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Review Article Left Atrial Volume - A New Index in Echocardiography Biswajit Paul Abstract Prediction of cardiovascular outcomes by non-invasive techniques remains a priority in patients with cardiovascular disease. The left ventricular ejection fraction, which is a parameter of systolic function has been the most sought after index for a long time. The importance of diastolic function has come to the foreground. Left atrial volume, is an index of long-standing diastolic function. This review discusses the methodology of estimation of left atrial volume and its clinical implications. F Introduction filling of LV is approximately 40%, 35% and 25% respectively.2 As LV relaxation gradually worsens, the relative contribution of LA reservoir and contractile function increases while conduit function decreases. But with advanced diastolic dysfunction the LA serves predominantly as a conduit. or long, the estimated ejection fraction (EF) of left ventricle (LV) determined by echocardiography has been used to predict cardiovascular outcomes. Recent evidence highlights the importance of left atrial (LA) volume with regards to prediction of cardiovascular outcomes. The LA volume has been compared to the “glycated hemoglobin of diabetes mellitus”, as it is a reflection of long-standing hemodynamic condition. This review aims to discuss the parameters used to determine LA phasic function, methodology of LA volume estimation and its usefulness in clinical practice. Assessment of LA size LA is not a symmetrically shaped three-dimensional (3D) structure. Furthermore, LA enlargement may not occur in a uniform fashion. Therefore anteroposterior measurement of LA by M-mode echocardiography is likely to be an insensitive assessment of any change in LA size. In contrast, LA volume by two-dimensional (2D) or 3D echocardiography provides a more accurate and reproducible estimate of LA size as compared to magnetic resonance imaging (MRI) and cine-computerised tomography (CT). The LA size is measured at the ventricular end-systole when the LA chamber is at its greatest dimension. It is imperative to avoid foreshortening of the LA for computing LA volume. The confluence of the pulmonary veins and LA appendage should be excluded, when performing planimetry.3 For assessment of left atrial ‘reservoir’, ‘conduit’ and ‘contractile’ function, LA volumes should be measured at specified phases of the cardiac cycle. They are a) end-systolic frame just before mitral valve opening ; b) end-diastolic frame just before mitral Mechanical Function of Left Atrium The mechanical function of LA has been traditionally described in three phases within the cardiac cycle: the ‘reservoir’, the ‘conduit’ and the ‘contractile’ machinery. During ventricular systole and isovolumic relaxation, the LA functions as a ‘reservoir’ receiving blood from pulmonary veins. The early phase of ventricular diastole sets the stage for LA operating as a ‘conduit’ for blood passing from the pulmonary veins into the LV. This is followed by atrial ‘contraction’ during which the LV stroke volume is augmented by approximately twenty percent.1 Various LA volumes have been used to describe LA phasic function. 1. Reservoir volume, is calculated as the difference between maximal and minimum LA volumes. Maximal LA volume occurs at ventricular end-systole just before the opening mitral valve; while minimum LA volume occurs at enddiastole, just before closure of mitral valve. 2. Conduit volume, is calculated as the difference between maximum and pre-atrial contraction LA volume. 3. Contractile volume, is calculated as the difference between minimum and pre-atrial contraction LA volume. The relative contribution of LA phasic function to LV filling is dependent upon the diastolic properties of LV. In subjects with normal diastolic function, the relative contribution of the reservoir, conduit and contractile function of the LA to the Consultant Cardiologist, Escorts Heart Institute & Research Centre, Okhla, New Delhi - 110025 Received: 8.11.2008; Accepted: 9.3.2009 © JAPI • JUNE 2009 • VOL. 57 Fig. 1: Panel A: LA volume by Simpson’s method. Panel B: LA volume by bi-plane area length method 463 pathology. In a non-compliant LV, as the LA is exposed to the pressures of the LV during diastole, LA pressure rises to maintain adequate LV filling.5 It is the increase in LA wall tension which leads to chamber dilatation. LA volume is therefore an expression of the chronicity of exposure to abnormal filling pressures.6 Thus LA volume reflects an average of LV filling pressures over time. It is thus useful for monitoring long-term hemodynamic control. Fig. 2: LA volume by 3-dimensional echocardiography valve closure; c) last frame just before mitral valve reopening i.e. pre-atrial contraction. Echocardiographic assessment of LA volume is done by Simpson’s method, area –length method and real time 3D echocardiography. Simpson’s method: Estimation of LA volume by Simpson’s method of disc is well validated and recommended by the American Society of Echocardiography (ASE) guidelines 3 (Figure 1A). 