INTRODUCTION
Chronic pulmonary thromboembolism (cPE) is a consequence of incomplete resolution of acute emboli in pulmonary arteries.
Physiopathology of pulmonary thromboembolism is complex and involves beyond pulmonary arteries.
Nowadays,
it is considered to be a manifestation of an extensive disease which encompasses deep venous system of the extremities and local mechanisms in pulmonary vasculature.
Therefore,
the term Venous Thromboembolic Disease (VTD) has been suggested to be more adequate.
The term Venous Thromboembolic Disease includes different manifestations of the thromboembolic disease:
- Deep vein thrombosis (acute and chronic).
- Acute pulmonary thromboembolism.
- Chronic pulmonary thromboembolism.
Little is known about cPE; although,
it is the least favourable final stage in the evolution of VTD.
The most worrisome evolution of cPE is development of pulmonary arterial hypertension (PAH).
Pulmonary hypertension is defined as a mean pulmonary arterial pressure >25 mmHg at rest or >30 mmHg with exercise and pulmonary capillary wedge pressure ≤15 mmHg measured by right heart catheterization.
Pulmonary hypertension has many causes.
When secondary to cPE it is called chronic thromboembolic pulmonary hypertension (CEPH).
This PAH is believed to develop due to organization of thrombotic material within the pulmonary arteries (obstructive factor),
which leads to a progressive vascular remodeling of the small vessels.
Both factors together,
obstruction and remodeling will predispose to a progressive elevation of pulmonary vascular resistance.
If it is not treated in time chronic thromboembolic pulmonary hypertension can lead to right heart failure.
Early diagnosis of cPE is challenging in current medicine in order to offer the most appropriate medical or surgical treatment to prevent progression and development of CEPH.
STATISTICS IN PULMONARY THROMBOEMBOLISM AND CHRONIC THROMBOEMBOLIC PULMONARY HYPERTENSION
The incidence of CEPH following an episode of acute pulmonary embolism is not well established.
In 2004 Pengo MD et al estimated that about 4% of patients with a symptomatic episode of pulmonary thromboembolism developed CEPH in the two years following the acute episode.
These ratios have been confirmed in recent studies.
A metaanalysis published in February 2017 (Ende-Verhaar YM et all),
brings a range of 0.52 - 3.2% of patients developing CEPH after acute pulmonary thromboembolism.
It is evident that diagnostic-techniques for detecting these patients in early stages are necessary.
The objective is to avoid development of pulmonary hypertension.
RADIOLOGY IN CHRONIC PULMONARY THROMBOEMBOLISM
Clinically,
cPE is usually undetectable.
It can be asymptomatic or little symptomatic,
due to compensatory mechanisms and coexistence,
many times,
of other cardiopulmonary diseases which mask cPE.
Cross-sectional imaging has become the main diagnostic tool in these patients:
Ventilation/perfusion scan: It is a scintigraphic examination of the lung that evaluates pulmonary perfusion and ventilation.
A normal ventilation scan with perfusion defects suggests pulmonary embolism.
However,
it expresses probability of PE,
and does not give a definitive diagnosis.
It is useful when the result is normal in both perfusion and ventilation because it helps to rule out PE.
Nevertheless,
any pathologic scintigraphic patterns require other confirming diagnostic tests.
Pulmonary arteriography: it is the most specific technique; but as it is invasive,
it should be only used in selected patients.
Computed tomography angiography (CTA); it is currently considered the “standard” imaging method for the diagnosis of PE.
It is able to confirm or rule out emboli in main pulmonary arteries,
as well as in lobar and segmental vessels.
However,
there are two main disadvantages of this technique:
- On the one hand,
specificity and sensitivity decrease significantly in small-size arteries.
cPE in subsegmental arteries may not be detected appropriately.
- On the other hand,
in the diagnosis of chronic pulmonary thromboembolism,
sometimes specific radiologic features are absent,
and radiologist may infer the diagnosis based on indirect signs.
These indirect signs might be misunderstood and erroneously interpreted.
Magnetic Resonance (MR) may also be used to measure lung perfusion,
based on classical sequences (contrast-enhanced MR and phase-contrast MR) and new sequences which allow vascular studies without contrast agent administration (Arterial Spin Labeling).
However,
MR of the lung is extremely challenging due to three main reasons: first,
the low amount of tissue only contains a small number of protons,
which is directly related to low signal.
Second,
the multiple air-fluid interfaces in the lung cause substantial susceptibility artifacts that result in a rapid decay of signal.
Third,
respiratory,
vascular and cardiac motion affect image registration and intrinsic resolution.