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Eur J Cardiothorac Surg 2003;24:59-65
© 2003 Elsevier Science NL

Assessment of left ventricular mass regression after aortic valve replacement – cardiovascular magnetic resonance versus M-mode echocardiography

Kim Rajappana,c*, Nicholas G. Bellengera, Giovanni Melinab, Marco Di Terlizzia, Magdi H. Yacoubb, Desmond J. Sheridanc, Dudley J. Pennella

a Cardiovascular MR Unit, Royal Brompton Hospital, Sydney Street, London, UK
b Academic Department of Cardiothoracic Surgery, Royal Brompton and Harefield NHS Trust, Sydney Street, London, UK
c Academic Cardiology Unit, St Mary's Hospital, NHLI, Imperial College School of Medicine, London, UK

Received 8 February 2003; received in revised form 20 March 2003; accepted 21 March 2003.

* Corresponding author. Tel.: +44-207-351-8800; fax: +44-207-351-8816
e-mail: kumaran.rajappan{at}ic.ac.uk


   Abstract
Top
 Abstract
1. Introduction
 2. Materials and methods
 3. Results
 4. Discussion
 5. Conclusions
 References
 
Objective: In patients with aortic valve disease, the presence of left ventricular hypertrophy (LVH) carries a significant risk of adverse cardiovascular events. Regression of hypertrophy after aortic valve replacement (AVR) is associated with a reduction in risk. In general, M-mode echocardiography has been used for quantitative assessment of left ventricular mass (LVM) and regression, but this technique is believed to have limitations from which cardiovascular magnetic resonance (CMR) does not suffer. The objective of this study therefore was to determine whether quantitative assessment of LVM and regression after AVR using the two techniques was comparable. Methods: Thirty-nine patients with aortic valve disease were studied before and 1 year after AVR. Transthoracic M-mode echocardiography and four different formulae were used to calculate left ventricular mass index (LVMI), and then compared with CMR measurements. Results: Overall, correlation between the techniques for single measurement of LVMI was moderate (r-values from 0.64 to 0.69), with a tendency for overestimation by echocardiography; there was no agreement in degree of regression (r-values from 0.004 to 0.18). The Bland–Altman limits of agreement ranged from 85 to 131% for single measurement of LVMI, and 328–470% for regression. The change in LVMI with CMR was 43±28 g/m2, vs. 27 to 54±19 to 41 g/m2 using echocardiography. Conclusions: M-mode echocardiography does not provide reliable quantification of regression of LVH in individuals, and for accurate measurement CMR is superior. The use of CMR in future studies may reduce costs since fewer subjects are needed to accurately detect significant changes in LVMI after AVR.

Key Words: Valve replacement • Hypertrophy • Regression • Echocardiography • Cardiovascular magnetic resonance


   1. Introduction
Top
 Abstract
 1. Introduction
2. Materials and methods
 3. Results
 4. Discussion
 5. Conclusions
 References
 
Left ventricular hypertrophy (LVH) in patients with aortic valve disease may initially be seen as an attempt to normalise wall stress and oxygen demand. However, as with hypertensive LVH, it is associated with unfavourable alterations in myocardial pathophysiology i.e. left ventricle (LV) systolic dysfunction, ischaemia, and arrhythmias, as well as adverse prognosis [1]. Regression of LVH has been demonstrated extensively after aortic valve replacement (AVR) [2], but the persistence of LVH is associated with a worse outcome [3], with patient-related factors, particularly systemic blood pressure, being significant causes of late residual LVH [4]. This has focused attention on the ability to achieve regression of LVH, and how best to measure the change.

