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European Heart Journal Advance Access originally published online on November 6, 2007
European Heart Journal 2008 29(10):1296-1306; doi:10.1093/eurheartj/ehm467
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Published on behalf of the European Society of Cardiology. All rights reserved. © The Author 2007. For permissions please email: journals.permissions@oxfordjournals.org

Real-time three-dimensional echocardiography in aortic stenosis: a novel, simple, and reliable method to improve accuracy in area calculation

Juan Luis Gutiérrez-Chico1,*, José Luis Zamorano2, Elsa Prieto-Moriche2, Rosa Ana Hernández-Antolín2, Marisol Bravo-Amaro1, Leopoldo Pérez de Isla2, Marcelo Sanmartín-Fernández1, José Antonio Baz-Alonso1 and Andrés Íñiguez-Romo1

1 Unidad de Cardiología Intervencionista, Hospital de Meixoeiro, Instituto Galego de Medicina Técnica, Crtra. de Meixoeiro s/n, 36204 Vigo (Pontevedra), Spain
2 Hospital Clínico San Carlos, Madrid, Spain

Received 21 April 2007; revised 10 September 2007; accepted 24 September 2007; online publish-ahead-of-print 6 November 2007.

* Corresponding author. Tel: +34 986 811163, Fax: +34 986 811727. Email: jlgutierrez{at}medynet.com/ juan.luis.gutierrez.chico{at}sergas.es


   Abstract
Top
 Abstract
Introduction
 Methods
 Results
 Discussion
 Conclusion
 Supplementary material
 References
 
Aims: The aim of the study was to validate a novel formula for aortic area, based on the principle of continuity equation (CE), that substitutes Doppler-derived stroke volume (SV) by SV directly measured with real-time three-dimensional (RT3D) echo and semi-automated border detection. RT3D has proved outstanding accuracy for left ventricular volume calculation. So far, however, neither this potential has been applied to haemodynamic assessment, nor RT3D has succeeded in the evaluation of aortic valve disease.

Methods and results: Aortic area was measured in 41 patients with aortic stenosis using Gorlin's equation, Hakki's formula, Doppler CE, two-dimensional Simpson's volumetric method, and by the novel RT3D method. RT3D has the best linear association and absolute agreement with Gorlin of all non-invasive methods r = 0.902, intraclass correlation coefficient (ICC) = 0.846, better than CE (r = 0.646, ICC = 0.626) and two-dimensional volumetric method (r = 0.627, ICC = 0.378). Linear and Passing–Bablok regression show that RT3D fits better to Gorlin (r2 = 0.814) than CE (r2 = 0.417) and two-dimensional method (r2 = 0.393). Its accuracy is comparable to Hakki's formula, routinely employed in catheter laboratories. Inter- and intraobserver agreements (ICC) were, respectively, 0.732 and 0.985, better than CE (0.662, 0.857). RT3D also grades most efficiently the severity of aortic stenosis as mild, moderate, or severe (weighted kappa = 0.932). RT3D underestimates aortic area (95% CI 0.084–0.193). ROC curves, however, show that the optimal cutoff point to consider aortic stenosis severity remains close to 1 cm2 (1.06 cm2).

Conclusions: RT3D is more accurate than CE and than two-dimensional volumetric methods to calculate area and to grade the severity of aortic stenosis. Area obtained by three-dimensional echo is slightly underestimated, but its range is clinically negligible.

Key Words: Aortic stenosis • Aortic area • Real-time three-dimensional echocardiography • Continuity equation • Doppler • Cardiac catheterization • Gorlin equation


   Introduction
Top
 Abstract
 Introduction
Methods
 Results
 Discussion
 Conclusion
 Supplementary material
 References
 
Different modes of three-dimensional echo have proved superior to biplane two-dimensional methods for volume calculation,1–8 including Real-time three-dimensional echocardiography (RT3D), currently appearing as the most accurate echocardiographic method commonly available to measure LV volumes.9–18 Volumes can be obtained by manually tracing the endocardial LV contour on several planes with off-line quantification, but this procedure is burdensome and time-consuming for the cardiologist, so it has not gained widespread acceptance in routine clinical practice. Semi-automated border detection (SABD), however, provides fast and accurate quantification of LV volumes with RT3D. RT3D+SABD has also been validated for LV volume calculation,19–21 appearing clearly superior to two-dimensional echo, although it does not reach the accuracy of RT3D with manual tracing.

