Precise Assessment of Myocardial Contractile Performance in Aortic Regurgitation: Biological and Management Implications

Strain is ‘‘stretching’’ in lay language, and ‘‘deformation’’ in technical terms, and can be used to measure and express myocardial deformation during contraction. Strain is a dimensionless quantity and is expressed as a percentage change in length from the original dimension. It is defined mathematically by the Lagrangian formula: E = L/ (Lo Li), where E is strain, Lo is the original length of myocardium, Li is the length of myocardium after it is stretched, and L is the change in length (Lo Li). It is not possible to measure Li in a beating heart, so end-diastolic length is used. Contraction generally results in negative strain, while relaxation results in positive strain. There are different types of left ventricular (LV) myocardial strain: LV long-axis strain measured in the base-to-apex orientation, circumferential strain (generally measured as LV midwall strain) along the circumference of the minor LVaxis, and radial strain as a measure of LV wall thickening. LV long-axis and circumferential strain is negative during systole, and radial strain is positive. These are overt simplifications, and it must be recognized that LV muscle fiber orientation is too complex to be resolved along different geometric planes cuttingthrough the left ventricle. Normal systolic longitudinal strain is in the range of 20%, similar to the amount of sarcomeric shortening of the myocyte during its contraction. Contractility is better expressed in terms of the systolic stress-strain relationship. LV circumferential and longitudinal strain used to be obtained traditionally using contrast left ventriculography, but the computation was cumbersome. Cardiac magnetic resonance imaging with myocardial tagging permits one to measure systolic myocardial strain at a temporal resolution of about 50 msec. Ultrasonic methods such as tissue Doppler and speckle tracking permit the measurement of both systolic and diastolic strain and strain rate at a high temporal resolution in the range of 5 to 10 msec. These tools are quite user friendly and potentially powerful for the assessment of both regional systolic and diastolic function. Myocardial systolic wall stress is a measure of how much force the myocardium must overcome to contract. Systolic wall stress is proportional to LV systolic pressure and size and inversely proportional to LV wall thickness, following the principles of the Laplace equation, with geometric corrections for wall stressesin different myocardialfiber orientations. Higher wall stresses make it difficult for the myocardium to contract. Hence, agivenstrainvaluemustbeinterpretedinthe contextofthe operating wall stress. A strain value of 18% at a systolic wall stress of 150 g/cm 2 indicates better contractile function than the same strain at a systolic wall stress of 75 g/cm 2 . In this issue of JASE, Iida et al. 1 report newer insights regarding altered myocardial mechanics in patients with aortic regurgitation (AR), despite normal LVejection fraction (EF), demonstrable by myocardial strain imaging. The salient findings of this study include reduced LV long-axis or meridional strain even in patients with moderate AR and reduced LV subendocardial circumferential and radial strain in patients with severe AR, with preservation of normal LV EF because of compensatory increases in subepicardial strain. Iida et al. describe the latter phenomenon as ‘‘transmural compensation,’’ a phenomenon of cross-fiber talk between different myocardial layers.