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Cardiopulmonary bypass
Cardiopulmonary bypass serves as a temporary substitute for a patient’s heart and lungs during the course of open-heartsurgery. The patient’s blood is pumped through a heart-lung machine for artificial introduction of oxygen and removal of carbon dioxide. Before its first successful application to operations on the human heart in the early 1950s, all heart operations had to be done either by the sense of touch or with the heart open to view but with the patient’s whole body held to a subnormal temperature (hypothermia). The latter procedure was feasible only for very brief periods (less than five minutes).
The first heart-lung machine (pump oxygenator) resembled only slightly the complicated apparatus currently used for correction of cardiac defects. With this machine the blood bypasses the heart and lungs so that the surgeon has an unobstructed view of the operative field. Cardiopulmonary bypass is accomplished by use of large drainage tubes (catheters) inserted in the superior and inferior venae cavae, the large veins that return the blood from the systemic circulation to the right upper chamber of the heart. The deoxygenated blood returning to the heart from the upper and lower portions of the body enters these tubes and by gravity drainage flows into a collecting reservoir on the heart-lung machine. Blood then flows into an oxygenator, the lung component of the machine, where it is exposed to an oxygen-containing gas mixture or oxygen alone. In this manner, oxygen is introduced into the blood, and carbon dioxide is removed in sufficient quantities to make the blood leaving the oxygenator similar to that normally returning to the heart from the lungs.
From the oxygenator, blood is pumped back to the body and returned to the arterial tree through a cannula (small tube) introduced in a major systemic artery, such as the femoral (groin) artery. Oxygenated blood then flows to the vital organs, such as the brain, kidneys, and liver. Meanwhile, the heart may be opened and the corrective operation performed. This procedure permits a surgeon to operate on the heart for many hours, if necessary.
The assemblage and sterilization of the components of the heart-lung machine are essential considerations, because the blood comes in contact with the apparatus outside of the body. Heart-lung machines have totally disposable tubing and plastic bubble oxygenators. Cardiopulmonary bypass is now more often carried out by using cardioplegic solutions designed to provide the heart with the necessary minimal nutrient and electrolyte requirements. Blood is also needed, and administration of an anticoagulant (heparin) prevents clotting of the blood while it is circulating in the heart-lung machine.
Repair of congenital cardiac defects
Most congenital cardiac defects can be repaired surgically. Operations are of two general types: those that can be performed without a heart-lung machine, such as surgeries for patent ductus arteriosus and coarctation of the aorta, and those, such as intracardiac abnormalities, that require a heart-lung machine.
Persistent (patent) ductus arteriosus
The ductus arteriosus is the channel in utero between the pulmonary artery and the first segment of the descending thoracic aorta. Before birth, blood flows from the right ventricle into the pulmonary artery and across the ductus arteriosus to the descending aorta. The ductus shunts blood away from the lungs because oxygen–carbon dioxide exchange begins only at the time of birth. Normally, functional closure of the ductus arteriosus is completed within the first few days after birth, although complete anatomic closure may not occur for several months. If it remains open, excessive levels of blood may flow through the lungs. Ligation of the ductus arteriosus performed by Robert E. Gross in Boston in 1938 was the first successful operation for congenital heart disease and initiated the modern era of cardiac surgery for congenital cardiovascular lesions. Today the ductus arteriosus can be closed with a nonsurgical prosthesis inserted by catheter.
Coarctation of the aorta
Coarctation of the aorta, which is a constriction of the aorta, usually in the same region as the ductus arteriosus, is one of the most common congenital cardiac defects. It was first successfully repaired by Clarence Crafoord in Sweden in 1944. In older children and adolescents the narrowed area is repaired by cutting out the constriction and stitching the two normal ends together. In infants a modified operation is used in which the left subclavian artery (the artery that supplies the left arm) is tied, divided, and used as a flap to repair the narrowed aortic area. With this procedure the stricture has less of a tendency to redevelop at that site. In adults it often may be necessary to bridge the narrowed area with a graft tube, which is attached to the aorta above and below the narrowed segment; the blood is thus able to bypass the constricted area to reach the organs below the defect.
Pulmonary valve stenosis
The most common congenital defect of the valves in children is a narrowing of the pulmonary valve (the valve opening to the pulmonary artery), called pulmonary stenosis. The valve cusps in this condition are not well formed, and, as a result, the valve cannot open normally. The valve cusps are thickened, and the size of the orifice varies in diameter from 1 mm to about two-thirds of the circumference of the pulmonary artery. As a result, blood flow from the right ventricle into the lungs is obstructed. Mild stenosis is usually compatible with normal activities and normal life, but moderate and severe stenosis may result in clear symptoms.
The surgical procedure used to correct this condition is usually performed on cardiopulmonary bypass, with the valve approached through the pulmonary artery and cut in three places to create a valve with three cusps. An alternative approach to surgery is the use of a special balloon catheter, which is passed from the femoral vein (the vein in the groin) into the right side of the heart and positioned across the pulmonary valve. A balloon at the tip of the catheter is then inflated to enlarge the valve orifice.
Aortic valve stenosis
Although mild aortic valve stenosis is manageable in children, deterioration may occur with growth. Severe aortic stenosis in infancy and childhood may be associated with either sudden death or heart failure. The usual basis for the stenosis is fusion of the valve, which is usually bicuspid rather than tricuspid. The valve is often both obstructed and incompetent (allowing blood to leak back from the aorta into the left ventricle). Patients with more than a trivial degree of aortic stenosis usually should not take part in competitive sports, such as swimming or football. In moderate to severe degrees of aortic stenosis, surgery usually is necessary and is performed using cardiopulmonary bypass. The aorta is opened just above the valve, and the surgeon incises the valve sufficiently to convert severe stenosis to a mild or moderate degree of obstruction. In older patients the valve is often thickened and calcified, and it may need to be replaced.
Atrial and ventricular septal defects
If atrial and ventricular septal defects require surgical closure, the patient’s circulation must be supported by the heart-lung machine. Atrial septal defects are usually repaired by sewing the tissue on either side of the defect together, although very large defects may require a patch of material to close the opening. Because of the frequency of spontaneous natural closure, small ventricular septal defects are observed for a period of time before the decision is made to perform surgery. Large ventricular septal defects are usually closed by a patch.
Cyanotic cardiovascular abnormalities
The first attempt to treat “blue babies” affected with cyanotic abnormalities was performed by the American physicians Alfred Blalock and Helen B. Taussig in 1944. This procedure transformed the outlook for cyanotic children and for the first time made survival possible. In the early 1950s, heart-lung cardiac surgery and procedures for repair were developed. Surgical treatment of the tetralogy of Fallot has been an important model for developments in more complex forms of cardiac surgery, and long-term results have been excellent. Most, but not all, forms of cyanotic congenital heart disease can now be repaired, and palliative surgery may produce considerable benefits for those in whom definitive treatment is not possible.