Hitherto, the advantages of RT3D for LV volume calculation have focused on LV systolic function. Practically, no publication addresses its potential for haemodynamic assessment. Only small studies have employed RT3D for stroke volume (SV) and cardiac output calculation, with promising results.22,23

The aortic area is commonly calculated by Doppler continuity equation (CE) in most echo laboratories, this being a well-validated and reliable method.24–27 Nevertheless, it has shown several well-known pitfalls in some subsets of patients, where the method has diminished accuracy.28–30 One of these limitations is the additive effect of cumulative errors in the measurement of all the parameters involved in the formula, mainly the LV outflow tract diameter, which appears squared, and therefore minimal mistakes often lead to considerable inaccuracy, and it is also extremely dependent on a good parasternal acoustic window.

We propose a new method to calculate the aortic area in patients with aortic stenosis, based on the principle of CE, applying the advantages of RT3D to improve the accuracy of strictly Doppler-based methods. We propose substituting the Doppler-derived SV by SV directly obtained with RT3D, which considerably simplifies the calculation, overcomes many potential sources of error, and does not depend on the quality of the parasternal acoustic window.


   Methods
Top
 Abstract
 Introduction
 Methods
Results
 Discussion
 Conclusion
 Supplementary material
 References
 
We studied 41 consecutive patients referred to the Invasive Cardiology Department, fulfilling any of the following inclusion criteria: (i) planned right catheterization, and maximal transaortic gradient >20 mmHg in prior non-invasive studies and (ii) known or suspected aortic stenosis undergoing cardiac catheterization. Exclusion criteria were (i) age < 18 years, (ii) subaortic stenosis, (iii) atrial fibrillation, (iv) bigeminism or frequent extrasystole, (v) any known condition precluding a proper transthoracic echocardiographic study, and (vi) clinical instability or any other circumstance discouraging the study as determined by the dealing cardiologist. Sample size was calculated considering a minimal r = 0.8, confidence 95%, and power 80%, and enlarged to fulfil the conditions to apply parametric tests and to obtain enough representation of mild, moderate, and severe aortic stenosis.

A left–right cardiac catheterization was performed, and the aortic area calculated according to the Gorlin and Gorlin formula.31 Cardiac output was measured with thermodilution through a Swan–Ganz catheter, considering the average of three different measurements. 5F catheters were used for left heart catheterization. Peak-to-peak and medium transaortic gradients were recorded in pullback from LV to aorta, positioning the catheter at the level of the aortic sinus. No simultaneous LV-aorta pressure recording was obtained, in order to reduce procedure invasiveness. The aortic area was also calculated with Hakki's simplified formula.32

An echocardiographic study was obtained prior to or following the invasive study, in a time gap <12 h, on a Philips ie33 © platform, using probes S51 and X3. A conventional two-dimensional Doppler study was performed and the aortic area calculated by CE. Doppler transaortic gradient was systematically searched in apical, right parasternal, and suprasternal windows, and the maximal measurement was employed for aortic formula calculations. Employing a non-imaging transducer for better Doppler recording was left to the investigator's discretion. LV volumes and systolic function were assessed in the two-dimensional study by Simpson's rule, tracing the endocardial border in apical two- and four-chamber views. Apical three-dimensional full-volume image was acquired, and LV volumes directly obtained using SABD. The semi-automated tracings were manually edited upon the echocardiographer's criterion on each case, adjusting the tracing to the outer limit of LV trabeculae, and including papillary muscles in the LV cavity.

The aortic area in the three-dimensional study was calculated with a novel method, dividing the SV directly obtained in the three-dimensional study by the Doppler time–velocity integral through the aortic valve (Figure 1). The same principle and the same formula were also tested with the SV obtained from Simpson's two-dimensional method (two-dimensional volumetric method).


Figure 1
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  		Figure 1 Formula for aortic area calculation with three-dimensional echo, proposed by Gutiérrez et al18

 
A single echocardiographer in each participating centre acquired and measured the two- and three-dimensional echo studies (Investigator 1). Twenty-five per cent of the studies were also measured by a second echocardiographer (Investigator 2) to assess interobserver variability, and again by Investigator 1 at least 15 days after the first measurement to control intraobserver variability. Invasive cardiologists, Investigator 1 and Investigator 2, were totally blinded to each other's results.

