Normal pulmonary circulation is a high-flow, low-resistance circuit capable of accommodating the entire right ventricular output at one fifth the pressure of the systemic circulation level. The thin-walled right ventricle functions primarily as a flow-generator pump and is particularly sensitive to increases in its afterload. Increased pulmonary artery pressure and pulmonary vascular resistance characterize pulmonary hypertension. See the images below.
Pulmonary hypertension may be divided into primary and secondary forms. Primary pulmonary hypertension (PPH) is a disease of unknown etiology, whereas secondary pulmonary arterial hypertension (SPAH) is due to either intrinsic parenchymal disease of the lung or disease extrinsic to the lung.
In an individual with suspected pulmonary hypertension, PPH is the diagnosis of exclusion. Hence, a designated algorithm should be used to exclude secondary causes of pulmonary hypertension. The following are proposed investigations:
Autoantibody tests, HIV test, liver function tests
Pulmonary function tests
Ventilation-perfusion (V/Q) scanning
Computed tomographic pulmonary angiography (CTPA)
Magnetic resonance imaging (MRI)
As a baseline, the first 5 tests are reasonable for substantiating the presence of increased pressures on the right side of the pulmonary vascular system. On the basis of the results, a more focused approach to establish the etiology of the pulmonary hypertension may then involve tests 6-9. Most patients with secondary pulmonary hypertension do not require right-heart catheterization before beginning a trial with vasodilators. However, select patients, such as those with collagen vascular disease, should undergo invasive investigations.
The purposes of imaging studies in individuals with pulmonary hypertension are as follows:
Detecting pulmonary hypertension (echocardiography, cardiac catheterization)
Differentiating the cause (echocardiography, V/Q scanning, CTPA, cardiac catheterization, pulmonary angiography, MRI)
Determining the severity (echocardiography, cardiac catheterization)
Evaluating the status of the right ventricle (echocardiography, cardiac catheterization, MRI)
Various imaging modalities, including chest radiography, CT, MRI, pulmonary function tests, echocardiography, and angiography, have variable success in detecting the presence and severity of pulmonary hypertension.
Electrocardiograms are useful for demonstrating signs of right ventricular hypertrophy and strain. These signs include the following:
R/S ratio greater than 1 in lead V1
Right atrial enlargement as indicated by an increased P-wave amplitude in lead II
Right bundle branch block
PPH is diagnosed if no underlying etiology is found.
Chest radiographs reveal a number of findings in pulmonary hypertension. (See the image below.)
A dilated right heart due to right ventricular enlargement is seen as a decrease in the retrosternal space on the lateral image. Right atrial enlargement may be present if significant tricuspid regurgitation is present. This finding may be recognized by the widened right heart border in the frontal projection and enlargement of the right atrial appendage in the lateral projection, which is seen as increased retrosternal opacity above the expected location of the right ventricle.
Enlarged central pulmonary arteries that taper distally may be found.
Peripheral vessel opacity-oligemic lung fields may be observed. This oligemia may be asymmetrical. This is also a useful finding suggestive of chronic pulmonary embolism (PE).
An increase in the transverse diameter of the right interlobar artery is indicative of pulmonary hypertension. The upper limit of the transverse diameter of the right interlobar artery from its lateral aspect to the intermediate bronchus is 15 mm in women and 16 mm in men.
The transverse diameter of the left interlobar artery is difficult to appreciate on the posteroanterior (PA) view. Using a lateral view, one can measure from the circular lucency of the left upper lobe bronchus to the posterior margin of the vessel; the upper limit is 18 mm.
Evidence of right atrial dilatation and right ventricular dilatation may be noted.
Kerley B lines may be present. These indicate the presence of pulmonary venous hypertension.
Causes of SPAH and their appearances
Atrial septal defect is characterized by cardiomegaly, enlargement of the right atrium and ventricle, and enlargement of the central pulmonary arteries. Also present is rapid tapering, with increased vascularity in the periphery of the lung.
Eisenmenger syndrome is characterized by peripheral oligemia, indicating the reversal of a left-to-right shunt due to increased vascular resistance. The peripheral vasculature is diminished, and the cardiac morphology is consistent with cor pulmonale.
CT scanning is a useful, noninvasive procedure for confirming the presence of pulmonary hypertension.
CTPA is useful in delineating the anatomic detail of the pulmonary vasculature (see the image below). Contrast-enhanced images may show intraluminal abnormalities in the arteries and veins, which are useful for confirming etiologies such as thromboembolic disease. [1, 2, 3, 4, 5, 6]
Sometimes, if the emboli are large, they may be seen in the pulmonary arteries on a routine contrast-enhanced CT scans.
