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Ann Thorac Surg 2003;76:1585-1586
© 2003 The Society of Thoracic Surgeons

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Original article: cardiovascular

Invited commentary

^a Division of Surgical Services (112D), San Francisco Veterans Affairs Medical Center, 4150 Clement Street, San Francisco, CA 94121, USA

e-mail: mark.ratcliffe{at}med.va.gov

	Introduction

Top Introduction Two-dimensional strain analysis Septal stretching Implications for PLV References

 
This paper by Setser and colleagues is probably the most in-depth mechanical analysis of partial ventriculectomy (PLV) data to date, and the authors are to be congratulated for their effort.

Briefly, the authors performed tagged MRI in 24 patients before 3 and 12 months after partial ventriculectomy. Two-dimensional circumferential strain was measured from the short axis tagged images as described by Yeon et al [1] and global and regional circumferential stress were calculated using the force balance equations of Falsetti et al [2] and Janz [3], respectively. Finally, event history analysis using the Cox proportional hazards model was performed with time to PLV failure (death, listing for transplantation) as the dependent variable.

The following is a list of significant findings:

1. There was significant regional heterogeneity in regional circumferential LV strain before PLV.
   
2. Regional circumferential strain increased and regional circumferential stress decreased uniformly after PLV. One consequence was that preoperative septal stretching (+ strain) turned to septal shortening (- strain) in some patients after PLV. Contrary to expectation, the changes in stress and strain had only weak correlation.
   
3. Preoperative regional shortening was greatest in the resected lateral LV wall.
   
4. Preoperative septal stretching was associated with longer time before PLV failure.
   

	Two-dimensional strain analysis

Top Introduction Two-dimensional strain analysis Septal stretching Implications for PLV References

 
The weak correlation between stress and strain may be a function of the two-dimensional analysis employed. The relationships between all six unique stress components and all six unique strain components are required to completely define mechanical properties of a solid. Short axis tagged images yield only three strain components (circumferential normal strain, radial normal strain, and shear strain in the short-axis plane), and these will become more inaccurate with distance from the apex if the through-short-axis-plane motion of the myocardium is not taken into account.

Moreover, the stress and strain components most directly related to myocardial function are the normal components aligned with the local ventricular muscle fiber orientation. However, muscle fiber orientation coincides with the circumferential direction only near the left ventricular midwall.

In addition, force balance equations such as those described by Falsetti et al [2] and Janz [3] yield only the average circumferential stress through the wall. In contrast, finite element models that are based on the conservation laws of continuum mechanics and take myocardial mechanical properties into account suggest that the transmural distribution of circumferential stress is heterogenous (ie, it varies significantly from endocardium to epicardium).

	Septal stretching

Top Introduction Two-dimensional strain analysis Septal stretching Implications for PLV References

 
A previous paper by Young et al [4], which analyzed a subset of the data presented here, also found septal dyskinesis. Although heterogeneity of regional LV shortening has been described in patients with dilated cardiomyopathy, the current report and the paper by Young are the first to focus specifically on septal stretching. Septal stretching may be caused by either high stress with normal contractile ability or by reduced contractility. Because of the weak correlation between stress and strain, Young et al [4] concluded that regional stress was not sufficient to explain the heterogeneity in strain and suggested that regional differences in oxidative metabolism might be responsible. Of note, the infarct borderzone (BZ) is an analogous situation, and recent finite element analysis has found that systolic BZ stretching is associated with a decrease in contractility [5]. Simulations of the sort employed by Guccione et al [5] in the borderzone analysis would probably help to identify the contractile ability of the dysfunctional septum in these patients.

	Implications for PLV

Top Introduction Two-dimensional strain analysis Septal stretching Implications for PLV References

 
Given heterogeneity of function, one might reasonably assume that the best PLV outcome would be obtained by resecting the portion of LV wall with reduced function. However, Setser and colleagues have shown that doing the opposite (specifically, resection of the lateral wall with the highest preoperative regional shortening and preservation of the septum which had the worst preoperative regional function), is associated with improved survival (freedom from death or relisting for heart transplantation). These findings are counter-intuitive and make the manuscript of significant interest. Setser and colleagues suggest that improvement in septal function "might help compensate for the loss of contractile tissue in the lateral wall." This may be true, however, before these interesting findings can be understood, the preoperative contractile ability of the septum must be determined and the effect of unloading by PLV on septal function calculated. Further mathematical modeling will be necessary to sort this out.

	References

Top Introduction Two-dimensional strain analysis Septal stretching Implications for PLV References

 

1. Yeon S.B., Reichek N., Tallant B.A., et al. Validation of in vivo myocardial strain measurement by magnetic resonance tagging with sonomicrometry. J Am Coll Cardiol 2001;38:555-561.[Abstract/Free Full Text]
2. Falsetti H.L., Mates R.E., Grant C., Greene D.G., Bunnell I.L. Left ventricular wall stress calculated from one-plane cineangiography. Circ Res 1970;26:71-83.[Abstract/Free Full Text]
3. Janz R.F. Estimation of local myocardial stress. Am J Physiol 1982;242:H875-H881.
4. Young A.A., Dokos S., Powell K.A., et al. Regional heterogeneity of function in nonischemic dilated cardiomyopathy. Cardiovasc Res 2001;49:308-318.[Abstract/Free Full Text]
5. Guccione J.M., Moonly S.M., Moustakidis P., et al. Mechanism underlying mechanical dysfunction in the border zone of left ventricular, aneurysm: a finite element model study. Ann Thorac Surg 2001;71:654-662.[Abstract/Free Full Text]

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