Definition of Segmental Pulmonary Embolism
Segmental pulmonary embolism (PE) is defined as a thrombus located in the segmental branches of the pulmonary arterial tree, which is more central than subsegmental arteries but more peripheral than lobar arteries.
Anatomical Classification of Pulmonary Embolism
Pulmonary embolism is classified based on the location of the thrombus in the pulmonary arterial tree:
- Central PE: Thrombi in the main pulmonary arteries
- Lobar PE: Thrombi in the lobar pulmonary arteries
- Segmental PE: Thrombi in the segmental branches of the pulmonary arteries
- Subsegmental PE: Thrombi located exclusively in the small peripheral branches (typically <1-2 mm in diameter) 1
Diagnostic Significance
The location of PE has important implications for both diagnosis and management:
Diagnostic accuracy: CT pulmonary angiography has excellent sensitivity for detecting emboli in main, lobar, and segmental arteries, but limited sensitivity for subsegmental and more peripheral arteries 2
Diagnostic confirmation: According to European Society of Cardiology guidelines, SDCT (single-detector CT) or MDCT (multi-detector CT) showing a thrombus at least at the segmental level is considered diagnostic of PE (Class I, Level A recommendation) 2
Treatment decisions: A segmental PE is generally considered clinically significant and requires anticoagulation, unlike some cases of isolated subsegmental PE where treatment decisions may be more nuanced 1
Imaging Characteristics
On CT pulmonary angiography, segmental PE appears as:
- Low attenuation filling defects within the segmental vessel
- May be partially or completely surrounded by opacified blood
- May present as a complete filling defect which leaves the distal vessel totally unopacified 2
Clinical Relevance
The location of the clot has prognostic implications:
Segmental PE is more likely to cause significant clinical symptoms compared to subsegmental PE
The 2025 European consensus statement notes that the location of acute PE is one of the key CTA findings that should be reported, as it may predict early hemodynamic collapse and death 2
According to the ESC Task Force, a CT showing a segmental or more proximal thrombus confirms PE with high confidence (Class I, Level A recommendation) 2
Diagnostic Challenges
There are several potential pitfalls in diagnosing segmental PE:
- Breathing artifacts can result in pseudo-hypoattenuating areas mimicking clots
- Prominent perivascular tissue may be confused with intravascular thromboembolic material
- Additional imaging tools such as cine viewing, multi-planar and three-dimensional image analysis may be helpful in challenging cases 2
Contrast with Subsegmental PE
It's important to distinguish segmental from subsegmental PE:
- Subsegmental PE involves only the small peripheral branches with no involvement of segmental or more proximal pulmonary arteries 1
- Management of subsegmental PE is more controversial, with some cases potentially being managed without anticoagulation based on risk factors and clinical assessment 1
- Segmental PE is generally always treated with anticoagulation 2
Understanding the definition and implications of segmental PE is crucial for appropriate diagnosis, risk stratification, and management decisions in patients with suspected pulmonary embolism.