Statistical analysis
The aortic area obtained with each echocardiographic method was compared with the area obtained invasively by Gorlin's equation. Linear association between the methods was measured with Pearson's correlation coefficient. Agreement between the methods was quantified through intraclass correlation coefficient (ICC) for absolute agreement, Lin's concordance correlation coefficient, and Bland–Altman analysis. Means were compared with t-test for paired samples to detect any possible bias. Finally, the methods were confronted in linear and Passing–Bablok non-parametric orthogonal regression. Interobserver and intraobserver variabilities were controlled with ICC for absolute agreement. A qualitative analysis of the agreement was also performed in order to study the ability of each echocardiographic method to correctly classify the degree of aortic stenosis as mild (2 cm2 > area > 1.5 cm2), moderate (1.5 cm2 ≥ area ≥ 1.0 cm2), or severe (area < 1.0 cm2). For this aim, weighted kappa coefficient was employed. ROC curves of the novel method were obtained. The statistical analysis was performed with SPSS 12.0 (SPSS, Inc., Chicago, IL, USA). Macro !PB for SPSS (A. Bonillo, J.M. Domenech, and R. Granero) was employed for Passing–Bablok regression. Macro !KAPPA for SPSS (J.M. Domenech, A. Bonillo, and R.Granero) was employed to calculate the weighted kappa coefficient. Macro !ROC (A. Bonillo, J.M. Domenech, R. Granero, and R. Sesma) was used for ROC curves.


   Results
Top
 Abstract
 Introduction
 Methods
 Results
Discussion
 Conclusion
 Supplementary material
 References
 
Sample description
Forty-five patients were initially assessed for inclusion, but four patients refused to give their consent for right heart catheterization. Forty-one patients were included in the study. One of them was excluded due to unexpected poor acoustic window. Invasive study of another patient was excluded due to inability to perform it within a time gap < 12 h after having obtained the echo for logistic reasons. In 39 patients, aortic area could be measured by all invasive and non-invasive methods. Age of the sample had a non-normal asymmetric distribution, mean 71.0, median 72.8, SD 10.5, minimum 40.8, maximum 86.3 decimal years. Of 41 patients, 23 were male (56.1%).

No severe concomitant valvular regurgitation was found in any of the patients of the sample. Only 10% of patients had moderate aortic regurgitation, 5% moderate mitral regurgitation, and 5% moderate tricuspid regurgitation. Eighty five per cent of patients had an ejection fraction >50%, whereas 15% had impaired LV systolic function (10% mildly depressed, 2.5% moderately, 2.5% severely). Most of the patients had severe aortic stenosis (70%) according to Gorlin's equation, but also moderate (20%) and mild (10%) aortic stenosis were represented in the sample. Clinical and baseline characteristics of the sample are displayed in Table 1.


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  		Table 1 Clinical and baseline characteristics of the sample

 
Correlation (linear association) and agreement
Gorlin shows better linear association with RT3D (r = 0.902, 95% CI) than with CE (r = 0.646) (Table 2). The correlation is similar to that observed with Hakki's formula (r = 0.901). Two-dimensional volumetric method worst correlated with Gorlin (r = 0.627).


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  		Table 2 Correlation and absolute agreement (expressed as ICCa and Lin's coefficient) between the different invasive and echocardiographic methods studied for aortic valve calculation

 
RT3D is the non-invasive method with best absolute agreement with Gorlin (ICC = 0.846, Lin's coefficient = 0.844), better than CE (ICC = 0.626, Lin's coefficient = 0.621) and two-dimensional volumetric method (ICC = 0.378, Lin's coefficient = 0.375). The absolute agreement of RT3D with Gorlin is close to the one observed with Hakki's formula (ICC = 0.857, Lin's coefficient = 0.852).

Bland–Altman analysis of each echocardiographic method compared with Gorlin's equation is shown in Figure 2. For a concordance of 95%, and a sample of n = 39, it is to expect that less than two points were out of the limits of agreement. Only one point in RT3D is in the limit zone, indicating good agreement with Gorlin. All the echocardiographic methods are significantly below the zero line, what means trend to underestimate with respect to Gorlin. This trend is more exaggerate in two-dimensional volumetric method. RT3D reduces the underestimation, appearing close to that of CE.


Figure 2
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  		Figure 2 Bland–Altman analysis of the echocardiographic methods and Hakki's formula, compared with Gorlin's equation, for aortic area calculation. See Supplementary material online for a coloured version of this figure

 
Comparison of means
Paired samples t-test confirms the trends observed in the Bland–Altman analysis (Table 3). CE shows a slight non-significant trend to underestimation compared with Gorlin, whereas two- and three-dimensional volumetric methods significantly underestimate the area. The two-dimensional volumetric method incurs a considerable bias (0.226–0.423 cm2); the three-dimensional method, however, considerably reduces this underestimation (0.084–0.193 cm2).


