Pulmonary hypertension

Dr Dan J Bell and A.Prof Frank Gaillard et al.

Pulmonary hypertension is defined as a resting mean pulmonary arterial pressure of 25 mmHg or greater at right heart catheterisation, which is a haemodynamic feature that is shared by all types of pulmonary hypertension in the Dana Point classification system. A resting mean pulmonary arterial pressure of 20mmHg or less is considered normal, while mean pulmonary arterial pressures ranging between 21-24 mmHg are considered abnormal requiring further investigation about the clinical course of disease.

The use of the term pulmonary arterial hypertension (PAH) is restricted to those with a haemodynamic profile in which high pulmonary pressure results from elevated precapillary pulmonary resistance and normal pulmonary venous pressure and is measured as a pulmonary wedge pressure of 15 mmHg or less. This corresponds to the haemodynamic profiles of groups 3, 4, and 5 in the Dana Point classification system, which was updated during the 4th World Symposium on Pulmonary Hypertension.

Epidemiology varies with the underlying cause, and risk factors (conditions that are associated with pulmonary hypertension, without however a definite causal relationship). Overall, there is a female predilection.

Risk factors include 3

Classical clinical presentation of pulmonary arterial hypertension is the combination of dyspnoea (especially with exercise) with symptoms and signs of elevated right heart pressures, including peripheral oedema and abdominal distention 2-3.

An ECG may demonstrate right ventricular strain and hypertrophy.

PAH is defined as a mean pulmonary arterial pressure >25 mmHg at rest 11 or >30 mmHg with exercise and pulmonary capillary wedge pressure ≤15 mmHg measured by cardiac catheterisation 3-4. It can result from either increased pulmonary venous resistance (most common) or increased pulmonary venous flow, such as with a left-to-right shunt 2.

Even in cases of increased flow, the main factor in generating severe pulmonary hypertension is an arteriopathy, which has four main components 3:

  1. muscular hypertrophy
  2. intimal thickening
  3. adventitial thickening
  4. plexiform lesions: focal proliferation of endothelial channels

The earliest change is muscular hypertrophy in muscular arteries which, over time, results in changes in the more proximal arteries. Eventually fibrosis of the wall occurs, at which point the process is irreversible 2.

In addition to cases of idiopathic pulmonary arterial hypertension, there are numerous known causes, and these can be divided in many ways. A simple unofficial classification is to divide causes of secondary PAH anatomically

See: causes of pulmonary arterial hypertension for a complete list.

Alternative classifications include:

By the time the diagnosis of pulmonary arterial hypertension is made, 90% of patients have an abnormal chest radiograph 3 although sensitivity and specificity is low 12. Features include:

  • elevated cardiac apex due to right ventricular hypertrophy
  • enlarged right atrium
  • prominent pulmonary outflow tract
  • enlarged pulmonary arteries
  • pruning of peripheral pulmonary vessels

High resolution CT (HRCT) chest, of course, is indispensable in assessing the lung parenchyma and to identify possible causative processes (e.g. interstitial lung disease, COPD, etc).

In pulmonary capillary haemangiomatosis small ill-defined centrilobular nodules and interlobular septal thickening may be apparent 3.

Additionally, routine CT chest and CT pulmonary angiography (CTPA) can also identify changes in the pulmonary vasculature and the heart. Features include:

  • enlarged pulmonary trunk (measured at pulmonary artery bifurcation on an axial slice vertical to its long axis)
    • >29 mm diameter is often used as a general predictive cut-off 6,10-11 however a recent study suggests 7:
      • 31.6 mm may be a more statistically robust cut-off in patients without interstitial lung disease (specificity 93%)
      • pulmonary trunk enlargement is a poor predictor of pulmonary hypertension in patients with interstitial lung disease (specificity ~40%)
      • a diameter of <29 mm does not necessarily exclude pulmonary hypertension
    • sex-specific reference values obtained from the Framingham Heart study suggest cut-off values 27 mm for women and 29 mm for men (on ECG-gated non-contrast CT ) 14 
  • main pulmonary artery (pulmonary trunk) to ascending aorta ratio
    • higher ratio correlates with higher PA pressure
    • the ratio obtained on the axial image at the bifurcation of the right pulmonary artery 14
    • adult: normal ratio is less than 1.0
    • children: normal up to a ratio of around 1.09 15
  • enlarged pulmonary arteries
  • mural calcification in central pulmonary arteries 6
  • evidence of previous pulmonary emboli
  • a segmental artery-to-bronchial diameter ratio of 1:1 11-1.25 13 or more in three or four lobes in the presence of a dilated (≥29 mm) main pulmonary artery and absence of significant structural lung disease has a specificity of 100% for the presence of pulmonary hypertension 11,13

ECG-gated CTPA ideally required:

  • right ventricular hypertrophy: defined as wall thickness of >4 mm
  • straightening or bowing (towards the left ventricle) of the interventricular septum
  • right ventricular dilatation (a right ventricle–to–left ventricle diameter ratio of more than 1:1 at the midventricular level on axial images)
  • decreased right ventricular ejection fraction
  • ancillary features
    • dilatation of the inferior vena cava and hepatic veins
    • pericardial effusion
  • centrilobular ground-glass nodules
    • especially common in patients with idiopathic pulmonary arterial hypertension
    • pathologically: represent cholesterol granulomas, which are caused by ingestion of red blood cells by pulmonary macrophages, a result of repeated episodes of pulmonary haemorrhage
  • neovascularity: tiny serpiginous intrapulmonary vessels that often emerge from centrilobular arterioles but do not conform to usual pulmonary arterial anatomy: can be seen as a manifestation of severe pulmonary hypertension

MRI and MR angiography have an increasing role in the management of pulmonary arterial hypertension, as dynamic imaging of both the heart and pulmonary circulation can be achieved.

Traditionally pulmonary angiography has been the gold standard to identify small peripheral occlusions and to better quantify the degree of stenosis. Increasingly this has been replaced by better quality CTPA and cardiac CT.

Care needs also to be taken when performing a pulmonary angiogram in patients with severe pulmonary hypertension.

Medical therapy includes 4

  • calcium channel antagonists
  • nitric oxide
  • prostanoids, e.g. epoprostenol, treprostinil, iloprost
  • endothelin antagonists e.g. bosentan, sitaxsentan, ambrisentan
  • phosphodiesterase inhibitors e.g. dipyridamole 

In selected cases, combined heart and lung transplantation can be performed 5. In patients with very high right heart pressures, an atrial septostomy has also been performed but is associated with high immediate mortality and reduces oxygenation due to the right-left shunt formed.

In cases where pulmonary arterial hypertension is due to proximal pulmonary emboli, pulmonary thromboendarterectomy is a surgical option.

Despite extensive research and recent advances in medical management prognosis remains poor, with a mean survival of only three years in untreated patients 5. Patients typically succumb to right heart failure or sudden death.

As a broad differential on plain film consider

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Article information

rID: 8843
Systems: Chest, Vascular
Section: Pathology
Synonyms or Alternate Spellings:
  • Pulmonary hypertension (PH)
  • Pulmonary arterial hypertension (PAH)
  • PAH
  • Pulmonary arterial hypertension

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