Heart-Lung Machine
Heart-Lung Machine
The heart-lung machine (HLM) is a device used to provide blood circulation and oxygenation while the heart is stopped. It is a means of keeping a patient alive while his/her heart is stopped or even removed from the body. Usually called the heart-lung machine, the device also is referred to as cardiopulmonary bypass, indicating its function as a means to substitute for the normal functions of the heart (cardio) and lungs (pulmonary).
It is the function of the heart to provide circulation of blood at all times. It pushes blood out into the body and through the lungs. It must function every minute of every day of life to maintain the health of the tissues throughout the body.
The heart can malfunction at times, and require surgery to correct the problem. Surgeons searched for a means to stop the heart so they could correct defects yet keep the patient alive by circulating blood by another means. For many years, no such means could be found. Some heart surgery was carried out while the organ still pumped, making delicate surgery virtually impossible. Surgeons then discovered that they could stop the heart by lowering the patient’s body temperature, a condition called hypothermia, and by flooding the heart with a cold solution. In its state of artificial hibernation the body needed less blood circulation, but at best that gave surgeons only a few brief moments to carry out the surgery. They were still limited as to the procedures they could do because of the severe time constraints.
History
At the beginning of the twentieth century, German scientists were studying isolated animal organs such as the liver and kidney and the effects that various drugs had on them. To do this they required the organ to be kept alive, meaning supplied with blood. They attempted various contrivances using syringes and pumps to maintain the viability of the organs. They experienced severe problems with blood clotting and changes in blood composition when the pumps damaged the blood cells. The researchers searched vainly for a means to provide oxygenated blood to their organ preparations. They filtered the blood through various screens and membranes and even pumped it through the lungs of dogs or monkeys, but their problem was not to be solved for decades, though this may be considered the beginning of research into a heart-lung device.
In 1931, American surgeon John H. Gibbon, Jr. (1903–1974) decided to build a heart-lung machine after a young female patient died of blocked lung circulation. At the time, experimental devices existed for pumping and oxygenating blood during perfusion (artificial circulation). Gibbon, who received his bachelor’s degree from Princeton University (New Jersey) in 1923 and his medical degree in 1927 from Jefferson Medical College (Philadelphia, Pennsylvania), began his heart-lung work in 1934 at Massachusetts General Hospital in Boston (Massachusetts). The work involved a research fellowship in which Gibbon had assistance from research technician Mary Hopkinson. They found the action of roller pumps gentle enough to minimize both clotting and damage to blood cells, and they employed centrifugal force to spread the blood in a layer thin enough to absorb the required amounts of oxygen. In 1935, the Gibbons went to the University of Pennsylvania’s School of Medicine and continued their experiments, reporting successful results with animals by 1939. In 1946, Gibbon became head of the surgical department at his alma mater and soon secured the backing of Thomas J. Watson, chairperson of International Business Machines Corporation (IBM). With the use of IBM laboratories and engineers, Gibbon’s heart-lung machine was perfected after the introduction of wire-mesh screens to enhance oxygenation, filters to block air bubbles or clots, and monitoring devices.
In 1953, at Jefferson Medical College, Gibbon connected the circulatory system of an 18-year-old female to a new machine, stopped the woman’s heart, and for 26 minutes performed surgery to close a hole in the wall of the heart between the left and right atria. It was the first successful use of a heart-lung machine and the beginning of a new era in cardiac surgery. The machine was not a sudden inspiration by anyone, but rather was the culmination of many years of dedicated research in many laboratories to find the means to oxygenate the blood and circulate it through the body.
That early machine, while functional, still was open to improvement. For one thing, it required many pints of blood to prime the machine and it was bulky and took up much of the room in the operating room. Since then, the size of the machine has been reduced and the need for blood to prime the machine has been dramatically reduced to only a few pints. In addition, the heart-lung machine was rapidly improved in other areas. Oxygenation, for example, was accomplished by more sophisticated methods. Once patients could be kept alive during heart surgery, a whole new range of operations became possible. Congenital heart defects could be repaired, diseased or damaged valves could be replaced, and coronary bypass surgery became possible through sewing in a replacement blood vessel to carry blood flow around a blocked section of artery. Thanks to Gibbon’s heart-lung machine, open-heart surgery—especially coronary bypass—has become routine throughout the world.
Function
To function, the heart-lung machine must be connected to the patient in a way that allows blood to be removed, processed, and returned to the body. Therefore, it requires two hook-ups. One is to a large artery where fresh blood can be pumped back into the body. The other is to a major vein where used blood can be removed from the body and passed through the machine.
In fact, connections are made on the right side of the heart to the inferior and superior vena cavae (singular: vena cava). These vessels collect blood drained from the body and head and empty into the right atrium. They carry blood that has been circulated
KEY TERMS
Atrium (plural: atria) —One of two upper chambers of the heart. They are receiving units that hold blood to be pumped into the lower chambers, the ventricles.
Isolated organs —Organs removed from an animal’s body for study. In this way, their function can be determined without influence by other organs.
Oxygenation —Supplying oxygen to blood to be circulated throughout the body.
through the body and is in need of oxygenation. Another connection is made by shunting into the aorta, the main artery leading from the heart to the body, or the femoral artery, a large artery in the upper leg. Blood is removed from the vena cavae, passed into the heart-lung machine where it is cooled to lower the patient’s body temperature, which reduces the tissues’ need for blood. The blood receives oxygen which forces out the carbon dioxide and it is filtered to remove any detritus that should not be in the circulation such as small clots. The processed blood then goes back into the patient in the aorta or femoral artery.
During surgery the technician monitoring the heart-lung machine carefully watches the temperature of the blood, the pressure at which it is being pumped, its oxygen content, and other measurements. When the surgeon nears the end of the procedure the technician will increase the temperature of the heat exchanger in the machine to allow the blood to warm. This will restore the normal body heat to the patient before he is taken off the machine.
See also Respiratory system; Thoracic surgery.