INTRODUCTION
Degenerative aortic valve disease affects over 25% of all patients over
65 1. However, most tend
to be asymptomatic until the valve is severely restricted. After the
onset of symptoms, survival rates decrease dramatically2. Patients with
valvular heart disease will acquire noninvasive testing such as ECG or
transthoracic echocardiography to evaluate the extent of the disease.
However, the less invasive doppler echocardiogram is the standard for
diagnosis 3. If there is
discordance between the symptoms and such testing, invasive evaluations
may be utilized 4.
Transesophageal echocardiography is especially useful in patients with a
poor transthoracic window or complex cardiac pathology. Although cardiac
catheterization is no longer recommended for the purposes of aortic
valve evaluation, it may be utilized for further evaluation and thus
optimizes the treatment strategy.
Echocardiographic estimation of aortic stenosis performed typically
correlates well with assessment by cardiac catheterization. However,
catheterization may differ slightly from the echocardiographic
assessment of certain individuals as echocardiography may overestimate
aortic stenosis severity5. Because the peak
aortic pressure is attained milliseconds later than the peak left
ventricular pressure, a peak-to-peak gradient is not an actual
physiological measurement during the catheterization procedure. On the
other hand, doppler measurements reflect peak instantaneousgradients. Thus, doppler-derived gradients may be of a greater value and
accuracy than catheter-derived gradients, where peak-to-peak gradients
are reported.
To excavate this inconsistency, the fluid dynamics of aortic stenosis
must be considered. Before the acceleration through the aortic valve,
there is a low dissipation of pressure and high stability, and laminar
flow of the blood as the static pressure (Potential Energy) is converted
to dynamic force (Kinetic Energy). Once accelerated through the valve,
there is much dynamic tension, which allows for the velocity through the
vena contracta (VC). At this point, the continuity equation corresponds
to the effective orifice area (EOA) or aortic valve area AVA). Upon
passage through this high-velocity point, a turbulent flow carries a
high dynamic pressure that should be transformed back to static pressure
upon deceleration. However, due to the instability and turbulence of
blood downstream from VC, some energy is lost due to heat and shear
force through the valve and the surrounding aorta. In valvular stenosis,
there is an increase in flow acceleration and velocity, resulting in a
higher-pressure gradient, as described by the Bernoulli Equation. The
peak gradient differences, mainly upstream and downstream from VC, are
measured using the Gorlin formula6-8.
In 2000, an article in circulation demonstrated the energy loss between
the left ventricular outflow tract and the ascending aorta. These
investigators established a relationship with the effective orifice area
(EOA), the aortic cross-sectional area (AA), and the energy loss in
terms of pressure difference across the valve instead of the
transvalvular pressure gradient (TPG)7:
\begin{equation}
\frac{\text{EOA\ X\ A}A}{AA-EOA}=\left(\frac{Q}{50\sqrt{EL}}\right)\nonumber \\
\end{equation}Based on this equation, Energy loss (EL) is a squared function of flow
rate. Therefore, at a given flow rate, energy loss (heat and other
forces on the aorta and the valve) increases with decreasing EOA and
increasing AA. The energy loss index (ELI) refers to the energy loss per
square meter of body surface area (BSA).9:
\begin{equation}
\frac{\frac{\text{EOA\ X\ A}A}{AA-EOA}}{\text{BSA}}\nonumber \\
\end{equation}Based on this theory, previous studies have shown the efficacy of ELI in
stratifying and reclassifying aortic stenosis patients in high,
moderate, or low-risk categories (ELI of <0.6cm2/m2 signifying
severe aortic stenosis). Interestingly, such studies highlighted the
overestimation of stenosis by doppler echocardiogram9. Furthermore, given
the significance of energy loss to the shear forces and turbulence
through the valve, we aim to study the longevity of the implanted
transcatheter aortic replacedĀ valves and use it to predict
post-transcatheter aortic valve replacement outcome. We, therefore,
hypothesized that one could predict the likelihood of all-cause
mortality utilizing ELI in patients who have undergone the TAVR
procedure.