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heart–lung machine

heart–lung machine Operating on the human heart poses problems which inhibited surgery on the heart until the early 1950s. Manipulation of the heart, and opening of its cavities' interferes with its function and its ability to sustain the circulation. The heart–lung machine is a system which takes over the function of the heart and the lungs with sufficient safety to maintain life while the heart is stopped or opened to allow surgery on the coronary arteries or the heart valves, or to allow repair of congenital abnormalities.

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.

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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.

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heart-lung machine

heart-lung machine, device that maintains the circulation of the blood and the oxygen content of the body when connected with the arteriovenous system; it is also called the pump oxygenator. The machine is used in open-heart surgery when it is necessary to effect a bypass of the circulatory system of the heart and lungs. The oxygenator repeatedly draws off the blood from the veins, reoxygenates it, and pumps it into the arterial system. The contractions of the heart are halted by running a potassium citrate solution through the coronary vessels. The surgeon is thus enabled to open the heart and make the necessary repairs while the heart is still and his view is not obstructed by blood.

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"heart-lung machine." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. 16 Oct. 2017 <http://www.encyclopedia.com>.

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"heart-lung machine." The Columbia Encyclopedia, 6th ed.. . Retrieved October 16, 2017 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/heart-lung-machine

heart-lung machine

heart-lung machine Apparatus used during some surgery to take over the function of the heart and lungs. It consists of a pump to circulate blood around the body and special equipment to add oxygen to the blood and remove carbon dioxide.

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"heart-lung machine." World Encyclopedia. . Retrieved October 16, 2017 from Encyclopedia.com: http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/heart-lung-machine

heart-lung machine

heart-lung machine n. an apparatus for taking over temporarily the functions of both the heart and the lungs during heart surgery. It incorporates a pump, to maintain the circulation, and equipment to oxygenate the blood.

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"heart-lung machine." A Dictionary of Nursing. . Retrieved October 16, 2017 from Encyclopedia.com: http://www.encyclopedia.com/caregiving/dictionaries-thesauruses-pictures-and-press-releases/heart-lung-machine