Ascending Aortic Length
Several studies have investigated the relationship between ascending aortic length and aortic dissection. Dr. Tobias Kruger was among the first to draw attention to this topic in his paper “Ascending aortic elongation and the risk of dissection.” Kruger et al. retrospectively studied CTAs of ATAAD patients and compared measurements to CTAs of healthy controls. They observed that hypertensive patients exhibited greater aortic diameter and length than non-hypertensive patients11. However, they concluded that the patients in their sample with hypertension were significantly older than non-hypertensive patients, therefore, definitive judgments on the impact of hypertension on aortic morphology could not be made. In our matched sample, we determined that hypertension was associated with greater tortuosity (P =0.044), but not with greater aortic length (P =0.266) or diameter at the STJ (P =0.655) and PAB (P =0.367). Kruger et al. also found that ATAAD patients had elongated ascending aortas when compared to healthy controls11. It is important to note that while Kruger et al. measured two-dimensional projections in the sagittal and frontal planes, therefore, length measurement values are not directly comparable with our data.
In 2018, Heuts et al. published a retrospective analysis of aortic morphometrics in ATAAD and propensity-score matched to healthy controls. They utilized CT scans to construct and measure a three-dimensional modelled aortic centerline. The results showed that the ATAAD group had an almost 1-cm greater average ascending aortic length, even after adjusting for dissection-induced morphologic changes by reducing length measurements 10%15. Our data show the ATAAD group had a 1.49-cm greater average ascending aortic length without adjusting length measurements, and a 0.72-cm greater average length when reducing ATAAD length measurements by 10%.
More recently, Wu et al. characterized the association between ascending aortic elongation and aortic adverse events (including aortic dissection). They utilized gated CT scans and a three-dimensional image analysis technique to measure the ascending aorta from the aortic annulus to the origin of the innominate artery. Their analysis showed that aortic elongation is strongly associated with increased risk for adverse aortic events—patients with very elongated aortas (\(\geq\)13 cm) had an almost 5-fold higher average yearly rate of adverse aortic events when compared with patients with shorter ascending aortas (<9 cm)16. The authors emphasized the importance of two distinct “hinge points” in ascending aortic length that were correlated with increased probability of adverse aortic events, ultimately proposing an ascending aortic length of 11 cm (measured from aortic annulus to origin of the IA) as a threshold for elective aneurysm repair. Our study parameters differed in that we defined ascending aortic length as the distance from the STJ to the origin of the innominate. The distance from aortic valve annulus to sinotubular junction is approximately 2 cm, so we can infer that any patients in our cohort with an ascending aortic length of 9 cm would meet or come close to meeting the threshold proposed by Wu et al.17 In our cohort, 13 of 67 cases (19.4%) and 0 of 67 controls had an ascending aortic length of 9 cm or greater. When length measurements in ATAAD patients were decreased by 10% as an inexact adjustment for dissection-induced morphologic changes (similar to the Heuts et al. paper), this number falls to just 6 of 67 cases (9.0%). As Wu et al. noted, the proposed 11-cm threshold for intervention is a “conservative” threshold, but it may identify patients who do not meet the diameter-based intervention standard. They observed that 70.4% of the patients in their cohort who experienced dissection at a diameter less than 5.5 cm exhibited an aortic length greater than 11 cm13. In our cohort, 16% of the patients who experienced dissection at a diameter less than 5.5 cm exhibited an aortic length greater than 9 cm. Of the six patients in our cohort who were estimated to have an ascending aortic length that would meet the Wu et al. length-based intervention standard, 5 of the 6 (83.3%) had a maximum ascending aortic diameter less than 5.5 cm. Out of 67 cases in our cohort, only 21 patients (31%) would have met either the length-based or current diameter-based intervention guidelines based on unadjusted post-dissection measurements (which are certainly greater than pre-dissection measurements).
Dissection pathology changes the normal anatomy of the ascending thoracic aorta. Rylski et al. noted a 5.4% average increase in ascending aortic centerline length after aortic dissection even when the ascending aorta was primarily nondissected, although results were not statistically significant9. Similarly, Wu et al. found an average ascending aortic length increase of 2.7% in 10 patients with pre-dissection CT scans16. In the absence of a CT scan conducted immediately prior to dissection, it is impossible to know the truest pre-dissection dimensions of the ascending thoracic aorta. Given the incredible obstacles to a prospective study design on this topic, we believe that an appropriately powered, matched retrospective analysis is sufficient for drawing some conclusions about ascending aortic length differences in ATAAD and control patients.
Our findings demonstrate that patients with ATAAD have greater path length of the thoracic aorta from the STJ to the origin of the IA when compared to matched controls. The proposed mechanism linking aortic length to aortic dissection investigates longitudinal forces on the aortic intima and the effect of aortic lengthening on tissue stress. Aortic dissections most frequently result in an intimal tear in the transverse (circumferential) direction, which indicates a pathologic stress in the longitudinal axis18,19. Tissue elasticity is critical in the ascending aorta, where pulsatile blood flow is transformed to a waveform through the damping effect of aortic distension and recoil. Since aortic elongation results in a loss of elasticity and a subsequent increase in wall stress on the intima, it seems likely that the increased degree of longitudinal stress in elongated aortas predispose patients to aortic dissection11,20,21. Regardless of the exact tissue mechanisms that link ascending aortic length and aortic dissection, establishing that this relationship exists is an important step toward developing better screening criteria.