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The optimal definition of ‘severe’ aortic stenosis (AS) remains a contentious issue, especially given the improved therapeutic options now available. Many of us have dedicated years of our lives to the elusive search for a number that defines ‘severe’ AS, eventually coming to the realisation that there is no single simple number. Instead, there is a dynamic interaction between the dysfunctional valve tissue, left ventricle and systemic vasculature with changing pressure-flow relationships over the patient’s lifetime and even during normal daily activities. There now is general agreement that the optimal reference standard for any definition of severe AS is prediction of clinical outcomes.
A schematic diagram of the mathematical relationship between aortic valve area (AVA), and maximum aortic velocity for a normal, low and high transaortic stroke volume index (SVI) is shown in figure 1. Of course, disease progression in an individual patient does not necessarily follow a single flow curve because as the degree of valve obstruction increases, volume flow rate may change simultaneously with changes in AVA. Thus, one patient might show progressive increases in aortic velocity (and gradient) as expected for each incremental decrease in AVA with maintenance of a normal SVI. A different patient might move from the normal to low flow curve as AVA decreases, resulting in a smaller increase in aortic velocity than would be expected if flow rate had remained normal.
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