Eur J Cardiothorac Surg 2006;29:S225-S230
© 2006 Elsevier Science NL
Left ventricular geometry in normal and post-anterior myocardial infarction patients: sphericity index and ‘new’ conicity index comparisons
Marisa Di Donato
a
,
*
,
Petar Dabic
b
,
Serenella Castelvecchio
b
,
Carlo Santambrogio
b
,
Jelena Brankovic
b
,
Luigi Collarini
b
,
Tammam Joussef
b
,
Alessandro Frigiola
b
,
Gerald Buckberg
c
,
d
,
Lorenzo Menicanti
b
, the RESTORE Group
a Deparment of Critical Care Medicine, University of Florence, Italy
b San Donato Hospital, Milano, Italy
c Option on Bioengineering, California Institute of Technology, Pasadena, CA, USA
d David Geffen School of Medicine at UCLA, Los Angeles, CA, USA
Received 2 February 2006;
received in revised form 23 February 2006;
accepted 1 March 2006.
* Corresponding author. Address: Cardiac Surgery Department, San Donato Hospital, Via Morandi 30, San Donato Milanese, Milano, Italy. Tel.: +39 0252774636; fax: +39 0252774615. (Email: marad{at}tin.it).
 |
Abstract
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Background: Anterior myocardial infarction leads a sequence of structural changes that alter the size and the shape of the left ventricle. Efforts to assess shape have been made by global left ventricular (LV) chamber analysis (sphericity index, SI) but this analysis does not detect regional shape abnormalities like those at the apical level, which precede global ventricular dilatation. Objective: The present study will introduce a new analysis of regional apical changes in 52 normal subjects and in 92 patients with previous anterior myocardial infarction. Methods: All patients had transthoracic echocardiogram and multiple views were obtained (long axis, 4CH, 2CH and short axis view). From the 4CH view the long and the short axes were measured and their ratio was calculated (sphericity index). In the same view, the apical axis length was also measured and the ratio between apical and short axis length was calculated (apical conicity index, ACI). Results: Patients had all the measured parameters significantly worse than normal, except the sphericity index which remained unchanged. Ventricular length and width increased following anterior MI but the ratio between the two measurements did not change. Conversely, apical conicity index is significantly different following anterior MI, thereby indicating anterior infarction produces a less conical shape. SI and ACI differed when correlations were made in the relationship of mitral valve function; SI correlates with the degree of mitral regurgitation (MR) and with the distance of papillary muscles, conversely ACI shows an inverse correlation with the determinants of mitral regurgitation. These observations reflect differences between apical versus global dilatation in ischemic cardiomyopathy, so that mitral function is better (lower tenting area and lower coaptation height) when the apex is markedly dilated in respect to the short axis (high conicity index). In contrast, mitral function is impaired (bigger distance between papillary muscles and higher degree of mitral regurgitation), when sphericity index is high. Conclusions: Sphericity index fails to detect regional apical shape abnormalities. To address this focal change, we introduce a simple new measure termed apical conicity index, which is abnormal in patients with myocardial infarction, and can be useful to evaluate changes induced by the subsequent surgical approach of ventricular re-shaping.
Key Words: Left ventricular geometry • Left ventricular shape • Sphericity index • Anterior myocardial infarction
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1. Introduction
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A sequence of structural changes following anterior infarction occurs as the heart dilates to compensate for the increased load produced from the nonfunctional akinetic or dyskinetic regions [1]. Prognosis is a surrogate of the extent of dilation [2,3]. Although a globular heart is described after dilation, there is limited detailed analysis of shape and geometry [4–6]. Anterior infarction is a regional event, yet efforts to assess shape have been made by global left ventricular (LV) chamber analysis by the sphericity index (SI), which measures the ratio of either (a) the long axis to short axis or (b) end diastolic volume to the volume of the sphere having the measured long axis diameter. Additionally, more complex and time-consuming Fourier [7] or radius curvature analysis is used to quantify left ventricular shape [8].
While global measurements of sphericity index may be useful for the global disease that occurs in nonischemic cardiomyopathy from valvular or diffuse myocyte origin, ischemic cardiomyopathy following anterior infarction begins in the apical region, with secondary global dilation on a time-related basis. Consequently, there is major limitation of sphericity index analysis of the entire chamber obtained by the global axis ratio, since this single plane ratio reflects a linear alteration in the two axes of the entire ventricular chamber. Consequently, it does not focus upon regional shape abnormalities in the apical region as these changes may precede secondary global ventricular dilation.
To overcome this problem, this study will introduce a new analysis of regional apical changes. There will be comparison between normal subjects with elliptical chambers and normal SI, and patients with post-anterior infarction cardiomyopathy to introduce apical conicity index (ACI) concept. Emphasis will be placed upon how these indices are altered following development of secondary mitral insufficiency, which frequently complicates myocardial infarction [9,10].
