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Physiology of Aortic Valve Stenosis
Study Questions:
Can pressure/flow dynamics of a stenotic aortic valve be assessed at rest, and how it is affected by transcatheter aortic valve implantation (TAVI)?
Methods:
This was a hemodynamic physiology study assessing pressure/flow dynamics of a stenotic aortic valve. A total of 16 patients undergoing TAVI for native valve severe aortic stenosis (AS) were enrolled and underwent physiological assessment prior to TAVI. This included placement of two separate 0.014-inch pressure wires in the aorta and left ventricle to measure simultaneous left ventricular-aortic pressures. In addition, cardiac output was measured using a 7F pulmonary artery catheter. Dobutamine was infused at graded doses via a central line, and pressure and flow measurements were made under conditions of stress. TAVI was then completed and physiologic assessment was repeated. Transvalvular gradient, flow, and aortic valve area were calculated at rest and with dobutamine infusion before and after TAVI. A stress aortic valve index (SAVI) was calculated and defined as the ratio of mean aortic/left ventricular systolic ejection pressure during peak stress.
Results:
Before TAVI, measured pressure difference did not relate to transvalvular flow. Observations during rest (flow or pressure-based models) did not correlate with stress observations. The unitless ratio of aortic to left ventricular pressures during systolic ejection under stress conditions correlated best with post-TAVI flow improvement.
Conclusions:
During peak dobutamine, the ratio of aortic to left ventricular pressures during systolic ejection provides a “fractional flow reserve” of the aortic valve that closely approximates the complex changing fluid dynamics. Because resting assessment cannot reliably predict stress hemodynamics, “valvular fractional flow” warrants study to explain exertional symptoms in patients with only moderate AS at rest.
Perspective:
This was a remarkable study attempting to better understand the complex physiology of AS beyond anatomy. The investigators showed that resting measures do not reliably predict hemodynamics when compared to stress conditions. Also, they introduced the idea of a “fractional flow reserve” across the aortic valve using “SAVI.” SAVI measures relative peak flow limitation across the stenotic valve under stress and may uncover severe stenosis, which may not be seen under resting conditions. Similar to the fractional flow reserve movement in coronary disease assessment, this paper suggests that anatomy and using aortic valve area may have limitations, and physiological assessment of aortic valve disease may be more clinically relevant. Although routine “functional” assessment of AS requires (much) further testing, steps forward would need expedient and safe ways of measuring SAVI. Of note, the study was stopped after a fatal cardiac arrest, which occurred 2 minutes after dobutamine infusion and prior to TAVI.
Keywords: Arterial Pressure, Aortic Valve Stenosis, Cardiac Output, Constriction, Pathologic, Coronary Artery Disease, Dobutamine, Fractional Flow Reserve, Myocardial, Heart Arrest, Heart Defects, Congenital, Heart Valve Diseases, Heart Valve Prosthesis, Hydrodynamics, Systole, Transcatheter Aortic Valve Replacement, Ventricular Pressure
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