Axial chest CT of the left and right main pulmonary arteries. Source: NEJM (doi:10.1056/NEJMoa1203830)

An increase in the size of the pulmonary artery relative to the size of the aorta, visible on a CT scan, is a strong predictor of the risk of exacerbation of chronic obstructive pulmonary disease (COPD), according to a study published online Sept. 3 in the New England Journal of Medicine.

Detection of pulmonary artery enlargement relative to the aorta (PA:A ratio of greater than one) may offer a way to identify a subpopulation at high risk for hospitalization for these acute events, according to study authors Mark Dransfield, MD, and colleagues at the University of Alabama at Birmingham.

“The metric is particularly valuable given that the measurement of this ratio requires minimal training and, when measured at the pulmonary artery bifurcation, appears to be reproducible,” wrote the authors.

The researchers noted acute exacerbations of COPD are critical events associated with accelerated loss of lung function and poor quality of life. As such, a reliable tool for predicting these events superior to those currently in clinical use—measurements of lung function, acid reflux or previous episodes of exacerbation, for example—is important. Since the aorta is typically larger than the pulmonary artery in patients without lung disease, the authors hypothesized that a PA:A ratio greater than one would be associated with severe COPD exacerbations.

Dransfield and colleagues tested this hypothesis by analyzing data from 3,464 patients enrolled in the COPDGene study and 2,005 patients enrolled in the Evaluation of COPD Longitudinally to Identify Predictive Surrogate End-points (ECLIPSE) study, which served as a validation cohort. Patients in the multicenter, observational trial were current and former smokers with COPD.

Results showed a significant independent association between a PA:A ratio greater than one and an increased risk of future severe exacerbations, with an odds ratio of 3.44 in the trial cohort and an odds ratio of 2.80 in the external validation cohort. Enlarged pulmonary artery relative to the aorta was also associated with a history of severe exacerbations at the time of enrollment in the trial, according to the authors.

"Our findings indicate that when the pulmonary artery becomes larger than the aorta, regardless of the underlying cause or other health conditions, the risk of exacerbation increases," Dransfield said in a release. "In particular, the risk of exacerbation requiring hospitalization increases dramatically."

The authors wrote that the PA:A ratio appears to outperform other risk factors for exacerbation including breathlessness, chronic bronchitis and other recently identified CT predictors. Another plus is that the measurement can be made from routine CT images without the use of vascular contrast or special software.

“Furthermore, the PA:A ratio, as compared with the pulmonary artery diameter alone, allows for adjustment for anthropometric differences between patients, corrects for CT acquisition and reconstruction algorithms, and provides an internal control that allows the ratio to be compared over time and across cohorts, a feature not shared by other CT markers of exacerbation risk,” they wrote.