Pulmonary Vasculature

Normal Pulmonary Vasculature

Pulmonary vessels are seen in the medial 2/3 of the lung. Vessels are generally not identified in the lateral third. The radiographic appearance of pulmonary vasculature is dependent on technique. Underexposure will lead to prominence of the vasculature, while overexposure will cause an apparent decrease in the vasculature.

Patient position may vary appearances greatly. In the erect position, there is reduced low to the upper lobes due to gravity. Alveolar pressure tends to collapse the upper lobe veins. The normal size ratio of upper to lower lung vessels is 1/2 or 1/3 on the erect PA radiograph. The hilar angle,which is the angle between the superior pulmonary vein and the descending pulmonary artery is normally concave.

a) Arterial

The main pulmonary artery is seen on the PA view, as its left border forms the pulmonary bay.The right pulmonary artery runs horizontally to the right within the mediastinum and is not seen on the frontal view. Its upper lobe branch is given off in the mediastinum. Thus, the descending branch of the right pulmonary artery is the vessel first identified, as it forms the lower part of the right hilum. On the lateral view, it is seen as a rounded density just anterior to the carina. The left pulmonary artery, along with its descending branch are identified on the plain film as it forms part of the left hilum and continues into the lower lobe. On the lateral view, it is seen over the left main bronchus, superior and posterior to the right pulmonary artery. The pulmonary arteries
within the lung parenchyma, are closely related to the bronchi and taper gradually, as they branch.

b) Pulmonary Veins

The right and left upper lobe or superior pulmonary veins descend lateral to the arteries, cross in front of the hilum, and enter the left atrium. The right inferior veins can be distinguished from the arteries as they follow a more horizontal course to the left atrium. On the left side, the inferior veins are more vertical.

Pulmonary Arterial System Changes
a) Pulmonary Plethora
With increased pulmonary arterial blood flow, pulmonary branches are visualized beyond the inner 2/3 of the lungs. Vessels in upper and lower lobes are dilated to the same degree. The number of end on vessels seen is 5 or more in both lung fields (or 3 or more in one lung field).

b) Pulmonary Arterial Hypertension

The features of pulmonary arterial hypertension are:

i)Central arterial enlargement, manifesting as an increased convexity of the pulmonary conus.

ii) Enlarged descending pulmonary artery, of more than 16 mm. Right descending arterial calibre of more than 25 mm is in keeping with Primary Pulmonary Hypertension.

iii) Sharp pruning of peripheral vasculature.

iv) Features of right ventricular hypertrophy and dilatation.

c) Pulmonary Oligemia
With reduced pulmonary flow, pulmonary vascular markings are markedly reduced, with vessels appearing attenuated. This is diagnosed radiographically only in patients with markedly decreased pulmonary flow.

Pulmonary Venous Congestion
In pulmonary venous hypertension, the earliest change is an increase in calibre of the upper lobe vessels. If the upper lobe veins measure more than 3 mm in the first interspace, they reflect an increase in pulmonary venous pressure.

Grading of Pulmonary Venous Hypertension
Grade 1: Diameter of upper zone vessels greater than or equal to lower zone; right hilar angle obliterated.

Grade 2: Interstitial pulmonary edema or pleural effusion; right hilar angle straightened;Kerley B lines and later Kerley A lines.

Grade 3: Alveolar edema; Right hilar angle convex.

Pulmonary Edema

When the capillary pressure exceeds the plasma osmotic pressure, fluid first accumulates in the interstitial spaces. The components of the interstitium (central and peripheral) are shown in Fig.

Components of the Interstitium

The central interstitium invests the bronchovascular bundle and extends from centre to periphery.Fluid accumulating in this perivascular and peribronchial interstitium causes an apparent increase in the size of vessels at the hilum, as well as loss of definition of vessels on the CXR.

The peripheral interstitium consists of subpleural, inter and intralobular septal components. One of the early manifestations of interstitial edema on the CXR are septal lines, commonly Kerley B lines. These are short, straight, horizontal lines, best seen in the lower zones, representing thickening of the interlobular septae .

Other lines described are Kerley A lines (4 to 6 cm, radiating from the hilum, more in the upper zones) and Kerley C lines (short, crisscrossing lines), all representing thickened interstitium.Further fluid accumulation results in edema of alveolar walls and alveolar edema. This characteristically has an “air-space” appearance with coalescent pulmonary opacities, resembling cotton wool. Air space opacification creates a contrast between air filled bronchi and the surrounding lung, and this may produce an air-bronchogram. Typically, there is a perihilar distribution, resulting in a “bats wing appearance”. Rapid clearing with antifailure measures is seen.