To date, most studies of LVH and its regression after AVR have used M-mode transthoracic echocardiography, a one-dimensional (1D) technique. This is readily available and the geometrical assumptions used have been validated in normal hearts [5], but it has some significant limitations, including being operator dependant, acoustic window dependant and subject to significant inter- and intra-observer variability. With the advent of three-dimensional (3D) tomographic techniques such as 3D echocardiography and ultrafast computerised tomography, it is now possible to address many of these limitations, but with cardiovascular magnetic resonance (CMR), it is possible to obtain high-resolution images in any desired plane without the need for ionising radiation or limitation by acoustic windows [6]. A stack of contiguous short axis slices that encompass the entire LV can be acquired and the precise volumes, mass and function calculated without the need for geometric assumptions. This results in measurements that are not only accurate compared to anatomical LVM determined by autopsy [7] but highly reproducible [8] and reduces the patient numbers required to prove a hypothesis in research studies [9]. Furthermore, the current fast sequences allow this to be achieved in a shorter imaging time than many of the other techniques [10].

Previous work from our institution has compared various techniques for measurement of cardiac volumes and function [11]. A few studies have compared echocardiography and CMR for assessment of LVM in various patient populations [12,13] but to date none have looked at quantitative assessment of regression of LVM. The aim of this work was therefore to determine how conventional 1D echocardiography compares with CMR, for the assessment of regression of LVM in a group of patients undergoing valve replacement surgery for aortic valve disease.


   2. Materials and methods
Top
 Abstract
 1. Introduction
 2. Materials and methods
3. Results
 4. Discussion
 5. Conclusions
 References
 
2.1. Study population
Thirty-nine patients (30 males) undergoing AVR were recruited to this study (aged 67±10 years), and 95% of them had predominant AS. The only exclusion criteria were those that prevented CMR. Of the 39 patients who had pre-operative investigations, 28 underwent both echocardiography and CMR 1 year post-operatively. Of the 11 patients who did not undergo CMR post-operatively four patients had permanent pacemaker implantation, one became claustrophobic, one had an epicardial pacing wire left in situ, and five were unable to attend/lost to follow-up. The Research and Ethics Committee approved the study, and patients gave informed written consent. Procedures were carried out in accordance with local guidelines.

2.2. Surgical technique
Twenty-nine valves were inserted in a standard manner. Eleven bioprostheses (five Baxter Carpentier Edwards, four Baxter Edwards Perimount, one Mitroflow and one St Jude) and 18 mechanical valves (nine Carbomedics 500 Type A, five St Jude, three Baxter Edwards MIRA and one Medtronic Hall) were used. Ten prostheses (three Medtronic Inc. Freestyle xenograft and seven homograft) were inserted as an aortic root replacement with reimplantation of the coronary arteries [14].

2.3. Echocardiography
Transthoracic echocardiography was performed using an ATL HDI 3000 (ATL Ltd, Bothwell, Washington, USA) cardiac ultrasound scanner with a 2.5 MHz probe, according to American Society of Echocardiography (ASE) guidelines [15]. LV dimensions were measured at end-diastole by 2D guided M-mode echocardiography taken at the level of the papillary muscle tip. LVM was calculated using the equations below and corrected for body surface area to give LVM index (LVMI). All echocardiographic values were the mean of three separate readings.

Bennett–Evans [16]: LVM=[(2xLVW+LVID)3-LVID3]x1.05
ASE [17]: LVM=0.83x[(IVST+PWT+LVID)3-LVID3]-0.6
Penn [5]: LVM=1.04x[(IVST+PWT+LVID)3-LVID3]-13.6
Teichholz [18]: LVM=1.04x{[7/(2.4+LVID+IVST+PWT)]x(LVID+IVST+PWT)3-[7/(2.4+LVID)]x(LVID)3}
where LVW is the left ventricular wall thickness measured from the posterior wall, LVID the left ventricular internal diameter, IVST the interventricular septal thickness, PWT is the posterior wall thickness. Established criteria were used to define the presence of LVH (LVMI>=134 or 110 g/m2 in men and women, respectively) [19].

2.4. Cardiovascular magnetic resonance (CMR)
Patients were imaged with a Picker Edge 1.5T scanner (Picker, Cleveland, OH), using the body coil and electrocardiogram (ECG) triggering, as previously described [20]. In brief, the cardiac short axis was determined from three scout images of the LV, the transverse, vertical long axis and breath-hold end-diastolic horizontal long axis. The basal short axis slice was positioned just forward of the atrioventricular ring, and all subsequent breath-hold cines were acquired in 1 cm steps towards the apex. A breath-hold segmented gradient echo fast low-angle shot (FLASH) sequence was used for each of the contiguous short axis slices. Parameters were as follows: echo time (TE) 3.8 ms, repeat time (TR)=RR interval, slice thickness 10 mm, field of view 35x35 cm, read matrix 256, phase matrix 128, frames 16, flip angle 35°, phase encode group 6–10. An average of 11 short axis segments was needed to encompass the entire LV.

