Pulmonary Artery : Aorta Ratio
Ratio of main pulmonary artery diameter to ascending aorta diameter on CT, a non-invasive marker of pulmonary hypertension.
The PA:Ao ratio compares the calibre of the main pulmonary artery with the ascending aorta at the level of the pulmonary artery bifurcation on axial CT. It is a simple, reproducible surrogate for elevated pulmonary artery pressure.
📐 Formula
PA:Ao = main pulmonary artery diameter / ascending aorta diameter
🎯 When to Use
- Screening for pulmonary hypertension on any chest CT.
- Risk-stratifying patients with COPD, ILD or chronic thromboembolic disease.
- Predicting COPD exacerbation risk.
⚙️ How It Works
- The main pulmonary artery is measured at its widest point at the bifurcation level, perpendicular to its long axis.
- The ascending aorta is measured on the same axial image.
- The ratio is the PA diameter divided by the aortic diameter.
📊 Interpreting the Result
- PA:Ao > 1 is associated with pulmonary hypertension (invasively confirmed mean PA pressure ≥ 25 mmHg) with good specificity.
- In COPD, a PA:Ao > 1 independently predicts severe exacerbations requiring hospitalisation.
- A main pulmonary artery diameter > 29 mm is itself abnormal and correlates with elevated pulmonary pressures.
⚠️ Limitations & Pitfalls
- A dilated aorta (aneurysm, aortic valve disease) can normalise the ratio despite pulmonary hypertension.
- CT is not a substitute for right-heart catheterisation, the diagnostic reference standard.
- Measurement varies with the axial level chosen and with contrast timing.
📚 References
- Ng CS, et al. Visual assessment of the size of the main pulmonary artery and aorta on CT. AJR / Clin Radiol, 1999; and Wells JM, et al. Pulmonary arterial enlargement and acute exacerbations of COPD. N Engl J Med. 2012;367:913–921.
- Truong QA, et al. Reference values for normal pulmonary artery dimensions by non-contrast CT (Framingham Heart Study). Circ Cardiovasc Imaging. 2012;5:147–154.
⚕️ For clinical decision support only. Always verify calculations and apply clinical judgment.