Patent Ductus Arteriosus

Charles E. Mullins

The patent ductus arteriosus (PDA) occurs normally as an essential structure in the fetus and becomes abnormal only when it persists after birth. In the term infant, the persistent patent ductus probably represents a structural abnormality in the ductus tissues present at birth. The persistent patency of the ductus in the premature infant is a more common problem and usually is a result of immaturity of ductal tissues. The ductus in the premature infant is covered in Chapter 53, Cardiovascular Disease in the Newborn, and is not discussed here. Persistent PDA is the second most common congenital heart defect, accounting for approximately 10% of all congenital heart defects in full-term infants.

The walls of the ductus arteriosus are composed of thick, spiraling elastic fibers and smooth muscles, which, when they contract, cause constriction of the lumen of the ducts. In the fetal circulation, the ductus allows right ventricular blood to bypass the nonexpanded and nonventilated lungs. Both the low PO₂ of the blood and a high level of circulating prostaglandins in the fetus inhibit constriction of the ductus. In the normal newborn, lung expansion occurs immediately with delivery. As a result, most of the right heart blood and, in turn, pulmonary artery blood is diverted immediately to the now lower-resistance pulmonary vascular bed. This obligatory flow through the lungs allows circulating prostaglandins in the fetus to be cleared by the most effective clearing system, the lungs, and immediately allows oxygenation of the blood, thereby increasing the circulating PO₂. Both the decreased prostaglandins and the increased blood PO₂ contribute to the normal constriction of the ductus. Normally, the ductus of a newborn functionally is closed by, at most, 72 hours of age and structurally is sealed by 3 months of age.

All factors resulting in persistent patency of the ductus are not understood. Factors such as high altitude or severe pulmonary disease, which cause persistent hypoxia, predispose to persistent patency of the ductus. Continued high prostaglandin levels, in the presence of a compromised or inefficient pulmonary clearing function (found in premature infants or in the marked decrease pulmonary flow occurring in some pulmonary atresia patients), contribute to the persistent patency of the ductus. Rubella (and possibly other viral infections) that occurs during the first trimester of pregnancy frequently results in patency of the ductus. Some evidence shows that a lower socioeconomic status, probably resulting in inadequate perinatal nutrition, may predispose to persistent patency of the ductus.

PATHOPHYSIOLOGY

When the ductus remains open and, with normal lungs, pulmonary resistance drops further, blood flows from the aorta through the ductus into the pulmonary arteries. Eventual flow to the lungs from the ductus depends on the size and shape of the ductus and on how close to normal levels the pulmonary vascular resistance drops. With normal pulmonary resistance, the flow through the ductus begins during mid to late systole and continues through diastole. This flow pattern corresponds to the timing of the maximal pressure gradient between the aorta and the pulmonary artery during the various phases of the cardiac cycle.

In the usual patient with a left aortic arch, the ductus arteriosus connects the junction of the main and left pulmonary artery to the descending thoracic aorta at a point just distal to the origin of the left subclavian artery. The persistent ductus varies in shape from very short and broad-based at both ends to long and tortuous. The diameter of the clinically detectable ductus ranges from less than 1 mm to more than 1 cm at the narrowest diameter and is independent of the shape of the ductus. Clinical finding depends on the final net flow through the lungs
into the left atrium, left ventricle, aorta, and back through the ductus.

The uncomplicated patent ductus places a pure volume workload on the left heart with little or no effect on the right heart. The blood from the aorta flows through the ductus into the pulmonary arteries, through the pulmonary vascular bed into the left atrium, into the left ventricle, and back into the aorta. The total additional workload on the left ventricle depends directly on the size of the persistent ductus arteriosus and the magnitude of the resultant flow through the ductus. The additional blood from the ductus does mix with the blood ejected from the right ventricle into the pulmonary artery; however, in the absence of increased pulmonary resistance, the extra blood does not add significantly to the work of the right ventricle. As a result, in the absence of increased pulmonary resistance, little or no additional volume or pressure work is placed on the right ventricle, and no physical or clinical laboratory signs are present to suggest any right-sided involvement.

