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Cardiothoracic
Demonstration data

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.

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📐 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.