Chapter 14 Pulmonary Vascular Abnormalities
The radiologic appearance of the pulmonary vessels depends on anatomic and physiologic parameters. In previous chapters, we have reviewed the normal pulmonary vascular anatomy on radiographs and on computed tomography (CT) scans. Knowledge of normal vascular anatomy should take into account the physiologic effects of gravity on the radiologic appearance of pulmonary vascularity. Gravitational differences in the lung are posturally dependent and are most striking in the erect position. Gravity has an important effect on the distribution of pulmonary blood flow, resulting in an increase in blood flow from the lung apices to the lung bases in an upright position. A gradual increase in the diameter of pulmonary vessels from the lung apices to the lung bases is seen on radiographs; this increase corresponds to the increasing distribution of pulmonary blood flow.
Gravitational effects are evident in other positions. In the supine position, there is a gradient between the anterior and posterior portions of the lung, which results in increased blood flow to the dependent, posterior portions. However, in contrast to the upright position, blood flow is fairly equivalent in the upper and lower lung zones. On supine chest radiographs, the caliber of vessels in the upper and lower lung zones are nearly equal. In the decubitus position, gravitational effects result in increased blood flow to the lung in the dependent position. A consequence of this phenomenon is unilateral pulmonary edema in the dependent lung of a patient who has been lying in a decubitus position.
In this chapter, we review the characteristic radiologic findings for several pulmonary vascular entities, including pulmonary artery hypertension, pulmonary venous hypertension, congestive heart failure, and pulmonary thromboembolism. When examining the pulmonary vasculature on a chest radiograph, the posturally dependent effect of gravity must always be considered. Subtle alterations in the pulmonary vasculature may be detectable only as a change in appearance from prior radiographs, and whenever possible, current radiographs should be compared with prior studies that were performed in the same position.
PULMONARY ARTERY HYPERTENSION
Pulmonary artery hypertension (PAH) is a condition of sustained elevation of pulmonary artery pressure. The diagnostic criterion for PAH is a systemic pulmonary artery pressure greater than 30 mm Hg or a mean pulmonary artery pressure greater than 20 mm Hg.
PAH may results from one of three basic mechanisms: increased flow of blood through the pulmonary vessels, decreased cross-sectional area of the pulmonary vasculature, and increased resistance to pulmonary venous drainage. These mechanisms provide a convenient framework for categorizing and understanding the large number of causes of PAH (Box 14-1). Another common way to categorize causes of PAH is to broadly divide them into precapillary causes (i.e., entities that result in increased blood flow or decreased cross-sectional area of the pulmonary vasculature) and postcapillary causes (i.e., entities that result in increased resistance to pulmonary venous drainage).
Box 14-1 Causes of Pulmonary Artery Hypertension
INCREASED PULMONARY BLOOD FLOW
Left-to-right shunts (atrial septal defect, patent ductus arteriosus, ventricular septal defect)
Increased total blood volume (thyrotoxicosis, anemia, pregnancy)
DECREASED CROSS-SECTIONAL AREA OF THE PULMONARY VASCULATURE
Resulting from Primary Disease in the Arterial Wall or Lumen
Primary pulmonary hypertension
Resulting from Pulmonary or Pleural Disease
INCREASED RESISTANCE TO PULMONARY VENOUS DRAINAGE
Most cases of PAH have a known cause; these cases are collectively referred to as secondary PAH. In a minority of cases, the cause remains unknown, and this is referred to as primary PAH. Primary PAH usually affects women younger than 40 years. A small percentage of patients with primary PAH have a familial form, which is inherited as an autosomal dominant trait. Affected patients usually present with symptoms of progressive dyspnea and easy fatigability. Approximately 10% of patients present with symptoms of Raynaud’s phenomenon. Because of the nonspecific presenting symptoms and the subtlety of clinical findings early in the course of the disease, the diagnosis is often delayed.
Treatment of primary PAH consists of supportive medical therapy and transplantation (lung or combined heart and lung). Pulmonary vascular disease with clinical and radiologic manifestations similar to primary PAH has been described in association with portal hypertension, human immunodeficiency virus (HIV) infection, cocaine abuse, and appetite-suppressant drugs.
Despite the wide variety of causes of PAH, the salient radiologic features are similar. There is usually marked enlargement of the main and hilar pulmonary arteries, which rapidly taper as they course distally (Fig. 14-1). The most striking feature of PAH is the disparity in size between the central and peripheral pulmonary arteries. Right-sided ventricular cardiac enlargement may occur and is best demonstrated on the lateral chest radiograph.
Figure 14-1 Primary pulmonary artery hypertension. A frontal chest radiograph demonstrates significant enlargement of the main and hilar pulmonary arteries, which rapidly taper as they course distally.
On chest radiographs, enlargement of the main pulmonary artery results in a prominent convex contour. However, the degree of pulmonary artery enlargement varies considerably among patients and conditions. A patient may have significant PAH despite a normal chest radiograph. Compared with chest radiographs, CT scans allow a more accurate determination of the size of the main pulmonary artery, and a diameter greater than 3 cm on CT usually is considered abnormal (Fig. 14-2).
Figure 14-2 Pulmonary artery hypertension resulting from multiple, peripheral pulmonic stenoses. A, A contrast-enhanced, helical CT image of the chest reveals significant enlargement of the main pulmonary artery (MPA), which is approximately 4.5 cm in diameter. The weblike stenosis of the right interlobar pulmonary artery (arrows) is associated with poststenotic dilatation. Numerous additional sites of stenosis were evident on other images. AA, ascending aorta; DA, descending aorta. B, A three-dimensional, shaded-surface display image that was reconstructed from a helical CT acquisition shows the marked disparity in size between the enlarged main pulmonary artery (M) and the normal-caliber ascending aorta (A).
