4 Discussion
According to the recent progress in treatment of AS16,
more accurate and adequate assessment is required for the severity of AS
from the prognostic aspect.17,18 Echocardiography is
the main tool for assessing AS to obtain Vmax, mPG, and AVA as the major
indexes of the severity of AS non-invasively and
repeatedly.3,4 However, one or two indexes of them may
sometimes discord with each other, and then require alternative
solution. One of the possible situations was in case of LFLG AS, which
requires dobutamine stress echocardiography for correct
diagnosis3–7, though we did not include such cases in
this study. Another situation may be due to technical difficulty, not
able to obtain good echocardiographic images, suboptimal Doppler signals
with ambiguous envelope in the real world.
Here, the ratio of AT to ET of transaortic Doppler flow signal, i.e.
AT/ET, is recently reported as a reliable index of AS
severity8-12, and positively correlated with peak V,
mPG, and negatively correlated with AVA or AVAi. This index has
potential to be added as a useful information of AS severity in such
cases.
Some investigators have already reported that the usefulness of AT/ET.
Nakamura et al reported that AT/ET correlated well with peak V and
catheter derived mPG in 1987.8 Zekry reported AT/ET
was useful in assessing dysfunction of prosthetic valve in aortic valve
position.13 AT/ET is an index of easy measurement,
easy recognition by eyes, and more importantly it is independent from
the transaortic flow rate. Therefore, AT/ET is useful in differentiating
true severe LFLG AS and pseudo AS, as Kamimura et al11and Gamaza-Chulián12 reported. Griguer et al reported
that prognosis was poor in patients with severe AS of
AT/ET>0.36.19
However, these reports were based on the data acquired from no matter
which acoustic window of the fastest transaortic velocity for a single
patient with AS or they assumed as the identity of the AT/ET obtained
from any acoustic windows. To obtain the true peak V of patients with
AS, multiple acoustic windows approaching is recommended, while time
interval measurement for AT/ET is able in apical or right parasternal
acoustic window.20,21 However, the results of time
interval measurement obtained from each acoustic window have not been
compared so far, therefore, we investigated the effects of acoustic
windows or approaching on AT/ET measurement by measuring AT, ET, and
AT/ET on continuous Doppler transaortic flow waves acquired from apical
(3C and 5C) and R acoustic windows in the same patients, and found the
non-uniform results between different acoustic windows.
In this study, there was no significant difference in cET between apical
(3C and 5C) and R approach in patients with AS. However, cAT was
significantly greater in R approach than apical approach, then AT/ET was
also greater in R approach than apical approach.
There are some possible reasons for greater AT/ET by R approach than
apical approach. One is the difference in peak V caused by approaches.
Thaden et al reported that the highest peak V of patients with severe AS
was achieved by R approach in nearly half of their
cases.22 Since severe AS was also more than half in
our 81 patients in our study and the higher peak V was obtained by R
approach rather than apical approach (3C or 5C), time interval
difference from earlier suboptimal peak V to later true peak V might
cause greater AT measurement value.
Another possibility was an error in time-interval measurement caused by
R approach. A flow characteristic of R approach is turbulent signal in
Valsalva sinus after severely restricted aortic valve as recognized by
color Doppler image. We cannot exclude the possibility of the effect of
the turbulent flow signal on the envelope of the continuous wave Doppler
signal which may make difficult to recognize the exact timing of the
peak velocity without clear envelope of Doppler signal. Because of this
reason, the variability of the inter-observer measurement difference was
greater in R approach than in apical approach. Therefore, we need to pay
an attention to measure AT/ET by R approach, though it is superior in
peak V detection by R approach to apical approach.
Additionally, we consider the effect of HR on ET or AT measurement since
ET or AT is affected by HR. Zoghbi et al23 reported
that ET shortened by 15 msec as 10 bpm increase in HR in patients with
AS in HR range of 55-110 bpm. Also, Reant et al24reported that ET shortened by 15 msec per 10 bpm increase in HR in AS
with reduced LV systolic function. However, the ratio AT/ET itself is
reported to be independent of HR19, thus the effect of
HR on AT/ET may be minor.
Finally, AT/ET is supposed to be a useful index of AS severity because
this is easy to measure, easy to recognize, and applicable to LFLG
AS.12 However, it needed to pay attention to the
effect of acoustic windows on the decision of AS severity by AT/ET as
revealed in the study.
Limitation of study: This study did not include AS patients
with decreased LVEF so-called LFLG AS, or patients with atrial
fibrillation, or patient with other significant valvular lesions. The
exact cause of the AT/ET discrepancy has not been determined in the
clinical situation though possible cause has mentioned in the text.