Assessment of left ventricular systolic function in aortic stenosis and prognostic implications.

Left ventricular (LV) ejection fraction, most commonly assessed with two-dimensional echocardiography, is an important prognostic determinant of many cardiovascular diseases. The decision of implanting specific cardiovascular devices, such as cardiac resynchronization therapy or implantable cardioverter defibrillator, recommending valvular surgery or chemotherapy relies on the LV ejection fraction. However, the accumulating evidence shows that LV ejection fraction is an imperfect measure of LV systolic function since it simply reflects the changes in the LV cavity volume and does not take into consideration the complex architecture of the LV. The LV myocardial fibre changes the three-dimensional spatial orientation from a right-handed helix in the subendocardial region to a left-handed helix in the subepicardium. This counterclockwise and clockwise spiral muscle configuration results in a characteristic three-dimenisonal LV deformation with shortening in the longitudinal and circumferential directions, thickening in the radial direction, and twist in the circumferential-longitudinal plane and permits LV emptying and filling with optimal mechanical efficiency. Thus, 15% of myofibre shortening during systole results in 40% myocardial wall thickening and .60% change in the LV ejection fraction in the normal heart. In addition, the extracellular collagen matrix of the myocardium supporting this myofibre arrangement is key to maintain the LV shape and size. Any change in the myofibre alignment or extracellular matrix may lead to LV dysfunction. Current imaging techniques, such as diffusion tensor and tagged magnetic resonance imaging and speckle tracking, permit noninvasive evaluation of the LV spiral muscle configuration and quantification of LV shortening, thickening, and twisting deformations. Particularly, speckle tracking echocardiography permits accurate assessment of multidirectional LV strain (longitudinal, circumferential, and radial) by tracking frame-to-frame natural acoustic markers equally distributed within the myocardium in two-dimensional grey-scale images. A large body of evidence has demonstrated a close correlation between the LV ejection fraction and global LV longitudinal strain. However, this correlation is not perfect, suggesting that global LV longitudinal strain assessed with speckle tracking and LV ejection fraction measured with Simpson’s method on two-dimensional echocardiography do not reflect the same aspects of LV systolic function. The subendocardial myocardial fibres are the main determinants of LV longitudinal strain, whereas the mid-myocardial and epicardial fibres, with a circumferential and longitudinal threedimensional disposition are the main determinants of the circumferential and rotational mechanics. At an early stage of myocardial disease, where the subendocardial layers may be affected by fibrosis or ischaemia and mid-myocardial and subepicardial layers may be unaffected, impaired longitudinal subendocardial shortening will be compensated by normal circumferential and rotational mechanics resulting in a preserved LV ejection fraction. Transmural myocardial injury or progression of the disease will impair myocardial circumferential and rotational mechanics leading to a reduced LV ejection fraction. Consequently, global LV longitudinal strain has been proposed as a more sensitive method than LV ejection fraction to detect subtle myocardial disease. Numerous studies have reported a subtle LV systolic dysfunction assessed with global LV longitudinal strain in multiple clinical conditions where LV ejection fraction may be preserved (diabetes mellitus, hypertension, valvular heart disease and use of chemotherapy). –7 In addition, the severity and extent of myocardial disease has been accurately evaluated with multidirectional LV strain speckle tracking echocardiography. For example, the magnitude of regional LV longitudinal, radial, and circumferential strain has been associated with myocardial scar transmurality and presence of viable myocardium in patients with ischemic heart disease. Furthermore, impaired global LV longitudinal strain has been independently associated with poor long-term outcome in