Evaluation of the hilar vessels on chest radiographs is usually a subjective assessment. An objective assessment of hilar vessel enlargement is the measurement of the diameter of the right interlobar artery. On posteroanterior, erect chest radiographs, the normal transverse diameter of the right interlobar artery as it descends adjacent to the bronchus intermedius is less than or equal to 16 mm.
In the setting of long-standing, severe PAH, the enlarged central pulmonary arteries may develop peripheral, atherosclerotic calcification. This is an unusual finding and is most frequently seen in patients with PAH caused by Eisenmenger’s syndrome (Fig. 14-3), a condition characterized by a reversal in the direction of a long-standing, severe left-to-right shunt.
Figure 14-3 Pulmonary artery hypertension in a patient with Eisenmenger’s syndrome. A, A frontal chest radiograph reveals massive enlargement of the main and hilar pulmonary arteries. Notice the presence of peripheral calcification (arrows) within the right interlobar pulmonary artery. B, The peripheral calcification (arrows) is seen in greater detail on the coned-down image of the right hilum. C, An axial image from unenhanced CT of the chest in the same patient demonstrates peripheral calcifications of both pulmonary arteries (arrows). Atheromatous calcification is an unusual finding for pulmonary artery hypertension, and it is most frequently seen in patients with Eisenmenger’s syndrome.
In cases of PAH caused by increased resistance to pulmonary venous return (i.e., postcapillary causes), radiographs show evidence of pulmonary venous hypertension. The most notable finding is cephalization of the pulmonary vasculature, also referred to as recruitment of upper lobe vessels. These terms refer to an increased caliber of the upper lobe vessels, which results from a diversion of blood flow. This is a reversal of the gravity-dependent increased caliber of lower lobe vessels seen on upright radiographs of normal individuals. Cephalization is best evaluated on upright radiographs.
An objective method for assessing cephalization is to examine the relative sizes of the anterior segment artery of either upper lobe and its adjacent bronchus, both of which are usually seen end-on on a posteroanterior, erect chest radiograph. In normal individuals, the diameter of the artery is about the same as that of the adjacent bronchus. Cephalization is diagnosed when the diameter of the artery is larger than that of the adjacent bronchus.
Cephalization of blood flow is not seen exclusively in postcapillary causes of PAH. It can be encountered in cases of PAH produced by precapillary causes. In precapillary PAH, recruitment of upper lobe vessels results from increased resistance in lower lobe pulmonary arteries.
In addition to cephalization, several other radiographic findings may be associated with pulmonary venous hypertension, including interstitial edema (described later), hemosiderosis, and pulmonary fibrosis. Hemosiderosis is related to recurrent episodes of alveolar hemorrhage. When these episodes are severe, hemosiderosis may be detected on chest radiographs as tiny, punctate opacities in the middle and lower lung zones. Pulmonary fibrosis is related to recurrent episodes of pulmonary edema and hemorrhage. Radiographically, it is characterized by reticular opacities in the middle and lower lung zones.
It is important to identify the underlying cause of PAH, because each case has a different treatment regimen. For example, the treatment of patients with primary PAH may include a transplantation procedure, whereas the treatment for patients with secondary PAH from chronic thromboembolism is thromboendarterectomy. In some cases, a careful examination of the chest radiograph for ancillary cardiac, pulmonary, and pleural findings may provide clues to the underlying cause of PAH. In many cases, however, additional imaging studies such as echocardiography, ventilation-perfusion imaging, and pulmonary angiography are necessary to diagnose the precise cause of PAH. In cases of suspected chronic thromboembolic disease, CT and magnetic resonance imaging (MRI) can also be helpful (discussed later).
CONGESTIVE HEART FAILURE
Pulmonary edema refers to the presence of excess extravascular fluid within the interstitial and alveolar compartments of the lung. Under normal conditions, these compartments are kept relatively dry by two main factors: a balance between capillary pressure and plasma oncotic pressure and the maintenance of normal capillary wall permeability. Pulmonary edema usually occurs by means of one of two mechanisms: elevated pulmonary microvascular pressure or increased capillary membrane permeability. The former is referred to as cardiogenic or hydrostatic pulmonary edema, and the latter is referred to as noncardiogenic pulmonary edema.
We focus on cardiogenic pulmonary edema in this chapter. The most common cause of elevated pulmonary microvascular pressure is elevation of pulmonary venous pressure caused by diseases of the left side of the heart. Examples include left ventricular failure, diseases of the mitral valve, and left atrial abnormalities (see Box 14-1).
Pulmonary edema usually follows a typical course. It begins in the interstitial compartment of the lung and extends into the alveolar compartment as it increases in severity. The first phase of pulmonary edema involves the interstitial compartment. It contains two major components: the peribronchovascular sheath and the interlobular septa. Fluid within the peribronchovascular sheath results in indistinctness of the pulmonary vessels and peribronchial cuffing (Fig. 14-4). Fluid within the interlobular septa results in Kerley lines (Table 14-1 and Fig. 14-5).
Figure 14-4 Interstitial pulmonary edema. A, A frontal chest radiograph reveals the presence of interstitial pulmonary edema, manifested by peribronchial cuffing, indistinctness of the pulmonary vessels, Kerley lines, and subpleural edema. B, The peribronchial cuffing (arrows)