To appreciate the abnormal patterns of pulmonary vasculature, it is important to be proficient with the details and nuances of normal pulmonary vasculature patterns.
The classic patterns of abnormal pulmonary vasculature include cephalization, centralization, collateralization, lateralization, localization, generalized decreased flow, and overcirculation vascularity.
There are radiographic signs of large and small pulmonary embolism as well as complications of pulmonary embolism, although final diagnosis resides with, usually, contrast-enhanced computed tomography scanning.
Recognition of other right-sided heart disease associated patterns such as those of large and small pulmonary arteries, dilation of the azygous vein, and enlargement of the superior vena cava is clinically useful.
Patterns of Pulmonary Vasculature
The influence of a cardiac lesion on the pulmonary vasculature is indicative of its hemodynamic consequence. Because the pulmonary vasculature is surrounded by air-filled lung, the pulmonary vessels are well defined. The distal one third of the intrapulmonary vessels are normally not apparent because of their small size. The arteries are normally slightly larger than their accompanying bronchi at any distance into the lung ( Graphic 5-1 ).
Normal Pulmonary Vasculature
The left hilum is normally a couple of centimeters more carinal than the right because the left pulmonary artery is slightly raised by the left mainstem bronchus. Pulmonary arteries to the upper lobes run medial and parallel to the veins. Normally, these veins are fairly well defined. Pulmonary veins from the lower lobes run more horizontally than the pulmonary arteries and enter the hila more inferiorly than the arteries leave the hila. The dependent portions of the lung receive greater flow; hence, in the erect chest radiograph, the lower lobe vessels are greater in size than the upper lobe vessels. In fact, the apical arteries are usually only faintly visualized because in the normal individual, there is little blood flow to the lung apices. The chest radiograph of a normal individual lying flat shows equal vessel size in the lower and upper lobes, because gravitational forces increase the flow and vessel size of the posterior lower and upper lobes. Bronchial vessels are normally not visible.
Abnormal Pulmonary Vasculature Patterns
The following categories are useful, because these radiographic descriptive patterns correspond to different pathophysiologic processes. Increased vessel size is noticeable only when vessels are at least twice normal size.
Cephalization: With pulmonary venous hypertension, pulmonary veins become dilated and more visible throughout the lung fields. However, dilated upper lobe pulmonary arteries are the most apparent feature of pulmonary venous hypertension, because lower lobe vessels are constricted (i.e., narrower than the apical vessels) as a result of local vascular reflexes initiated by raised intravascular pressure (>10–15 mm Hg). To maintain pulmonary perfusion in the face of constricted lower lobe vessels, upper lobe vessels become recruited. Such a pattern strongly suggests elevated pulmonary venous pressure. It is not often seen in left-to-right intracardiac and extracardiac shunts. It is a reliable sign only for erect (nonsupine) radiographs, because, for the reasons mentioned previously, equal vessel sizes to the upper and lower lobes are to be expected in supine radiographs.
Centralization: Increased size of the main pulmonary artery and proximal pulmonary arteries with some reduction of peripheral pulmonary vessels is referred to as centralization. It is found in states of precapillary pulmonary hypertension ( Figs. 5-1 to 5-8 ).
Collateralization: This is suggested by oligemia of the lung fields, with evidence of
Bronchial collateralization (vessels seen in the upper and medial lung zones near their origin from the descending aorta)
Intercostal collateralization (indicated by rib notching) (see Chapter 6 )
Lateralization of flow : This is suggested by segmental oligemia and segmental preserved flow. This may be seen in the clinical setting of a pulmonary embolus (Westermark sign), or pulmonary artery anomalies ( Fig. 5-9 ).
Localization of pulmonary vasculature : This usually indicates a pulmonary arteriovenous malformation ( Figs. 5-10 to 5-12 ).
Generalized decreased flow: This occurs with severe congenital right-sided obstructive lesions or advanced pulmonary vascular disease. Reduced blood flow is suggested by abnormal lung lucency and small vessel size.
Overcirculation vascularity: This refers to uniform increase in prominence of central and intrapulmonary vessels. The pattern may be symmetric or asymmetric (asymmetry may be because of a congenital cause or a surgical corrective procedure). Causes of overcirculation vascularity include
Intracardiac (e.g., atrial septal defect, ventral septal defect)
Extracardiac (e.g., patent ductus arteriosus, truncus, transposition, anomalous pulmonary venous return, surgical shunts, other)
High flow states (e.g., hyperthyroidism, pregnancy, anemia)
Obviously, plethoric pulmonary vasculature is always due to a shunt of greater than 2:1 pulmonary-to-systemic flow ratio (Qp:Qs). A less obvious appearance may be from any hyperdynamic state.
Pulmonary Artery Sizing
The most frequent visible branch of the pulmonary arteries is the right descending pulmonary artery. This component of the pulmonary arterial system is the most visible because it is surrounded by air-filled lung and lies free of other radiopaque structures in the heart. It is usually well visualized near the right hilum, lateral and parallel to the lower lobe bronchus and is measured there.
Interlobar Pulmonary Artery
An increase in the pulmonary artery size is seen in states of increased pressure, increased flow, or turbulent flow (i.e., poststenotic dilatation, pulmonary hypertension, atrial septal defect). Normally, the diameters of the bronchi and pulmonary arteries are roughly comparable at any distance into the lung. The interlobar (right) pulmonary artery is typically silhouetted on its medial and lateral aspects on the frontal radiography and can be measured:
Normal: 9–14 mm
Abnormal: ≥17 mm (male or female) (<14 mm is unlikely to have significantly increased flow)
Upper limit normal (males): 15 mm
Upper limit normal (females): 16 mm