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  		Table 3 Paired samples t-test for comparison of means

 
Linear regression
Figure 3 shows the linear regression analysis of the echocardiographic methods (x) with respect to Gorlin's equation (y). RT3D is the method that best fits to Gorlin, with r2 = 0.814 in the sample, adjusted r2 = 0.809 in the population, whereas CE only attains r2 = 0.417, adjusted r2 = 0.401. The predictive power of RT3D method is comparable with Hakki's formula (r2 = 0.809, adjusted r2 = 0.804). The estimate of constant {alpha} is lower with RT3D than with CE (0.168 vs. 0.280), and the estimate of β is closer to 1 (0.961 vs. 0.757).


Figure 3
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  		Figure 3 Linear regression of the echocardiographic methods and Hakki's formula (x) with respect to Gorlin's equation (y). Lines represent regression line (red, centre), 95% CI for the mean (red, inner bounds), and 95% CI for the individuals (blue, outer bounds)

 
Passing–Bablok non-parametric orthogonal regression
When the gold standard itself is subject of certain variability, linear regression is not the most appropriate, since it would imply the assumption that Gorlin measures aortic valve area without error. Figure 4 shows the analysis employing non-parametric orthogonal regression, more suitable for this kind of study, which underscores the previous findings: the three-dimensional method has the highest accuracy of all non-invasive methods (narrow interval between upper and lower bounds), comparable with that of Hakki's formula, although the area calculated by the three-dimensional method is significantly underestimated with respect to that calculated by Gorlin (estimated {alpha} = 0.454).


Figure 4
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  		Figure 4 Passing–Bablok non-parametric orthogonal regression of the echocardiographic methods and Hakki's formula (x) with respect to Gorlin's equation (y). See Supplementary material online for a coloured version of this figure

 
Inter- and intraobserver variability
The three-dimensional method is the non-invasive method with the best interobserver (ICC = 0.732) and intraobserver agreement (ICC = 0.985), surpassing that of CE (0.662 and 0.857, respectively).

Qualitative analysis
Three-dimensional echo is more efficient than CE for grading the severity of the aortic stenosis, considered as an ordinal variable with three categories (mild, moderate, and severe). The weighted (quadratic) kappa coefficient of three-dimensional echo contrasted vs. Gorlin is 0.932 (95% CI 0.831–1), clearly superior to CE (weighted kappa = 0.553; 95% CI 0.188–0.917). The two-dimensional Simpson's volumetric method has an extremely poor agreement (weighted kappa = 0.376; 95% CI 0.090–0.663) (Table 4).


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  		Table 4 Level of agreement between the echocardiographic methods and Gorlin's equation to grade the severity of aortic stenosis

 
ROC curves of RT3D to diagnose severe aortic stenosis (defined as an aortic area < 1 cm2 by Gorlin) are displayed in Figure 5. RT3D proves to be an excellent diagnostic tool, with an area under the curve value of 0.971 (95% CI 0.860–0.999). Notwithstanding the small bias detected in the quantitative analysis, the optimal cutoff point remains close to 1 cm2 (1.06 cm2). For this point, sensitivity = 100% (95% CI 87.66–100), specificity = 81.82% (95% CI 58.22–97.72), likelihood ratio = 5.5. We define ‘optimal cutoff point' as the value that, considered as cutoff to decide if aortic stenosis is severe, classifies correctly a largest proportion of subjects, that is to say, that renders the best efficiency. Neither positive nor negative predictive values are given, since the prevalence of severe aortic stenosis in the sample is not representative of the prevalence among patients primarily referred to the echo laboratory for diagnosis.