CLINICAL MANIFESTATIONS AND COMPLICATIONS

The clinical histories of patients with persistent patent ductus range from florid heart failure in the young infant to the presence of an incidental murmur in an otherwise perfectly healthy child or occasionally, even, in an adult. Occasionally, a patent ductus is discovered coincidentally on an echocardiogram with no associated symptoms or signs, thus suggesting a patent ductus—the so-called “silent ductus.” The patient with a patent ductus presents as early as the newborn period and anytime thereafter, including into late adulthood. The most common presentation of the patient with a persistent patent ductus is a heart murmur discovered incidentally in an asymptomatic young child being examined for some other reason. The infant or child with a moderate-to-large patent ductus may be prone to, or more susceptible to, secondary lower respiratory tract infections after an initial upper airway respiratory infection. This susceptibility presumably is due to the decreased compliance of the lungs associated with the significantly increased pulmonary blood flow.

The murmur and associated clinical finding of a patent ductus usually are characteristic and often pathognomonic of the defect. The typical murmur of a patent ductus is continuous (sounding like machinery) and is maximal in intensity in the first and second left intercostal spaces in the left midclavicular line. The murmur begins with the first heart sound, then crescendos throughout systole until the second heart sound. The murmur peaks in intensity at the second sound before trailing off during diastole. Depending on the shape and size of the ductus, the intensity of the murmur varies from grade 1 to grade 6, and the quality ranges from high-pitched and blowing to low-frequency and rough.

The pulmonary component of the second sound is increased in intensity and delayed in occurrence, which widens the splitting and increases the overall intensity of the second sound, giving it a “slapping sail” quality. The second sound is maximal in intensity in the second and third left intercostal spaces. In the larger ductus with a significant increase in pulmonary flow, an associated left ventricular lift and an apical diastolic flow rumble occur, both of which are caused by the increased volume flow across an otherwise normal mitral valve.

Peripheral pulses are bounding in quality as a result of both the increased left ventricular stroke volume and the diastolic runoff through the ductus into the lungs. This combination generates a wide pulse pressure. In the patient with a large persistent ductus, pulses often are visible in the suprasternal, carotid, axillary, and femoral areas.

Several situations with an isolated ductus exist in which physical findings of the ductus are atypical. In the patient with higher pulmonary resistance, the diastolic runoff into the lungs is decreased or stopped entirely, so the clinical findings resulting from this runoff are not present. The diastolic component of the murmur, in particular, decreases or disappears, the bounding pulses no longer are present, and no mitral flow murmur occurs. These atypical findings may occur in the presence of significant bronchospastic disease or a superimposed severe lower respiratory tract infection.

At the other extreme, in the young infant with a very large ductus and low pulmonary resistance, much of the flow through the ductus into the pulmonary arteries occurs during systole, or the ejection phase of the blood, with little continued flow occurring during diastole. In these patients, the murmur is localized further down the left sternal border, and the diastolic component is decreased markedly or is absent. The tip-off to the presence of a patent ductus in these patients is the persistence of the split and “slapping” second sound and the bounding pulses caused by the wide pulse pressure. The atypical persistent patent ductus requires an accurate echocardiogram and/or, often, a detailed cardiac catheterization for confirmation of the diagnosis.

The electrocardiogram in the patient with the uncomplicated ductus is normal or, in the larger ductus, shows left ventricular hypertrophy and left atrial enlargement. The chest radiograph shows cardiomegaly proportionate to the flow through the ductus, with a prominent main pulmonary artery segment, large ascending aorta and arch, increased pulmonary vascular markings, and a “left ventricular” contour to the heart shadow, with possible left atrial enlargement.

DIAGNOSIS

The diagnosis is supported by the echocardiogram. The ductus usually is seen on two-dimensional echocardiogram. Turbulent flow by Doppler interrogation in the main pulmonary artery supports the echocardiographic findings. Intracardiac lesions other than persistent ductus are ruled out by the echocardiogram, as are lesions that can be confused with the ductus. Continuous-wave Doppler can detect very small streams of abnormal flow in the pulmonary artery. Even the very tiny ductus (the so-called silent ductus), which is too small to be audible or visualized by echocardiogram, can be detected by continuous-wave Doppler, and/or the flow can be seen using color Doppler.

When all clinical findings of the ductus are assimilated and are absolutely characteristic, the diagnosis is established without further study. If even one atypical feature in any part of the clinical assessment is present, then the diagnosis should be established by cardiac catheterization. A cardiac catheterization for a patent ductus should be performed only in a catheterization laboratory capable of treating, as well as diagnosing, the ductus in the catheterization laboratory.

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