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.
Heart-Lung Machine
Heart-lung machine
The heart-lung machine 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 heart is stopped or even removed from his 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 malfunctions at times and requires 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.
At the turn of the 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 blood cells were damaged by the pumps. 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 1953, at Jefferson Medical College in Philadelphia, Dr. John Gibbon connected the circulatory system of an 18-year-old female to a new machine, stopped the woman's heart, and for 26 minutes he 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.
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 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.
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.
heart–lung machine
While in theory it is only necessary to bypass the function of the heart, it soon became apparent that in practice it is simpler to bypass the function of both the heart and the lungs. The main components of a heart–lung machine are a pump (to provide the driving force to the blood in the arterial system), an oxygenator (for exchange of oxygen and carbon dioxide), and a heat exchanger (to allow control of temperature of the body). The connecting tubing and filter are other components of the heart–lung bypass circuit.
Venous blood is siphoned from the body via a tube in the right atrium of the heart, or via two tubes in the major veins which converge on the heart. It is pumped through the oxygenator and heat exchanger, and returned via a plastic tube into the arterial system of the body — usually at the upper portion of the ascending aorta (see blood circulation).
The design of pump which is in most common use today is the roller pump — a simple rotating arm carrying rollers which compress a loop of polymeric tubing against a solid surface. Speed of rotation of the roller-bearing arm is controlled to allow a pumping rate similar to that of the normal heart at rest (about 2.4 litres/min/m2 body surface — or typically about 5 litres/min in an adult).
There are two main types of oxygenator in use at present. ‘Bubble oxygenators’ expose the passing blood to a stream of gaseous bubbles composed of 95% oxygen and 5% carbon dioxide. Gas exchange with the blood occurs on the surface of the bubbles and results in reasonably normal levels of oxygenation of the blood and maintains carbon dioxide in the normal physiological range. The bubble oxygenator has a sponge-like filter and reservoir to enable gaseous bubbles to be removed from the oxygenated blood before it is pumped back to the body.
Membrane oxygenators consist of a series of fine tubes which allow diffusion of oxygen and carbon dioxide between the blood flowing through them and the ventilating gas surrounding them (or vice versa).
The oxygenator also combines with a heat exchanger — a system of tubes through which the blood passes, surrounded by circulating water at controlled temperature. This allows the blood temperature to be maintained (counteracting the heat loss during the passage of blood through the heart–lung machine). It also allows deliberate cooling and subsequent rewarming of the blood, giving the surgeon the option of reducing, or even stopping, the circulation of the blood around the body for a period of time with safety, because the oxygen requirement of the body is reduced by hypothermia.
The connecting tubes, the oxygenator, and the pump tubing are all filled with a physiologically compatible fluid (priming fluid) prior to final connection with the circulation of the body. Avoidance of air bubbles in the heart–lung circuit is of vital importance. Exposure of blood to the foreign surfaces of the heart–lung machine initiates the natural clotting mechanisms of the body, and this must be inhibited by giving the drug heparin to the patient before allowing the circulation to be taken over by the heart–lung machine. Normal blood clotting is restored after the operation by the administration of protamine, which neutralizes the heparin.
The heart–lung machine has made virtually all the advances in cardiac surgery possible. With the function of the heart and lungs taken over temporarily by artificial means it is possible to stop ventilation of the lungs, and to stop the heart, and open the coronary arteries or the cavities of the heart for repair or replacement of the heart valves, or to undertake the correction of congenital abnormalities of the heart.
For periods of up to two or three hours (usually adequate for most surgery) the heart–lung machine is safe; beyond this time there is a risk of increasing damage to the red cells of the blood. Exposure of blood to the foreign surfaces of the artificial circuit initiates an inflammatory response throughout the body, and there is an impairment of function of many organs for a short period after surgery. Nevertheless, the heart–lung machine has become a safe and crucial component of virtually all surgery on the heart and on the major blood vessels around the heart.
D. J. Wheatley
Bibliography
Millner, R. and and Treasure, T. (1995). Explaining cardiac surgery: patient assessment and care. BMJ Publishing Group, London.
Heart-Lung Machine
Heart-Lung Machine
One of the major milestones in medicine was the development of artificial circulation, also known as heart-lung bypass. Before the heart-lung machine was invented, heart surgeons operated blindly, with the heart still pumping, or by slowly chilling the patient's body until circulation nearly stopped, or by connecting the patient's circulatory system to a second person's system during the operation. All of these methods were extremely risky.
Gibbon's Research
An American surgeon named John H. Gibbon Jr. (1903-1974), began pursuing the goal of total artificial circulation in 1931 after a young female patient died of blocked lung circulation. Gibbon realized that it was necessary to keep oxygenated blood circulating without use of the heart, especially to the brain, to carry out careful operations on the heart under direct vision. His pursuit was to last for almost three decades.
After years of intensive experiment, John Gibbon, his wife, Mary, and others were able to construct a heart-lung machine to allow such artificial circulation. On May 6, 1953, surgery using the heart-lung machine was successfully performed on the first human, 18-year-old Cecilia Bavolek, to close a hole between her upper heart chambers. Gibbon's original heart-lung machine was massive, complicated, and difficult to manage. It damaged blood elements and caused bleeding problems and severe consumption of red blood cells. Because of its ability to permit corrective operations to be performed inside the human heart for the first time, however, these drawbacks of heart-lung bypass were considered acceptable. The era of open heart surgery had begun.
Product Improvements
Gradually, the safety and ease of use of heart-lung equipment improved. With today's state-of-the-art machines, minimal blood trauma occurs during heart-lung procedures. It is now commonplace for surgeons to stop the heart from beating for several hours while the circulation is maintained by heart-lung equipment.
Now that patients can be kept alive during heart surgery, a whole new range of operations has become possible. Congenital heart defects (those occurring at birth) can be repaired. Diseased or damaged heart valves can be replaced. Coronary bypass surgery, in which a replacement blood vessel is used to carry blood flow around a blocked section of artery, is commonplace. Thanks to Gibbon's heart-lung machine, open-heart surgery, especially coronary bypass, has become routine throughout the world.
How It Works
Blue blood withdrawn from the upper chambers of the heart is siphoned into a reservoir. It is then pumped through an artificial lung to expose it to oxygen. When the blood is passing through the lung (or oxygenator), it comes into close contact with the surfaces of the machine itself. Oxygen gas is delivered to the interface between the blood and the machine, allowing the blood cells to absorb oxygen molecules directly. The blood is now red in color, owing to its rich content of oxygen. The heart-lung machine next pumps the red blood back into the patient through a tube. The heart-lung circuit is a continuous loop: as the red blood goes into the body, blue blood is drained into the pump.