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2. Methods
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2.1 Patient groups
The normal reference group was comprised of 52 subjects (26 men and 26 women; mean age 43 ± 14 years) that were considered to be normal following clinical and EKG examination. Each underwent a transthoracic echocardiographic study. The dilated ischemic anterior cardiomyopathy group included 92 patients (75 men and 17 women, mean age 65 ± 9 years). Each patient was hospitalized at our institution, underwent pre-operative transthoracic echocardiographic study, and subsequent coronary artery by-pass grafting, mitral valve repair (when indicated), and left ventricular reconstruction.
2.2 Ultrasonographic images and measurements
Vivid 7 GE Medical System ultrasound instrument was used. Parasternal long and short axis views, 2CH and 4CH were obtained in all cases. LV end diastolic and end systolic volumes, indexed by body surface area (EDVI and ESVI, respectively) were measured with the Simpson's method in 4CH and 2CH views and EF automatically calculated. Parasternal long axis view was used to calculate end diastolic and end systolic internal diameters (EDD and ESD in mm), tenting area of the mitral valve (TA in cm2), and the height of mitral leaflet coaptation point (H in mm). The distance between the two papillary muscle (PMM) in mm was measured from short axis view at the level of PMM heads.
Long axis (L) length was measured in 4CH view from the apex to the mid-point of the mitral valve and short axis (S) length was measured as the axis that perpendicularly intersects the mid-point of the long axis. From the same echocardiographic view, the diameter of the sphere that best fits the apex was measured as the apical axis (Ap) (Fig. 1
). Diastolic and systolic measurements were obtained, and the percent change of each single axis was also calculated. For precision, all measurements were done in triplicate and displayed as an average value.
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Fig. 1. Four chamber view used to measure the apical axis and conicity index. The apical axis is taken as the diameter of the hypothetical sphere that best fits the apical curvature.
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2.2.1 LV shape estimation
Sphericity index as short to long axis ratio, and apical conicity index as apical to short axis ratio (Fig. 1) was calculated in systole and diastole. Sphericity index was also calculated by volume ratio whereby the theoretic sphere volume was calculated as 1/6
x
L
3 (where L is the long axis of the LV) and the ratio between the ventricle and the theoretical volume defined SIV.
2.2.2 Statistical analysis
Data are presented as mean ± SD. Means were compared with an unpaired, two tailed Student's t-test.
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3. Results
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Summaries of comparisons between normal and ischemic anterior cardiomyopathy patients are shown in Table 1
reflecting axis measurements and fractional shortening of each axis, Table 2
displays echocardiographic parameters, and Table 3
defines shape considerations.
The adverse effects of ischemic cardiomyopathy are statistically evident in every parameter except global sphericity, which remained unchanged between normal patients and those with dilated hearts after anterior infarction. Although ventricular length and width increased following anterior infarction, the dimensionless ratio between length and width did not change, so that SI in diastole (0.53 ± 0.10 vs 0.52 ± 0.06) and SI in systole (0.46 ± 0.12 vs 0.45 ± 0.06) was unaltered.
Conversely, apical conicity index during systolic and diastolic phases of the cardiac cycle is significantly different in anterior ischemic cardiomyopathy compared to normal subjects (ACI 0.76 ± 0.19 vs 0.60 ± 0.09 in diastole and 0.86 ± 0.34 vs 0.52 ± 0.10 in systole p
= 0.000), thereby indicating anterior infarction produced a less conical shape. These ACI changes confirm the Fourier analysis changes (Fig. 2
) described previously [3], whereby post-anterior MI patients display a less conical apical shape (flattened curvature), together with inferior apex displacement towards the mitral plane.
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Fig. 2. Mean left ventricular contours as reconstructed by Fourier analysis in normal and patients with chronic anterior myocardial infarction. Curvature analysis starts from the anterior aortic corner (point 0) and proceeds clockwise in 90 equidistant steps to the mitral plane (point 90). In order to reconstruct the mean ventricular contour we performed the Fourier shape analysis according to the method adopted by Kass. A rounded ventricular apex is well evident in patients as well as its clockwise rotation.
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Visual comparisons of SI are shown in Fig. 3
, demonstrating how the long and the short axes proportionally increase in normal and anterior ischemic cardiomyopathy, so that the SI ratio between the two remains constant. In contrast, Fig. 4
demonstrates how the apical axis increases more than the short axis following anterior ischemic cardiomyopathy, so that the diastolic ACI ratio is significantly higher (0.76 ± 0.19 vs 0.60 ± 0.09, p
= 0.000) than occurring in normal patients.