3. Atrial fibrillation: It is difficult to establish a causal relationship between atrial fibrillation and LA volume. Structural alterations in LA, may be related to the underlying pathophysiology or solely to the arrhythmia itself. Data from experimental animal studies do lend evidence to the fact that atrial arrhythmias induce structural remodeling.9 4. Volume overload: Chronic volume overload associated with large shunts, valvular regurgitation and high output states including athletic heart10 can contribute to LA enlargement. However myocardial relaxation physiology is usually normal as compared to abnormal myocyte relaxation seen in pressure overload situations. Atrial fibrillation (AF) is a serious cardiac arrhythmia associated with increased morbidity and mortality. Data from the Framingham11 and Cardiovascular Health Study12 have incriminated an increased anteroposterior LA diameter as the harbinger of AF. It has been confirmed that LA volume represents a superior measure over LA diameter for predicting outcomes inclusive of AF. The prognostic information provided is incremental to clinical risk factors. Increased LA volume is also a predictor of stroke and death. An indexed LA volume of ≥32 ml/m2 is associated with an increased risk of stroke independent of age and other clinical risk factors for cerebrovascular disease.13 An increased LA volume is also the predictor of first stroke in elderly who are in sinus rhythm and without any history of ischemic neurological events, AF or valvular heart disease. Real time 3D echocardiography:4 Full volume 3-dimensional images are obtained with the matrix array transducer. Zoom function gain adjustments are used to clarify the endocardial border. The three- dimensional dataset is transferred to a Q-LAB system for offline analysis. For calculation of LA volume, a semiautomated tracing of the LA endocardial border is performed by marking five atrial points : the anterior, inferior, lateral, septal mitral annuli and the LA apex (Figure 2). Modifications are made to correct automatic tracings if necessary. LA volume is the barometer of LV filing pressure and reflects the burden of diastolic dysfunction. Because a large number of individuals with LV dysfunction are in a pre-clinical phase of the disease, methods to quantify the risk of progression to symptomatic heart failure would be clinically useful. LA volume ≥ 32 ml/m2 is associated with increased incidence of heart failure which is independent of age, myocardial infarction, diabetes mellitus, hypertension, LV hypertrophy and mitral inflow velocities.14,15 Even in subjects with a normal ejection fraction, an increment in LA volume is observed from baseline to the diagnosis of heart failure. Reference Values Normal indexed LA volume has been determined in several studies involving several hundred patients using the preferred biplane technique. Most trial results indicate a value of 22 ± 6 ml/ m2 as the normal range and the same is recognized by ASE.3 Determinants of LA size 464 Anthropometry and age: LA size should be indexed to body surface area as body size is a major determinant of LA size. It is the variation in body size which accounts for gender difference in LA size.7 The left atrial volume index (LAVI) is independent of age from childhood onwards.8 The age related LA enlargement is a reflection of the pathophysiologic changes that accompany advancing age rather than a consequence of chronologic aging Prediction of Cardiovascular Outcomes Biplane area- length method: Orthogonal apical views, apical four and two–chamber views are obtained for determination of LA area and length. The length is determined from the middle of the plane of mitral annulus to posterior wall. Left atrial volume is calculated on the basis of the algorithm 0.85 × A1 × A2 ÷ L ; where A1 and A2 are the areas of LA in four and two chamber views and L is the shortest of the lengths obtained from the orthogonal views and indexed to body surface area.3 This method has also been recommended by the ASE guidelines for estimation of LA volume (Figure 1B ). 1. 2. LAVI is a predictor of survival after acute myocardial infarction. An exponential increase in mortality with increasing LA volume has been documented.16 Moreover, the prognostic information is incremental to clinical data and standard Left ventricular filling pressures: The increase in LA volume is a reflection of elevated LV filling pressures in the absence of congenital heart disease, mitral valve or primary atrial © JAPI • JUNE 2009 • VOL. 57 echocardiographic measures of LV systolic and diastolic function. dimensional echocardiography to measure left atrial volume: comparison with other echocardiographic techniques. J Am Soc Echocardiogr 2005;18:991-7. There is evidence that LAVI ≤ 28 ml/m2 is strongly predictive of normal stress echocardiogram.17 Although a robust data in this perspective is awaited, it holds promise to provide a simple means of identifying patients with low ischemic risk. LA volume is intimately related to LV mass/hypertrophy, systolic and diastolic dysfunction. The incremental value of each parameter for the prediction of death is expected to diminish when considering others. But LAVI derives its importance in providing incremental value in predicting mortality. 