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.