient, the blood oxygen content, and other vital parameters. During this time, the surgeon is then freed up to concentrate on the technical aspects of the heart operation itself. The operation of the heart-lung machine is an excellent example of team work in action.
In this photograph, it is easy to see the plastic tubing used to connect the heart-lung machine to the patient during an actual surgery. As the surgeon repairs the heart, the perfusionist maintains the flow of oxygen-rich red blood to the rest of the body.
After the technical reconstruction of the heart is completed, the heart beat is restored. As the strength of the hearts own contractions gets stronger, the heart-lung machine can be gradually withdrawn. Then the tubes are removed from the patients circulation, and the anticoagulation medication is reversed with an antidote drug called "protamine".
For a further details on the surgical treatment of coronary artery disease, visit the page discussing Coronary Artery Bypass Surgery in The Learning Center.
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