Effect of Aortic Arch Position on the Bronchial Length Ratio

Abstract Background Bronchial length ratio (BLR) is widely used to differentiate heterotaxy with right or left isomerism from situs solitus or inversus. During daily experience, we found that the bronchial length is also affected by the position of the aortic arch and wondered whether the bronchial length ratio is a reliable indicator of the thoracic situs. Objective To examine the influence of the position of the aortic arch on the BLR and assess the accuracy of the BLR in defining the body situs. Material and Methods The study population was selected from the radiologic database of 2001–2022 using the key words, left aortic arch, right aortic arch, double aortic arch, situs inversus, heterotaxy and isomerism. From the patients identified, the patients having good quality computed tomographic angiograms (CTA) or magnetic resonance angiograms (MRA) were chosen. Using a 3D workstation, the lengths of the main bronchi were measured and the ratio between the longer and shorter bronchi was calculated. The relationship between the main bronchi and the branch pulmonary arteries and the position of the aortic arch relative to the trachea was recorded. The BLR and the broncho-pulmonary arterial relationship were compared among the groups having different combinations of body situs and aortic arch position. Results The study population consisted of 69 patients with situs solitus and left aortic arch (Group 1A), 20 with situs inversus and right aortic arch (Group 1B), 43 with situs solitus and right aortic arch (Group 2A), 2 with situs inversus and left aortic arch (Group 2B), 11 with situs solitus and double aortic arch (Group 3), and 14 with heterotaxy with right or left isomerism (Group 4). Group 2 (2A + 2B) showed a significantly smaller BLR than Group 1 (1A + 1B) (p = 0.001). Group 3 also showed a smaller BLR than Group 1 but without statistical significance (p = 0.26). Group 4 showed a significantly smaller BLR than Groups 1–3 (p < 0.001). In Group 1, there was no patient having the BLR fall in the range of heterotaxy group (Group 4). In Group 2, 5 patients (11%) had BLR within the range of heterotaxy group. In Group 3, one patient (9%) had BLR within the range of heterotaxy group. In all patients, the broncho-pulmonary arterial relationship was consistent with the situs of the rest of the body. Conclusion The BLR is affected by the position of the aortic arch. The ratio is smaller when the aortic arch is in an unusual position for the given situs, for example, right or double aortic arch in situs solitus and left aortic arch in situs inversus, than when the aortic arch is in a usual position. Therefore, the BLR is of limited value in situs determination. The broncho-pulmonary arterial relationship should be used as the primary criterion for situs determination.