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Fig. 3. The drawing shows the diastolic short to long axis ratio in normal and patients. Note the average proportional increase ±SD of the two axes which lead no changes in sphericity index.
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Fig. 4. The drawing shows the increase of the apical axis in patients and the significant increase in the apical to short axis ratio (conicity index). Diastolic values are reported.
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Fractional shortening is significantly reduced in anterior ischemic cardiomyopathy at each axis level (Table 1), but the greatest reduction (12% vs 33%) occurred at the apical axis.
These indices differed when correlations were made in the relationship of mitral valve function regarding predictors of normal and abnormal function as gauged by tenting area and mitral coaptation point, degree of mitral insufficiency and dimensions between papillary muscles.
In anterior ischemic cardiomyopathy patients, anterior conicity index shows a weak but significant inverse correlation with the tenting area (r
= –0.35, p
= 0.01; 95% CI –0.5600/–0.1130) and with the mitral coaptation point (r
= –0.30, p
= 0.001; 95% CI –0.5169/–0.0052), while sphericity index does not. In contrast, sphericity index shows a weak direct correlation with the degree of mitral regurgitation (MR) (r
= 0.28, p
= 0.02; 5% CI 0.029–0.499) and with the distance of PMM (r
= 0.42, p
= 0.001; 95% CI 0.1675–0.6226), while anterior conicity index does not.
These observations reflect differences between apical versus global dilation in ischemic cardiomyopathy patients, so that mitral function is better (lower tenting area and lower coaptation height) only when the apex is markedly dilated with respect to the short axis (highest conicity index). In contrast, mitral function is impaired (bigger distance between PMM and higher degree of MR) when sphericity index is high. Table 4
shows global sphericity index and conicity index in patients with and without mitral regurgitation. In the subset of patients with MR, SI is significantly higher than normal (more spherical ventricle) (SI 0.58 ± 0.09 in diastole and 0.51 ± 0.08 in systole, p
= 0.0007 and 0.036 vs normal, respectively) in the MR cohort, while apical conicity index does not correlate with the MR component. LV volumes were not significantly different in MR and no MR patients.
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4. Discussion
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This study compared measures of ventricular geometry relating to LV size and shape, echocardiographic changes and global and apical shape indices in normal patients and a cohort of patients with ischemic anterior dilated cardiomyopathy. The major findings include: (1) left ventricular geometry becomes abnormal following chronic anterior MI; (2) long and short ventricular axes increase proportionally in patients without mitral regurgitation so that (3) sphericity index only increases after mitral regurgitation; and (4) a new measurement termed the apical conicity index (ACI) is introduced, which alters significantly in anterior ischemic patients. This ACI change is independent of associated changes in sphericity index, thereby reflecting the major abnormalities in apical shape produced by anterior infarction.
4.1 Left ventricular remodeling
The underlying pathology of anterior infarction is necrosis and scarring of the anterolateral ventricle and septum, followed by progressive changes in remote muscle to cause progressive dilation of compensatory remote muscle and evolution of heart failure [3]. Adaptive changes within the myocardium, during ‘LV remodeling,’ are viewed as a ‘neuro-hormonal model,’ whereby heart failure progresses as a result of the over expression of biologically active molecules that are capable of exerting deleterious effects on the heart and circulation [11]. Adverse events affect the biology of the cardiac myocyte, the volume of myocyte and nonmyocyte components of the myocardium, and thereby alter the geometry and architecture of the LV chamber. New mechanical burdens for the failing heart create the consequent increase in LV size and resultant change in LV shape.
These observations led to the recent introduction of the ‘biomechanical model’ of heart failure, which predicts that at some point, heart failure will progress independently of the neuro-hormonal status. This implication led to consideration of therapeutic strategies that interrupt the vicious cycle of myocardial dysfunction and/or cardiac remodeling in order to reverse the natural history of heart failure progression [11,12]. The surgical approaches to prevent, retard, and reverse remodeling include cardiomyoplasty, which has largely been abandoned [13], mitral valve surgery [14], volume reduction surgery (partial left ventriculectomy (so-called Batista procedure)) [15], endoventricular circular patch plasty/surgical anterior ventricular restoration (so-called Dor procedure) [16], and cardiac assist support devices [17,18]. Each of these options addresses changing the index event that ultimately dilates the ventricle, so that quantification of structural geometric abnormalities stemming from ischemic, valvular, or nonischemic processes may allow proper evaluation of surgical results.