Figure 5
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  		Figure 5 ROC curves of the aortic area calculated by three-dimensional echo to diagnose severe aortic stenosis (defined as area ≤ 1 cm2 by Gorlin)

 

   Discussion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
Conclusion
 Supplementary material
 References
 
The results of this study fully validate RT3D as an accurate tool for aortic area calculation and clinical decision-making. Until now, RT3D had not succeeded in proving any clinical advantage for aortic valve disease assessment. The RT3D approach to aortic stenosis had focused mainly on anatomic valvular orifice planimetry, emulating transoesophageal echocardiography, with poor results. RT3D planimetry has succeeded in the mitral valve,33 but failed in the aortic valve. Our formula changes this pure anatomic concept and combines the accuracy of volumetric RT3D with Doppler measurements. Similar rationale had been previously applied to two-dimensional echo34 and cardiac magnetic resonance35,36 with excellent results. Prior studies have underscored the outstanding accuracy of RT3D for LV volume calculation,6–9,11,18–21 whereas CE depends on the unbiased estimation of several parameters, in turn dependent on the quality of both apical and parasternal acoustic windows. RT3D reduces the potential sources of error (direct anatomic SV calculation) and relies only on the quality of the apical acoustic window.

RT3D yields statistically significant aortic area underestimation. Many investigators have reported the trend of RT3D to underestimate LV volumes with respect to cardiac magnetic resonance.8,14,18–21 This bias vanishes when the endocardial contour is manually traced on eight planes or more,18 but increases when SABD is employed.21 Nonetheless, we intentionally chose SABD for the study, because it has gained widespread acceptance in routine clinical practice and it makes more sense to validate the tool the clinician will most likely have available for decision-making. Besides, there are hydrodynamic reasons to expect the aortic area by Gorlin to be slightly larger than by any echocardiographic method.37 Finally, the more precise a method is, the more prone to unmask its own bias. The underestimation does not seem, however, clinically relevant, since RT3D is the method that most efficiently grades the severity of aortic stenosis in the qualitative analysis, and the optimal cutoff point to diagnose severe aortic stenosis remains close to 1 cm2. The accuracy and consistency of the measurements somehow outweigh this evident trend towards underestimation.

In our study, CE yielded substantially worse results than those reported on classical studies,24–27 but comparable with those reported in other studies performed in similar clinical settings.28–30 Analysis of residuals reveals one far outlier and four outliers, responsible for the poor performance of CE. CE is an excellent and reliable method for most patients, but unsuitable for others (outliers), which greatly spoils its overall accuracy. RT3D seems more solid and less sensitive to the effect of outliers.

Limitations
Considering invasive assessment of the aortic valve area by Gorlin's equation as the gold standard could be questioned, especially when pullback measurements substitute the ideal simultaneous LV-aorta recordings. Admitting its limitations, Gorlin's equation is the best gold standard currently available, and the reference used for validation of all non-invasive techniques in aortic stenosis. Pullback recordings were intentionally chosen in order to reduce the invasiveness of the procedure. In routine practice, when aortic stenosis can be thoroughly evaluated in non-invasive studies, double arterial catheterization for such a study is hardly justifiable and gives rise to ethical problems. Simultaneous recordings in the femoral artery are even less reliable. To counterbalance the fact that our gold standard was subject to variability, we performed a more suitable statistical analysis for this scenario: Bland–Altman and Passing–Bablok non-parametric orthogonal analysis.

This is a general validation study, which includes all kinds of patients with aortic stenosis. The proportion of patients with moderate or severe concomitant valvular regurgitations was low in the sample, favouring agreement. Further research is needed to clarify to what degree severe valvular regurgitations could affect the reliability of the proposed formula. Likewise, most of the patients had preserved LV systolic function. Although systolic dysfunction was not found to impair accuracy in our study, and theoretically the aortic area should remain stable regardless of LV systolic function, a specific study focusing on this challenging subgroup should be addressed.

Our design (inclusion of patients referred for cardiac catheterization) propitiated an overrepresentation of severe aortic stenosis in the sample, which must be taken into account when interpreting our results. Mild and moderate stenoses were also sufficiently represented (otherwise validation would have been impossible) but their proportions differ by far from the prevalence among patients referred to echo laboratories for diagnosis. Therefore no valid estimation of predictive values can be inferred from this study.

Patients with atrial fibrillation or arrhythmia were excluded from the study, therefore no valid inference is possible in this subset of patients whose assessment remains difficult and challenging.


   Conclusion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conclusion
Supplementary material
 References
 
Three-dimensional echo is more accurate than the CE and than two-dimensional volumetric methods to calculate area and to grade the severity of aortic stenosis. The area obtained by three-dimensional echo is slightly underestimated with respect to that obtained by Gorlin, but the optimal cutoff point to consider the stenosis severe remains, however, close to 1 cm2.


   Supplementary material
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conclusion
 Supplementary material
References
 
Supplementary material is available at European Heart Journal online.

Conflict of interest: none declared.


   References
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conclusion
 Supplementary material
 References
 

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