Image analysis was performed on a personal computer using in-house developed software (CMRtools© Imperial College). End-diastolic (phase 1) and end-systolic (phase 4–7 depending on number of phase encoded groups) images were chosen as the maximal and minimal cross-sectional areas in each cine. Short axis end-diastolic epicardial and endocardial borders were traced manually for each slice (Fig. 1 ). From the area within the contours and the slice thickness, the epicardial and endocardial volumes were calculated, the difference representing myocardial volume. Mass was derived from this volume multiplied by the specific density of myocardium (1.05 g/cm3). Papillary muscles were included in the mass. Care was taken not to include atrial slices at end-systole secondary to apical movement of the heart.



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  		Fig. 1. Calculation of volumes using a CMR stack of contiguous short axis slices according to Simpson's method. ED – end-diastolic images; ES – end-systolic images. The epicardial borders are defined on the ED images, and the endocardial borders are defined on both the ED and ES images. This allows accurate measurement of the area of the blood pool and myocardium for each short axis slice, and subsequent calculation of ventricular volumes and mass. Note the signal loss (white arrow) around the sternum caused by sternal wires in this patient who had a sternotomy. This does not affect LV volume assessment.

 
2.5. Statistical analysis
The mean differences in LVM and LVMI were calculated between each technique and Student's t-test was used to determine statistical significance. A line of identity on a scattergram was drawn between each technique to allow a visual assessment of agreement. The correlation coefficient was then calculated to assess the strength of the relation. However, since correlation does not necessarily represent agreement, Bland–Altman plots were constructed to compare echocardiographic and CMR data [21]. Results are presented as mean±1 standard deviation.


   3. Results
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Results
4. Discussion
 5. Conclusions
 References
 
Pre-operative LVMI (N=39), post-operative LVMI (N=28) and both absolute reduction as well as percentage reduction of LVMI (N=28) are shown in Table 1 for CMR and each of the echocardiographic formulae. Change in LVM by CMR was 43±26, 54±41 g/m2 with the Bennett–Evans (BE) formula, 42±32 g/m2 with the ASE formula, 52±40 g/m2 with the formula of Devereux with Penn correction (Penn) and 27±20 g/m2 with the Teichholz formula. Individual changes are shown in Fig. 2 . For all the scans performed both pre- and post-operatively, significant correlation was found between LVMI assessed with CMR and echocardiography using all the equations. However, discrepancy in a few high-end LVM measurements did influence the steepness of the slope of correlation to some extent (Fig. 3 ). Bland–Altman plots showed that despite the significant correlation between LVMI using all four echocardiographic formulae and CMR and the effect of any particular points (Fig. 3), the Bland–Altman limits were very wide for each formula, ranging from 99 to 152 g/m2 (85–131%) (Table 2).


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  		Table 1. Echocardiographic and CMR values of LVMI

 


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  		Fig. 2. Pre-operative (pre op) and post-operative (post op) LVMI values using CMR and each of the echocardiographic formulae.

 


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  		Fig. 3. Scattergrams and Bland–Altman plots of combined pre- and post-operative values of LVMI measured with CMR and echocardiography using the BE, ASE, Devereux with Penn correction (Penn) and Teichholz formulae. The line of unity (dashed line), linear regression line (solid line) and r values are shown on each scattergram.

 

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  		Table 2. Differences, correlation and Bland–Altman limits for mass and regression measurements

 
There was no correlation between the change in LVMI after AVR assessed with any of the echocardiographic formulae and CMR, either when the absolute reduction in LVMI or the percentage reduction in LVMI was considered (Table 2 and Fig. 4 ). Bland–Altman plots suggested that for increasing LVMI, BE, ASE and Penn echocardiography tended to overestimate reduction in LVMI compared to CMR, but overall the plots and wide Bland–Altman limits of agreement confirmed the lack of any useful relationship between echocardiographic and CMR LVMI regression assessment (Fig. 4).