5. Greenberg B, Chatterjee K, Parmley WW, Werner JA, Holly AN. The influence of left ventricular filling pressure on atrial contribution to cardiac output. Am Heart J 1979;98:742–51. 6. Tsang TS, Barnes ME, Gersh BJ, Bailey KR, Seward JB. Left atrial volume as a morphophysiologic expression of left ventricular diastolic dysfunction and relation to cardiovascular risk burden. Am J Cardiol 2002;90:1284 –9. 7. Knutsen KM, Stugaard M, Michelsen S, Otterstad JE. M-mode echocardiographic findings in apparently healthy, nonathletic Norwegians aged 20–70 years. Influence of age, sex and body surface area. J Intern Med 1989;225:111–5. 8. Pearlman JD, Triulzi MO, King ME, Abascal VM, Newell J, Weyman AE. Left atrial dimensions in growth and development: normal limits for two-dimensional echocardiography. J Am Coll Cardiol 1990;16:1168 –74. 9. Allessie M, Ausma J, Schotten U. Electrical, contractile and structural remodeling during atrial fibrillation. Cardiovasc Res 2002;54:230–46. Limitations The major limitation in any imaging modality is its image quality. The atria are located in the far field of apical views. The image quality of LA is therefore not optimal. Modification of gain settings may not improve image quality as increase in gain will further reduce LA lumen size. Inadvertent planimetry of a foreshortened LA would introduce considerable errors in volume estimation. Therefore it is necessary that maximal LA size be obtained during volume estimation. These pitfalls can not be negated by 3-D imaging owing to decreased resolution thus making LA trace problematic. 10. Hoogsteen J, Hoogeveen A, Schaffers H, Wijn PF, van der Wall EE. Left atrial and ventricular dimensions in highly trained cyclists. Int J Cardiovasc Imaging 2003;19:211–7. 11. Vaziri SM, Larson MG, Benjamin EJ, Levy D. Echocardiographic predictors of nonrheumatic atrial fibrillation. The Framingham heart study. Circulation 1994;89:724 –30. Certain questions need to be answered. Does regression of LA size with therapy translate into improved outcomes?18 What is the natural history of LA remodeling? Future studies are warranted to further our understanding. 12. Gardin JM, McClelland R, Kitzman D, et al. M-mode echocardiographic predictors of six- to seven-year incidence of coronary heart disease, stroke, congestive heart failure, and mortality in an elderly cohort (the Cardiovascular Health Study). Am J Cardiol 2001;87:1051–7. Conclusion LA volume is a valuable tool for clinical and prognostic implications. It should be routinely incorporated in clinical practice. It is an evolving science and more data is required to understand the natural history of LA remodeling and the impact on outcomes after LA size reduction. 13. Barnes ME, Miyasaka Y, Seward JB, et al. Left atrial volume in the prediction of first ischemic stroke in an elderly cohort without atrial fibrillation. Mayo Clin Proc 2004;79:1008 –14. 14. Takemoto Y, Barnes ME, Seward JB, et al. Usefulness of left atrial volume in predicting first congestive heart failure in patients ≥ 65 years of age with well-preserved left ventricular systolic function. Am J Cardiol 2005;96:832– 6. References 1. Mitchell JH, Shapiro W.Atrial function and the hemodynamic consequences of atrial fibrillation in man. Am J Cardiol 1969; 23: 556-67. 2. Prioli A, Marino P, Lanzoni L, Zardini P. Increasing degrees of left ventricular filling impairment modulate left atrial function in humans. Am J Cardiol 1998; 82: 756-61. 3. 4. 15. Gottdiener JS, Kitzman DW, Aurigemma GP, Arnold AM, Manolio TA. Left atrial volume, geometry, and function in systolic and diastolic heart failure of persons ≥ 65 years of age (the Cardiovascular Health Study). Am J Cardiol 2006;97:83–9. 16. Moller JE, Hillis GS, Oh JK, Seward JB, Reeder GS, Wright RS, Park SW, Bailey KR, Pellika PA.Left atrial volume: a powerful predictor of survival after acute myocardial infarction. Circulation 2003 ; 107 : 2207-12. Lang RM, Bierig M, Devereux RB, et al. Recommendations for chamber quantification: a report from the American Society of Echocardiography’s guidelines and standards committee and the chamber quantification writing group, developed in conjunction with the European Association of Echocardiography, a branch of the European Society of Cardiology. J Am Soc Echocardiogr 2005;18: 1440–63. 17. Alsaileek AA, Osranek M, Fatema K, McCully RB, Tsang TS, Seward JB.Predictive value of normal left atrial volume in stress echocardiography. J Am Coll Cardiol 2006 ; 47: 1024-28. 18. Tsang TS, Barnes ME, Abhayaratna WP, et al. Effects of quinapril on left atrial structural remodeling and arterial stiffness. Am J Cardiol 2006;97:916–20. Jenkins C, Bricknell K, Marwick TH. Use of real-time three Announcement 7th National Conference of Cardiology, Diabetology, Electrocardiology, Echocardiography and Critical Care 3rd - 4th October, 2009 at Hotel Jehan Numa Palace, Shamla Hills, Bhopal, Madhya Pradesh For details contact Conference Secretariat Dr. P.C. Manoria E-5/103, Arera Colony, Bhopal 462 016, Madhya Pradesh Tel.: (0755) 2422299 • Mobile: 9893042229 • Fax: (0755) 2532405 • e-mail: [email protected] © JAPI • JUNE 2009 • VOL. 57 465