ReviewComputed tomography signs of pulmonary hypertension: old and new observations
Introduction
Raised pulmonary artery (PA) pressure can be an idiopathic disorder or more commonly the consequence of many disparate clinical conditions. A mean PA pressure (mPAP) of >25 mm Hg at rest in the presence of a normal left atrial pressure is a widely accepted criterion for the diagnosis of pulmonary hypertension (PH),1 with right heart catheterization (RHC) the reference standard test for its diagnosis. The presenting symptoms of PH such as dyspnoea or peripheral oedema are entirely non-specific1 and in certain instances (such as in patients with interstitial lung disease) the symptoms can frequently be difficult to distinguish from those of the underlying condition. Because many patients with these non-specific symptoms or with predisposing underlying respiratory conditions routinely undergo thoracic computed tomography (CT), the radiologist may be in a position to be the first to suggest a diagnosis of PH, particularly when the limitations of other non-invasive tests are appreciated. What follows is a review of the role of CT in the detection of PH, in particular the value of and pitfalls associated with certain CT signs of raised PA pressure. At the outset the importance of diagnosing PH is considered and a brief review of the reliability of other tests used in the investigation of PH is also provided.
Section snippets
The incidence and importance of PH
The incidence of PH depends on the population under consideration. In the whole population PH is a rare disease with 2.5 to four cases of idiopathic pulmonary arterial hypertension (IPAH; formerly referred to as primary PH) per million individuals in the UK.2 By contrast, if a population of adult patients with known congenital heart disease is studied, as many as one third of patients will have raised PA pressure.3
PH is of importance when considering patients with underlying predisposing
Methods of detecting PH
The most accurate test for determining mPAP remains RHC; however, before carrying out this invasive test, numerous non-invasive investigations are available that may suggest the diagnosis. Prior to discussion of the CT signs of PH, this article will briefly review the role of chest radiography, echocardiography, and magnetic resonance imaging (MRI) in detecting PH.
The classic chest radiographic changes in PH are dilatation of the central PAs and pruning of the peripheral arteries.11, 12
CT signs of PH
Unlike other tests, CT is capable of portraying various parts of the pulmonary vasculature starting with the right ventricle and main pulmonary artery, extending out to the peripheral PAs and pulmonary parenchyma.
Accuracy issues
PH occurs as a consequence of abnormalities principally sited at the level of the distal pulmonary arterial tree. For example, classically in IPAH the cause is occlusion and remodelling of vessels <0.2 mm in diameter.68 This is of importance given that new therapies aimed at treating PH work by their action on the microcirculation, yet the state of the small vessels is not reliably assessed by RHC, or indeed any test, short of lung biopsy. Nevertheless, RHC is regarded as the reference standard
Applicability of CT signs of PH
Of the several CT signs of PH described in this review, the size of the main PA at the level of the PA bifurcation is probably the most widely used, and most reliable. However, in the setting of patients with no known underlying predisposing condition it is uncommon for a radiologist to be the first to confidently diagnose PH using this, or indeed any other CT sign, in isolation. In cases where, for example, the main PA is massively dilated or there is severe reflux of contrast on CTPA into the
Conclusion
Knowledge of the various CT signs of raised PA pressure, as well as the caveats associated with them may enable the radiologist to suggest the diagnosis of PH, especially in patients with predisposing underlying diseases. Information from CT can be used in conjunction with other tests to stratify patients for further investigation, and with the advent of new therapeutic agents to treat PH, the threshold to perform RHC is now lower than it was previously. Given the lack of a single reliable
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