4.2 The remodeled heart and sphericity index
Ventricular size and shape are the two geometric aspects that change in dilated failing hearts. The quantitative anatomic observations of Lindbach, in 1960 described the consistent finding of a more spherical shape within the expanded size of the abnormal geometry of remodeled ventricle [19]. This concept of increased sphericity has led to the development of the sphericity index, as a way to quantify the abnormal geometric changes that accompany heart failure in dilated failing left ventricles.
Although the data in this report of ischemic cardiomyopathy after chronic anterior infarction document profound changes in ventricular size and geometry (larger volumes, bigger longitudinal and transverse diameters, greater tenting area and height of leaflet coaptation) sphericity index always differ from normal hearts. The long and short axis increased at the same proportion to maintain a constant ratio, except when patients with anterior infarction developed mitral regurgitation. These observations led to an evaluation that focuses upon a regional shape abnormality as the index event of anterior infarction. Simultaneously, it uncovers how the secondary events of late mitral insufficiency influences the global chamber to alter SI.
The LV apex is primarily involved in anterior MI, so that the regional changes affect the anterior, septal, and inferior ventricular components. Conversely, sphericity index evaluates the entire LV chamber, and fails to detect such regional abnormalities. For that reason, the apical conicity index was introduced, since it measures the ratio of the apex to the short axis. This ACI is significantly greater in anterior ischemic patients compared to normal patients, since this index quantified the changes in enlarged, less conical apex. The contrast to normals was apparent, since the apex of the ventricle is conical under physiologic conditions, has lower diastolic wall stress due to greater radius of curvature, and a lower internal radius. Conversely, the regional shape abnormality in anterior ischemic patients decreases curvature and increases internal radius. These architectural changes become coupled with a thinner wall to allow markedly deformed apical shape to increase wall tension. The ACI was used since this index is easily measured by echocardiography, instead of regional 3D image technology that provides better analysis.
The sphericity index, addresses global chamber changes, since this index uses a single plane to analyze the ratio of the short to long axis, and compares the volume of the ventricle and the volume of the theoretical sphere (defined by using the measured long axis as the diameter of the sphere). A higher SI occurred only when global chamber changes emerged following secondary mitral regurgitation. This valvular complication of AMI [10] stretches remote muscle and increases global sphericity.
4.3 Mitral regurgitation and geometric abnormalities
Mitral regurgitation was present in 46 of our patients (50%); in 19 of these, it was grade 3 or 4+, representing the 21% of the overall group. This complication occurs with the largest LVESV volumes, as reported previously [20]. Sphericity index significantly correlated with the degree of mitral regurgitation, and was significantly different from normal only in anterior ischemic patients with mitral regurgitation. Conversely, MR was absent in patients with dilated ventricles, but without an increased global sphericity. These observations suggest that the geometric changes of sphericity may potentially displace the papillary muscles postero-laterally [21] and thus impair leaflet coaptation, as described previously after circumflex infarction [22]. In contrast, mid-ventricular chamber dilation occurred in these anterior MI patients without circumflex infarction, and this secondary event in noninfarcted remote muscle dilation thereby allowed SI to correlate with the distance of papillary muscles. The short axis is a vital component of measurements of the underlying feature that contributed to MR and with the short axis in systole and diastole obtained from the mid-point perpendicular to the long axis in four chamber view.
For example, the length of the ventricle does not differ in patients with and without MR but short axis does (SI diastole: 56 ± 8 mm vs 49 ± 10 mm, p
= 0.007). Conversely, apical conicity index does not address the short axis (higher CI) and negatively correlates with mitral geometric parameters (tenting area and height of leaflet coaptation). These changes indicate mitral geometry is preserved when the apex is markedly dilated, but the short axis is not dilated to the same extent.
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5. Limitations
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A major limitation of the study is that measurements are obtained by 2D echocardiography on a single plane. However, reference to normal subjects evaluated with the same ultrasound views, may allow a reliable data comparison.
In conclusion, the present study provides LV geometric measurements in a large series of normal subjects and compares these values to a cohort with ischemic cardiomyopathy after chronic anterior MI. Although anterior ischemic cardiomyopathy patients have marked geometric ventricular abnormalities, the significant elongation is accompanied by a proportional increase in width. Consequently, the sphericity index is maintained within normal range, because the global ratio remains constant, except for patients with mitral regurgitation.
Following anterior infarction, deformation occurs primarily at the apical level (regional shape abnormality) and is thus not detectable by the sphericity index. To address this focal change, a simple new measure, called the apical conicity index is described. This index detects apex abnormalities, and can be useful to evaluate changes induced by the subsequent surgical approach of ventricular re-shaping. We suggest that application of this simple index will detect the apical shape of normalities that precede global dilation.
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Footnotes
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Read at 10th RESTORE meeting in San Francisco, California, April 9, 2005.
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Abstract
1. Introduction