CTPA, and often conventional pulmonary arteriography, is necessary for an adequate evaluation of chronic PE. The technique of CTPA includes a rapid infusion at 3-4 mL/s for a total of 80-120 mL. Careful review of the images on a workstation is the optimal method of detecting subtle emboli. Evaluation of the lung windows to detect abnormalities of perfusion (mosaic perfusion) is necessary, as occlusions of major pulmonary arterial branches may not be present in a subset of patients with chronic pulmonary thromboembolism.
High-resolution CT (HRCT) scanning of the chest has a role in the evaluation of pulmonary hypertension in patients with suspected diffuse lung disease, eg, in patients with collagen vascular disease.
Kuriyama et al determined that a main pulmonary artery of 29 mm or larger, as shown on a CT scan, has a sensitivity of 69% and a specificity of 100% for predicting pulmonary hypertension. The widest portion of the main pulmonary artery within 3 cm of the bifurcation was used to determine the value. In addition, Tan et al demonstrated that individuals with intrinsic lung disease can be identified as having pulmonary hypertension if the main pulmonary artery is greater than 29 mm.
The upper limit of normal for the diameter of the pulmonary artery is 28.6 mm. A value greater than 28.6 mm suggests increased pressures in the pulmonary system. (See the image below.)
Degree of confidence
CTPA is the best method for demonstrating emboli. HRCT is useful for demonstrating lung disease, which may account for secondary pulmonary hypertension.
Magnetic Resonance Imaging
MRI has capabilities similar to those of echocardiography in the diagnosis and treatment of patients with pulmonary hypertension. MRI is useful for measuring the mass, volume, and overall function of the right ventricle. MRI is also useful for detecting shunt lesions contributing to pulmonary hypertension. Acute and chronic pulmonary thromboembolic disease can be confirmed by using this imaging modality.
Degree of confidence
The disadvantages with MRI include limitations in individuals with cardiac pacemakers and defibrillators, its limited availability and cost, and difficulty in assessing estimate PA pressures with MRI.
In evaluating pulmonary hypertension, echocardiography can be used to identify secondary causes, such as congenital, valvular, and myocardial disease. In addition, one may estimate pulmonary artery systolic pressure with this method.  Specifically, the World Health Organization (WHO) has defined pulmonary hypertension as a systolic pressure greater than 30 mm Hg; this corresponds to a tricuspid regurgitant velocity of 3.0 m/s on echocardiography.
The following findings for pulmonary hypertension are confirmed on echocardiography: (1) right atrial and ventricular enlargement, and (2) paradoxical movement of the interventricular septum, and tricuspid regurgitation. (If right ventricular end-diastolic pressures are greater than 20 mm hg, the risk of sudden death is relatively high.)
Doppler echocardiography is the most reliable noninvasive method for estimating pulmonary artery pressure. Tricuspid regurgitation is often present in pulmonary hypertension. It is detected in more than 90% of patients with severe pulmonary hypertension, with a correlation of more than 95% when the pressure is measured by means of catheterization. [11, 12]
Radioisotope perfusion lung scanning is performed with intravenous injection of particles of albumin labeled with technetium-99m. As these particles perfuse the lung, the lungs are imaged by using a gamma camera to obtain anterior, posterior, lateral, and oblique views. One would expect a normal distribution of these particles, which produce 2 blackened lung-shaped shadows.
In PPH, the V/Q scan is usually normal.
In secondary pulmonary hypertension due to chronic thromboembolic disease, emboli are responsible for blocking the branches of the pulmonary artery. The lung tissue peripheral to the block is not perfused; this block results in a defect on the scan. When findings in the perfusion scan are abnormal, a ventilation scan is obtained next by using inhalational radioactive xenon-133. A number of lung diseases, including pneumonia and COPD, can cause alterations in the ventilation component, whereas uncomplicated PE does not. Thus, a patient with a high likelihood of PE has an abnormal perfusion scan with a normal ventilation component. (See the image below.)
Right heart catheterization may be required. Pulmonary angiography is the most accurate modality for evaluating the anatomy and pathophysiology of pulmonary hypertension. Berberich and Hirsch obtained the first pulmonary angiogram in 1923, and Robb and Steinberg later optimized the technique in 1931. This examination is the criterion standard for the diagnosis of pulmonary hypertension. (See the images below.)
Pulmonary angiography should be performed when the results of V/Q scanning cannot exclude chronic thromboembolic disease as an etiology for the elevated pulmonary pressures. The angiogram reveals large central pulmonary arteries with marked peripheral tapering.
The disadvantage of pulmonary angiography is that it is an invasive procedure as one cannulates the right side of the heart and the pulmonary artery. Complications include (1) transient rhythm disturbances that respond to the removal of the catheter, (2) cardiac perforation leading to pericardial tamponade, and (3) nephrotoxicity. In patients with an elevated creatinine level (1.5 X normal), the use of high-osmolar contrast material poses a risk of nephrotoxicity. This problem is less commonly seen with the selective use of small amounts of low-osmolar nonionic contrast material. The frequency of complications with pulmonary angiography is limited to < 5% of cases examined.