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Table of Contents
ORIGINAL ARTICLE  
Year : 2013  |  Volume : 16  |  Issue : 1  |  Page : 23-27
Echocardiographic quantification of mitral valvular response to myocardial revascularization


1 Department of Anaesthesia, Critical Care and Pain Medicine, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA, USA
2 Mayo Clinic, Rochester, MN, USA

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Date of Submission10-May-2012
Date of Acceptance07-Sep-2012
Date of Web Publication2-Jan-2013
 

   Abstract 

Aims and Objectives: Mild and/or moderate ischemic mitral regurgitation (IMR) may resolve after isolated coronary artery bypass grafting (CABG). It has been shown that the loss of saddle shape of the mitral valve is associated with IMR and is determined by an increase in the nonplanarity angle (NPA). The aim of this prospective, observational study was to test the hypothesis that NPA might decrease immediately after CABG alone in patients with mild to moderate IMR. Materials and Methods: This prospective, observational study was conducted in an academic, tertiary care hospital. Twenty patients underwent 2D and 3D transoesophageal echocardiography (TEE) and mitral valve assessment before and immediately after the CABG. NPA, circularity index, and other geometric variables were obtained. They were compared using paired t test. The SPSS (Version 15.0, Chicago, IL, USA) was used for statistical analysis. P <0.05 was considered significant. Results: The NPA was similar in the pre- and post-bypass periods (148° ± 15°, 148° ± 19°, P = 0.88). Circularity index (0.93 ± 0.13, 0.97 ± 0.11, P = 0.41) also was similar. Conclusions: There was no change in the mitral valve NPA with revascularization alone in patients with mild or moderate IMR. Mitral valve does not change its planarity (NPA) with revascularization alone in patients with IMR.

Keywords: Circularity index, Coronary revascularization, Ischemic mitral regurgitation, Mitral valve geometry, Nonplanarity angle

How to cite this article:
Govindan S, Hayward G, Mahmood F, Subramaniam B. Echocardiographic quantification of mitral valvular response to myocardial revascularization. Ann Card Anaesth 2013;16:23-7

How to cite this URL:
Govindan S, Hayward G, Mahmood F, Subramaniam B. Echocardiographic quantification of mitral valvular response to myocardial revascularization. Ann Card Anaesth [serial online] 2013 [cited 2019 Jul 20];16:23-7. Available from: http://www.annals.in/text.asp?2013/16/1/23/105366



   Introduction Top


Ischemic mitral regurgitation (IMR) is a complex disease resulting from the effects of both ventricular remodeling and altered geometry of the mitral valve apparatus. [1] Reduced leaflet closing forces due to left ventricular (LV) dysfunction, [2] increased systolic mitral leaflet tethering due to papillary muscle displacement, [3] and annular dilatation [4] have been implicated in the development of IMR. It has been shown that mitral annulus dilates, flattens, and assumes a more spherical shape, resulting in reduced leaflet coaptation and mitral regurgitation. IMR with an incidence of 19-50% [5] is an independent predictor of mortality and has a lower 5-year survival. [6]

Mild to moderate IMR may or may not resolve after isolated coronary artery bypass grafting (CABG). [7] At present, patients undergoing CABG with incidental mild to moderate IMR may not get their regurgitant lesion surgically corrected. [8],[9],[10] Recent advances in echocardiographic techniques and data analysis software [11] have allowed for a better understanding of the mitral valve's three-dimensional (3D) structure and function. Using 3D transoesophageal echocardiography (TEE), we studied the mitral valve's geometry and function in patients who underwent CABG to see if there were any changes in its geometry in the immediate post-bypass period following surgical revascularization. We were particularly interested in the nonplanarity angle (NPA) and the circularity index. Loss of saddle shape of the mitral valve is associated with IMR and can be assessed by an increase in the NPA. [12],[13] Circularity index is a measure of the extent of circularity attained by the annulus as part of the annular remodeling secondary to IMR. We hypothesized that patients undergoing coronary revascularization alone with incidental mild to moderate IMR will show a decrease in NPA with or without changes in other geometric parameters.


   Materials and Methods Top


After obtaining institutional review board approval, 32 patients scheduled for elective CABG were assessed for eligibility in this prospective, observational pilot study. These patients had mild to moderate mitral regurgitation that was not surgically treated. Patients with incomplete data, lack of intraoperative echocardiographic data and/or associated valvular lesions were not considered eligible for the study.

Data acquisition

After patients were induced with standard dose of intravenous fentanyl and propofol, intubation was facilitated with intravenous rocuronium. Once the airway was secured, an orogastric tube was passed to ensure esophageal patency and also to aspirate the gastric fluid as per our institutional standard practice. Then, a 3D TEE probe was inserted. 3D TEE was performed before the start of surgical procedure after hemodynamic stabilization. The studies were performed on the Philips Medical Systems IE-33 TEE machine (Andover, MA, USA). The 3D images were reconstructed and analyzed using the mitral valve analysis software package by TomTec (Unterschleissheim, Germany).

