From the Guidelines
The upper limit of normal pulmonary artery diameter is 29 mm, as an MPA diameter of 29 mm on CT is 87% sensitive and 89% specific, with a positive predictive value (PPV) of 97% for PH. This measurement is typically taken at the level of the pulmonary artery bifurcation on a transverse CT image, and it is essential to consider the pulmonary artery to ascending aorta ratio (PA:A ratio) as well, with a ratio greater than 1:1 suggesting abnormality even when absolute measurements are borderline 1.
In clinical practice, pulmonary artery enlargement should prompt further evaluation with echocardiography or right heart catheterization to assess pulmonary pressures directly. The measurement threshold is based on population studies correlating pulmonary artery dimensions with hemodynamic parameters, though individual variation exists based on body size, age, and other factors 1.
Some key points to consider when evaluating pulmonary artery diameter include:
- The presence of parenchymal lung disease, which can decrease the sensitivity and specificity of the MPA diameter measurement 1
- The ratio of the MPA to the adjacent ascending aorta, which is an extremely sensitive CT finding for PH 1
- Additional findings of PH on CT, such as true right and left descending PA diameters of 16 mm and 21 mm, respectively, and enlargement of the right ventricle 1
- The importance of evaluating with RHC to confirm PH detected on CT before initiating therapy 1
When pulmonary artery enlargement is detected, the underlying cause should be investigated, as it may indicate conditions ranging from pulmonary arterial hypertension to chronic thromboembolic disease. High-resolution CT can help characterize various pulmonary etiologies that cause PH, including IPAH, pulmonary capillary hemangiomatosis (PCH), pulmonary veno-occlusive disease (PVOD), and many diffuse lung diseases 1.
From the Research
Upper Limit Pulmonary Artery Diameter
The upper limit of pulmonary artery diameter can be an indicator of pulmonary hypertension.
- According to a study published in 2013 2, the main pulmonary artery diameter on chest computed tomography (CT) can predict borderline pulmonary hypertension, with a cutoff value of ≥2.9 cm.
- The study found that patients with borderline pulmonary hypertension (mean PA pressure 21-24 mmHg) had significantly larger main PA diameters compared to patients without pulmonary hypertension (3.16 ± 0.53 vs. 2.78 ± 0.43 cm, P = 0.001).
- The main PA diameter on CT correlated with mean PA pressure (r = 0.496, P < 0.001) and pulmonary vascular resistance (r = 0.445, P < 0.001).
Diagnostic Criteria
The diagnosis of pulmonary hypertension is typically made using right heart catheterization, which measures the mean pulmonary artery pressure.
- A mean pulmonary artery pressure of ≥25 mmHg is considered diagnostic of pulmonary hypertension 3.
- Borderline pulmonary hypertension is defined as a mean PA pressure of 21-24 mmHg 2.
Clinical Significance
Pulmonary hypertension can lead to right heart failure and death if left untreated.