Assessment of Left Atrial Size
- Teresa S.M. Tsang, MD
Left atrial remodeling: an echocardiographic diagnosis
Left atrial (LA) enlargement is an important echocardiographic finding. The size of the left atrium has profound clinical implications, in terms of both understanding disease processes that lead to enlargement and predicting future diseases and outcomes. LA size cannot be assessed by clinical examination. However, it is easily and reproducibly quantitated by echocardiography. This chapter will focus only on assessment of size and its clinical implications; function is discussed in another chapter. Echocardiography is the best method for assessment and follow-up of LA size, because this imaging modality is radiation free and does not require contrast enhancement. It is safe, portable, comfortable for patients, relatively inexpensive, and readily available in large and smaller centers, and it could be used more widely for population screening.
Definition of left atrial remodeling
LA remodeling refers to a spectrum of complex pathophysiologic changes that occur in response to external stressors. LA dilatation, a hallmark of LA structural remodeling, is often the result of pressure or volume overload. Diastolic dysfunction, tachycardia, ischemia, and valve diseases such as mitral stenosis and aortic stenosis can lead to LA enlargement. The left atrium may respond with a range of adaptive and maladaptive changes, such as myocyte hypertrophy, apoptosis, necrosis, changes in the composition of extracellular matrix, alterations in cellular energy balance, and neurohormonal disturbances. LA pressure increases to maintain adequate filling of the LV in conditions associated with increased stiffness or decreased compliance of the LV. The increase in LA wall tension leads to dilatation. Functional remodeling with reduction of LA pump function, and electrical remodeling with reduction of L-type Ca 2 + current and shortening of action potential duration.
Diastolic dysfunction and left atrial size
Diastolic dysfunction is highly prevalent in the community. Progressive worsening of diastolic function with increased filling pressures leads to enlargement of LA size. In the absence of mitral valve disease, atrial fibrillation (AF), and conditions with higher cardiac output such as athlete’s heart, there is an excellent correlation between the severity of diastolic dysfunction and LA volume. One of the potential downstream deleterious effects of LA dilatation is progressive increase in interstitial fibrosis and deterioration in LA function. It portends development of AF, stroke, and heart failure. LA enlargement may be unrelated to diastolic dysfunction. Relative volume overload of LA in mitral regurgitation, arteriovenous fistula, and high-output states such as anemia can also contribute to LA enlargement. High cardiac output in athlete’s heart leads to physiologic hypertrophy and chamber enlargement. Unlike pathologic conditions in which diastolic dysfunction accompanies enlargement, athlete’s heart is usually associated with normal diastolic function. Large long-terms studies of these individuals are not yet available, although athletes are at increased risk for developing atrial fibrillation, thought to be also related to LA remodeling.
Left atrial enlargement for outcome prediction and prognostication
LA enlargement is an early echocardiographic finding of hypertension heart disease. It is highly predictive of first and recurrent AF. It also predicts the likelihood of postoperative AF, success in cardioversion, likelihood of maintenance of sinus rhythm following AF ablation, and heart failure. In different types of cardiomyopathy, it was found to be an excellent prognostic indicator. In patients with dilated cardiomyopathy, maximal LA volume was found to be an independent predictor of mortality and need for cardiac transplantation. In patients with hypertrophic cardiomyopathy, increased LA volume and rapid dilatation of LA with an increase of LA volume of more than 3 mL per year were both independent predictors of unfavorable outcomes, defined by the occurrence of sudden death, need for cardiac transplantation, or invasive therapy for reduction of LV outflow tract obstruction.
LA volume also predicts stroke, coronary events, and premature death. The strong relationship between LA enlargement and increase in mortality has been demonstrated in patients with underlying cardiovascular diseases as well as in the general population.
Echocardiographic methods of left atrial size assessment
LA size can be assessed by multiple methods using two-dimensional (2D) or M-mode echocardiography. LA area and M-mode linear dimensions had been used for the quantification of LA size, but these parameters are no longer regarded as standard. The paradigm shift in LA assessment over the past decade has been toward volume assessment, whether by 2D or three-dimensional (3D) methods. Use of LA diameter is passé. LA volume, compared with area and linear dimension, is a superior predictor of outcomes, including first occurrence of AF, heart failure, stroke, transient ischemic attack, myocardial infarction, coronary revascularization, and cardiovascular death ( Fig. 45.1 ). By 2005, the American Society of Echocardiography had established LA volume as the standard method for LA size assessment, regardless of whether it was obtained by the method of discs or the biplane area-length method.
It is well recognized that volumetric assessments correlate well across various imaging technologies, including computed tomography (CT) and magnetic resonance imaging (MRI). Absolute volume differs. LA volumes by echocardiography are smaller than those estimated by CT or MRI. Hence the reference values for these modalities are different. Normality and degree of enlargement of any individual’s LA volume should be described in the context of the reference values used to establish that method. So if a laboratory chooses to use the biplane area-length method for assessing LA volume, then the reference values for determining normality and the degree of departure from normality should be based on the reference values established for biplane area-length method and not the reference values for MRI biplane area-length, or even echocardiographic single plane area-length method.
Body habitus is a major determinant of LA size, and therefore LA volume should be indexed to body surface area. Gender differences in absolute LA volume are largely accounted for by variation in body size. LA size is independent of age, and enlargement has been shown to be related to age-associated cardiac conditions or risk factors.