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  		Fig. 4. Scattergrams and Bland–Altman plots of absolute reduction in LVMI measured with CMR and echocardiography using the BE, ASE, Devereux with Penn correction (Penn) and Teichholz formulae. The line of unity (dashed line), and linear regression line (solid line) with equation, r and P values are shown on each scattergram.

 

   4. Discussion
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Results
 4. Discussion
5. Conclusions
 References
 
The main results of this study are that, although there is some correlation between quantification of baseline LVMI with 1D M-mode echocardiography and CMR, the agreement is not acceptable in the assessment of LVM regression in individuals – an end-point used widely in the literature while assessing the effect of therapeutic interventions such as AVR.

4.1. Assessment of regression of LVMI
Although there was some correlation between echocardiography and CMR for LVMI assessment and agreement in mean differences in LVMI after AVR, there was no correlation in assessing regression in individuals. There are several key reasons for this. CMR has fundamental advantages over other imaging techniques, in that it produces accurate and reproducible 3D tomographic, static or cine images of high spatial and temporal resolution in any desired plane without exposure to contrast agents or ionising radiation. However, there is significant initial cost in setting up a CMR scanner and there are some people for whom it is not suitable e.g. those with permanent pacemakers. Compare this to M-mode echocardiography that, although widely available, cheap on a per scan basis, and portable, is operator and acoustic window dependent, and only measures a single segment assuming this is representative of the entire LV. The subsequently derived volume and mass measurements from formulae based on certain geometric assumptions do not hold true in the presence of regional variations. Studies on hypertensive patients with presumed concentric hypertrophy reported that only 8% of 165 hypertensive patients had typical concentric hypertrophy to which the various geometric echo formulae may be applied [22]. As the LV volume increases the LV becomes more spherical and the relation between length and diameter is altered. As a result, as the LV diameter increases, the 95% confidence interval of prediction of LV volume from the diameter rapidly increases [23]. 2D echocardiography overcomes some of these problems but still extrapolates data from limited views, which are very dependent on correct angulation of the probe, gain dependent edge identification and good endocardial border definition. Furthermore, there is significant intra-observer variability in measurement of wall thickness and internal diameter (6.9 and 2.3% for posterior wall thickness and end-diastolic diameter, respectively) [24]. These errors are cubed by the echo formulae and can result in changes in LV mass of approximately 8–15%. Errors are also exacerbated by the fast heart rate of some patients; a 2.7% decrease in end-diastolic diameter and a 2.7–3.6% increase in wall thickness occurs with each 10 beat/min increase in heart rate in normal subjects [25].

To date, most studies have used echocardiography to investigate regression of LVM after AVR. Although M-mode echocardiography is widely available and has been validated in the past [5], it is clear that newer 3D techniques such as CMR has overcome many of the inaccuracies associated with a 1D technique like M-mode. Previous studies have demonstrated that in comparison to CMR, echocardiography does not provide a reliable method of quantifying LV mass in individual patients [12]. In our study comparing a 3D with a 1D imaging modality, the lack of any relationship between the two techniques in the assessment of regression itself, suggests a particular limitation of M-mode echocardiography in assessing this marker of prognostic significance. M-mode echocardiography may allow semi-quantitative or qualitative assessment, but for accurate quantitative measurement CMR, and other 3D techniques such as 3D echocardiography are superior.


   5. Conclusions
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Results
 4. Discussion
 5. Conclusions
References
 
The implications of our findings are twofold. First, that in research studies the use of 3D techniques such as CMR should now be seen as the gold standard for LVM regression measurement. This may actually reduce costs because the increased accuracy allows the reduction of the number of subjects needed to detect significant changes. Secondly, future clinical studies and assessment should also involve the use of newer techniques for all the same reasons.


   References
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Results
 4. Discussion
 5. Conclusions
 References
 

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