Mitral valve imaging

A basic, comprehensive 2D and color Doppler TEE examination was performed. A full-volume 3D loop of the mitral valve was obtained before and after the revascularization during a stable hemodynamic period. The images acquired were immediately reconstructed and analyzed using the software package by TomTec. The analysis was performed in a sequential manner as published before [14] and is given in [Table 1]. The geometric variables acquired both before and after cardiopulmonary bypass were then compared.
Table 1: Sequential 3D analysis of the mitral value


Click here to view


The geometric variables measured are:

  1. Antero-posterior (AP) diameter - the distance between the anterior and the posterior landmarks.
  2. Commissural diameter - the annular diameter at the level of the commissures.
  3. Anterolateral-posteromedial (AL-PM) diameter - the diameter between the anterolateral and posteromedial landmarks.
  4. Anterior annulus length - the curve length of the anterior annulus between the two commissures.
  5. Posterior annulus length - the curve length of the posterior annulus between the two commissures.
  6. Annular circumference.
  7. Annular area.
  8. Nonplanarity angle - the angle subtended between the anterior and the posterior landmarks at the commissural diameter [Figure 1].
  9. Circularity index - automatically generated ratio of AP diameter/AL-PM diameter.
Figure 1: Nonplanarity angle as assessed by the mitral valve quantification software

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Data analysis

Paired t test was used to compare the geometric variables obtained before and after myocardial revascularization. The continuous data are expressed as mean and SD. Proportions are expressed as percentages. The SPSS (Version 15.0, Chicago, IL, USA) was used for statistical analysis. P <0.05 was considered significant.


   Results Top


Nine patients were excluded due to incomplete data acquisition. Three more patients were excluded due to associated valvular lesions such as aortic regurgitation or aortic stenosis. Twenty patients who had complete data sets were included to analyze mitral valve geometry before and immediately after revascularization before leaving the operating rooms. The demographic data for these 20 patients are shown in [Table 2]. These patients had an average age of 65 ± 10 (mean ± SD) years. Of these 20 patients, 70% were males, 25% had previous myocardial infarction (MI), and 15% had previous history of congestive heart failure. None of these patients had a known chronic mitral valve disorder. In the pre-CPB period, three patients had moderate mitral regurgitation (MR) and the rest were less than moderate. These were similar in the post-CPB period (moderate MR = 3, less than moderate = 17). All patients had left ventricular (LV) ejection fraction (EF) ≥40% in the pre- and post-bypass periods. The changes in the geometric variables after the CABG procedure are given in [Table 3].
Table 2: Demographic data (n = 20)


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Table 3: Geometric data


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Nonplanarity angle

The NPA was similar in the pre- and post-CPB periods (148° ± 15°, 148° ± 19°, P = 0.88). Circularity index (0.93 ± 0.13, 0.97 ± 0.11, P = 0.41) also was similar.

Other geometric parameters

Annular circumference (12.5 ± 1.1 cm, 11.7 ± 1.4 cm, P = 0.01) and annular area (10.1 ± 1.9 cm 2 , 9.3 ± 2.2 cm 2 , P = 0.04) were significantly reduced after coronary revascularization. Both the anterior annulus length (4.7 ± 0.9 cm, 4.2 ± 0.8 cm, P = 0.06), and posterior annulus length (7.8 ± 1.4 cm, 7.5 ± 1.2 cm, P = 0.33) did not change between the pre-CABG and post-CABG phases. AL-PM diameter and the commissural diameter were not statistically different between the pre- and post-bypass periods. Coaptation length, both 2D and 3D, did not undergo any statistically significant change.


   Discussion Top


The principal finding in our study is that both the NPA and the circularity index were similar in the pre-bypass and the immediate post-bypass periods following coronary revascularization alone in patients with incidental mild to moderate IMR. Both NPA and circularity index assess the saddle shape of the mitral annulus and its ability to protect against IMR. Mean NPA was increased in the pre-bypass period in our study and there were no immediate improvements in the mitral valve geometry (NPA and circularity index) following coronary revascularization. The secondary findings include a reduction in mitral annulus area and circumference.

Consensus opinion on the management of severe (grade 3 or 4) IMR points to remodeling annuloplasty using an undersized ring. [15] As for mild to moderate IMR, studies have shown varying results. While some studies have shown no change in the IMR after revascularization, [16] some have shown improvement in IMR with revascularization alone. [17] Reliable preoperative prediction of improvement of mild to moderate IMR after CABG alone cannot be done as of now. On the other hand, combined CABG and valve annuloplasty is associated with higher mortality. [10] We attempted to demonstrate any immediate mitral valve geometric changes in incidental mild to moderate IMR in patients of coronary artery disease after myocardial revascularization alone.

