Macro design, structure, and mechanics of the left ventricle

Physiologic understanding of LV function can facilitate definition of the appropriate role for new procedures and help explain successes and failures.Thus enthusiasm for LV volume reduction has waxed and waned rapidly with little understanding of why the operation succeeded in some patients and failed in others, leaving uncertainty about who is an appropriate candidate for such surgery.Looking ahead, advances in surgical capacity to remodel and rearrange ventricular anatomy and an avalanche of new tools, including skeletal muscle grafts, mechanical assist devices, maze operations, pacing therapies for heart failure, and molecular and laser approaches to myocardial revascularization, will challenge our ability to understand new surgical tools and define their appropriate clinical role.The ability to meet this challenge begins with functional cardiac anatomy.This review summarizes current understanding of the relation between structure and function of the LV.Some areas in which such knowledge is incomplete and analytic methods are flawed are discussed.Illustration of how current understanding may contribute to further progress in clinical surgery is an underlying goal. Technical issuesThe heart has been described as "a complex threedimensional and fiber-wound structure with mechanical properties that are nonlinear, anisotropic, time varying, and probably spatially inhomogeneous."33 The LV, the heart's most structurally complex region, has proven remarkably resistant to analysis of functional anatomy.Studies have generally been based on quantitative histology or dynamic measurements.Quantitative histology requires fixed tissue that accurately reflects conditions in vivo.Proper relation of sample dimensions to three-dimensional LV architecture is also essential.Dynamic studies are compromised by difficulty defining the location of the endocardial surface when that surface buckles and thickens during systole.When resolution is coarse, as in echocardiography, dynamic techniques provide measurements averaged over a slice several millimeters thick.Such data are not directly comparable with histologic sections, which are a frac-T he functional anatomy of the left ventricle (LV) has been defined by anatomic dissection, 1 light 2,3 and electron microscopy, 4,5 x-ray contrast angiography, 6,7 two-dimensional echocardiography, 8,9 nuclear magnetic resonance (NMR) tagging, 10,11 implanted radiopaque markers, 12,13 sonomicrometry, 11,14,15 and other methods.16,17 Early in the history of cardiac surgery, interest in functional anatomy was spurred by inconsistent results of valve replacement [18][19][20] and ventricular aneurysm surgery and by issues in correction of congenital heart disease.Although some inconsistency could be attributed to problems with myocardial protection or technical errors, other problems appeared to reflect incomplete understanding of LV functional anatomy.The importance of papillary muscle function in mitral valve surgery [21][22][23][24][25][26] and of ventricular size and shape in LV aneurysmectomy 27,28 have been emphasized in subsequent studies.Maturation of intraoperative two-dimensional echocardiography further focused surgical interest in acute changes in function.The marked, intraoperative decrease in LV ejection fraction after valve replacement for chronic mitral regurgitation 29 spurred study of valve repair and chordal preservation.[21][22][23][24][25][26] Improving surgical materials, skill, and knowledge have made novel procedures like cardiomyoplasty, 30 LV volume reduction, 31,32 and patch ventriculoplasty 27 feasible, but they also challenge surgical investigators to define related changes in global and regional function.