The standard 2D echocardiographic methods recommended by the American Society of Echocardiography (ASE) are the biplane area-length method ( Fig. 45.2 ) and the Simpson biplane method ( Fig. 45.3 ). Normal LA volume for these methods has been established as 22 ± 6 mL/m 2 . LA volume beyond 1 standard deviation (SD) of the mean, which means that a volume larger than 28 mL/m 2 , has been considered enlarged. Many echocardiographic parameters have set the cutoff for enlargement at 2 SD above a normal mean value. In the case of LA enlargement cutoff, the 2005 guidelines had adopted the cutoff for mildly, moderately, and severely enlarged at cut points of 29, 34, and 40 mL/m 2 , based principally on prior outcome studies rather than SD. If cutoffs at 2 SD from the mean were to be used, then the cutoff for enlargement would be at 34 mL/m 2 . It is important to recognize that LA volume is a continuous variable. The larger the LA size, the higher the risk for adverse outcomes, and nothing magical exists at 32 or 34 mL/m 2 . In fact, in the ASE guidelines for evaluation of left ventricular diastolic function and filling pressures, 34 mL/m 2 was used as the cutoff for enlargement. This partition simply provides a more specific but less sensitive discrimination between abnormal and normal LA volume when assessing diastolic function.
Pitfalls of left atrial volume measurement
Accuracy of echocardiographic LA volume assessment is increased with the use of more than one plane because of the inherent asymmetry of the atrial cavity as well as asymmetric changes in LA size that occur with disease. Biplane area-length method and biplane Simpson method of discs, using orthogonal views, have been validated against reference standards such as angiography, CT, MRI, and 3D echocardiography. 3D echocardiography may prove to be the preferred method of LA volume assessment ( Fig. 45.4 ), based on the simple fact that 3D volume assessment is not affected by the geometric assumptions inherent with 2D assessment and volume calculation. However, 2D assessment is still the mainstay in today’s clinical laboratories.
With 2D assessments, optimal contours should be obtained orthogonally around the long axis of the LA using 2D apical views. It is important to obtain planes for measurement that do not foreshorten the long axis of the left atrium. In area-length (AL) method, we use the formula:
The length (L) is the distance from the midpoint of the mitral annulus plane to the superior margin of the left atrium. In the orthogonal apical views, the two lengths should be nearly equal. A slight discrepancy may exist because of the variability of chamber orientation and limitation of image projections. However, a difference greater than 5 mm should call attention to the possibility of measurement error or a foreshortened LA in at least one view. The longer, the shorter, and the average of the two lengths from the two views have all been used in different studies for the calculation of volume. The use of the longer of the two lengths will yield a slightly smaller volume, whereas the use of the shorter length will yield a slightly larger volume calculation. The ASE has recommended the use of the shorter of the two lengths for calculation of LA volume, which minimizes the underestimation of LA volume by echocardiography when compared with CT or MRI assessments. We routinely ensure optimal visualization of the LA without foreshortening, and check that the two lengths measured in the orthogonal views are within 5 mm of each other. We have used the average of the two lengths for calculation of LA volume in our previous studies because any small error in measuring the length is equally likely for both views. With these criteria, the relative difference in LA volume between one calculated with the shorter length and another using the average of the two lengths is less than 5%.
Newer modalities of echocardiographic left atrial volume assessment
Acknowledging the inherent geometric assumptions, conventional 2D echocardiography is the most commonly used imaging modality in clinical cardiology for LA volume assessment. Real-time 3D echocardiography has been shown in a study to provide a more accurate and reproducible assessment of LA volume than 2D echocardiography, with cardiac magnetic resonance imaging as the gold standard. In one study, 3D minimal LA volume was found to provide the best prediction of death, myocardial infarction, and stroke during the 45-month follow-up period, superior to that provided by maximal LA volume, LV volume, LV mass, and LV ejection fraction.
Reverse remodeling of LA
LA size and function can improve and reverse to more normal states. Reverse remodeling has yet to be analyzed for its impact on improving outcomes. In a double-blind, randomized controlled study, the effects of quinapril on LA size and function was examined in patients with isolated diastolic dysfunction and a maximal LA volume of at least 32 mL/m 2 . Subjects treated with quinapril for 1 year demonstrated a mean LA volume reduction of 4.2 mL/m 2 , whereas the mean LA volume in the placebo group increased by 5.5 mL/m 2 . These changes were independent of blood pressure response, suggesting that LA structural remodeling can be reversed with the use of medical therapy. Similar results have been reported by the Losartan Intervention for Endpoint Reduction in Hypertension (LIFE) trial, in which patients with electrocardiographic evidence of LV hypertrophy were randomized to treatment with losartan or atenolol. The baseline LA diameter predicted the incidence of cardiovascular events. Greater reduction in LA diameter was associated with a greater decrease in LV hypertrophy, and absence of new-onset AF or mitral regurgitation during follow-up. These studies highlighted the potential role of LA size as a therapeutic target and the reversibility of LA remodeling.
Future directions
LA remodeling predicts adverse cardiovascular events. Measurement of LA volume and function should be incorporated into risk stratification models in clinical practice. LA enlargement is potentially reversible, at least in its early stages, and echocardiography is the best modality for monitoring the degree of remodeling and its reversal, and for assessing therapeutic effects. Future research should be aimed at defining the best therapies for reversing LA remodeling and for determining if such reversal translates to improvement in cardiovascular outcomes.
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