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.