Our understanding of the pathogenesis of IMR has improved significantly with advances in echocardiography. [18],[19] Multiple factors are known to contribute to the pathogenesis of IMR. Imbalance in the mitral valve tethering and closing forces, mitral valve annular changes, and dys-synchronous LV contraction have been implicated in its pathology. [20] Two opposing forces act on the mitral valve leaflets at any given time during systole. A tethering force generated by the papillary muscles and the chordae works to pull the leaflets away from the annular plane and a closing force generated by the LV contraction moves them in the opposite direction. LV remodeling after MI increases tethering leading to MR by altering the mitral valve geometry. [21]

The inherent nonplanarity (saddle shape) of the mitral valve reduces the stress exerted on the valve leaflets during systole. [22],[23] Mitral annulus is saddle shaped with the highest points located anteriorly and posteriorly, and the lowest points located medially and laterally, at the commissures. Mitral valve annulus becomes larger and more planar in IMR. [24],[25] The loss of nonplanarity directly correlates with the loss of coaptation in MR. It has been shown that as the annular height commissural width ratio (AHCWR) increases, the peak leaflet stress decreases becoming maximally reduced once the ratio exceeds 0.2.[23] Peak stress decreases when the saddle height increases. When the annulus is flat, peak stress become very high. Salgo, et al., [23] have studied and described this concept elegantly in their study involving simulation experiments, and human and baboon mitral valve 3D echo images.

Mitral annular area decreases during systole and increases during diastole in a normal mitral valve. Different studies have variously described the time points at which maximal and minimal valve area are attained. [26] Studies have demonstrated that as mitral annulus enlarges, it loses its ability to change its position and shape. [4],[27] Several echocardiographic indices are utilized in measuring the degree of mitral regurgitation. Tenting height, tenting area, and tenting volume provide information about overall tethering effect. Regional leaflet tethering information can be obtained from individual leaflet angles. [20] In the current study, we measured NPA, circularity index, and annular area as a means to quantify the change in annular shape immediately after revascularization. NPA, [11],[28] the angle subtended between the anterior and posterior horns subtended at the commissural diameter, has been shown to quantify the mitral annulus nonplanarity. NPA has been shown to correlate well with AHCWR [28] The smaller the NPA, more nonplanar the annulus. We measured the change in the NPA in order to assess the effect of revascularization on IMR. There were no statistically significant changes in the NPA post revascularization. Circularity index is the ratio of the AP and the AL-PM diameter of the mitral valve at the annular plane, and is an automatically generated value. [29] A normal mitral valve has a circularity index less than 1 as the AP diameter is less than the AL-PM diameter. As the annulus dilates in the AP direction, circularity index increases to 1 and mitral annulus appears perfectly circular.

Limitations

This is a pilot study with a small sample size which gives a snapshot of mitral valve geometry immediately post-bypass in patients undergoing myocardial revascularization with mild to moderate MR. The same analysis if done at the time of discharge and after 6 months may throw more light into late remodeling. Statistically significant reductions in the annular area and annular circumference were found which could be due to the altered loading conditions caused by both anesthesia and CABG. However, the role of annular area change in the etiology of IMR is still being debated. Mitral valve function is affected by various factors like the cardiac rhythm, preload, LV systolic function, and degree and duration of MR. Various factors have been shown to predict the course of IMR after CABG and include LVEF, presence of viable myocardium, sinus rhythm, lower LV dimensions, and smaller annular diameter of which LVEF is an independent predictor. [7],[30] We have not analyzed the influence of these factors on MR. In addition to this, patients undergoing revascularization procedures are also subjected to alterations in volume and vascular tone due to anesthesia and the CPB procedure itself. Although, this study was not continued into the recovery period; echocardiographic follow-up of patients into the recovery period will allow us to quantify any persistent changes in mitral valve dimensions and shape.

To conclude, there was no evidence for mitral valve nonplanarity or circularity change immediately after revascularization in patients undergoing coronary revascularization alone with incidental mild to moderate IMR. Immediate mitral valve geometrical analysis following coronary revascularization surgery does not help to predict the progression of the incidental mild to moderate ischemic MR in the immediate postoperative period.

 
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Correspondence Address:
Balachundhar Subramaniam
One Deaconess Road, Harvard Medical School, West Campus CC-470, Boston, MA 02215
USA
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Source of Support: Institutional , Conflict of Interest: None


DOI: 10.4103/0971-9784.105366

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