Dear Editor,

The article by Saetung et al. [1] on ‘main pulmonary artery-to-descending aorta ratio in computed tomography’ was quite informative and made for interesting reading.

The pulmonary artery to aorta ratio (PA/A) has been considered a proxy for pulmonary hypertension (PHT) and has been validated in many previous studies [2-4] not only for diagnosis but also for prognostication of patients [4].

The ascending aorta has been used most commonly for calculation of the pulmonary artery to aorta ratio and is well-validated; however, in cases where ascending aortic diameters cannot be used, an alternate aortic reference diameter is needed. The authors have identified the descending thoracic aortic diameter as an alternate aortic reference diameter with high specificity (91%) and PPV (88%).

Proximal aortic pathologies (arch hypoplasia, aortic interruption, coarctation of aorta) and involvement of DTA (e.g. vasculitis) can effect DTA diameters precluding use of DTA diameter. However, DTA diameters should only be used in cases where ascending aortic diameters cannot be used due to pathological involvement, and they can be a valuable alternative in this case. Considering the lack studies validating the MPA to DTA ratio and the small sample size (45) in the current study, more studies are needed with larger sample sizes before this can be used in routine clinical practice.