Role of echocardiography in the evaIuation of atrial ...

MlNERVA CARDIOANGIOL

2010;58:379-97

Role of echocardiography in the evaIuation of atrial function and diseases Y. GUNES, Y.

This article describes four essential components (hemodynamic, bioelectrical, hormonal and metabolic) of atrial function. And then discusses the traditional and more recent echocardiographic measures that have been employed to evaluate atrial function. Conventional parameters include the study of the various phases of atrial activity using atrial volume measurements, the peak A wave velocity, its velocity time integral (VTI), fraction of atrial contribution and atrial ejection force. Newer parameters for atrial function assessment inelude Doppler tissue imaging (DTI) ineluding segmental atrial contractility using color Doppler tissue imaging (CDTl) and esrimates of atrial strain and strain rate. Furthermore, the authors relate various particular types of atrial function in physiological and in selected pathological conditions. Key words. Atrial function

- Echocardiogra-

phy - Heart diseases.

I

nthe field of cardiology, heart ventricles have been of great interest while atria have received rather lıtüe attention. Consequently, the function of the atria is much less known than the function of the ventricles. There is currently no widely accepted non-invasive "gold standard" to evaluate atrial function. In comparison to ventricular function that has been extenCorresporıding author: YGunes, Yuzuncu Yil University, Faculty of Medicine, Cardiology Department, Van, Turkey. E-mail: yilı[email protected]

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v.ı,

H. A. GUMRUKCUOGLU,

M. TUNCER

University, Faculty of Medicine Cardiology Department, Van, Turkey

sively investigated, there is a relatively paucity of literature regarding the evaluation of atrial function.

Anatomy of atria Left atrium

Left atrium (LA) is a thin walled chamber and has ovoid structures, localized behind aortic root, superior of left ventricle and back of the heart. It's surrounded by left and right pulmonary veins superiorly, by atrial appendix anterolaterally and by mitral orifice inferiorly. The left atrium receives oxygenated blood from the lungs through the pulmonary veins.! Right atrium Right atrium (RA) is thin walled and obliquely-formed quadrangle in shape. it forms the majority of the right border of the heart. The right atrium is slightly elongated; the superior part is marked by the orifice of the superior vena cava. The inferior end of the chamber has the tricuspid orifice. it receives blood from superior vena cava, infe-

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rior vena cava, coronaıy sinus, venae cordis minimae, anterior cardiac veins and right marginal vein.! Function of the atria Atria have hemodynamic, bioelectrical, hormonal and metabolic functions. Hemodynamic function

of atria

Hemodynamic function of the atria corısists of three components: reservoir, conduit and booster pump.s When the atrioventricular valves are dosed during ventrieular systole and isovolumic relaxation, the atria perform as easily distensible reservoirs. Blood flowing from the veins to the atrial chambers is unable to pass through the dosed atrioventricular valves being stored in the atria. Atrial reservoir function ends when the atrioventricular valves open.> During ventricular diastole, after atrioverıtricular valves opening, blood starts to flow from the atria to the ventrides. The first phase of ventricular filling is the equivalent of atrial passiye emptying. In this phase energy stored previously in the elastic atrial chamber walls is used for blood flow. In addition, blood coming into the atrial chambers from veins passes the atrioventricular ostia and enters dilating ventrides that work now as suckers. This phase is called plateau or diastasis because there is a balance between blood inflow and outflow and atrial volumes do not change markedly now in which atria work mainly as a conduit. 4 Atrial pump function is usually subdivided into passiye and active components. The passive portion represents the amount of blood leaving the atria due to energy stored during the atrioventricular valve dosure, but not ejected by the atrial contraction. The active portion represents the amount of blood ejected from the atria due to their musde work. During active stage, the atria contract expelling actively the blood contained in their chambers to the ventrides serving as a booster pump. During the active atrial emptying phase, blood is ejected into both the ventrides (forward flow) and pulmonaıy veins (backward flow). The ratio of

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these flows depends on the ventricular erıddiastolic pressure.s In healthy individuals, the atria are of minor importance as contractile organs. Atrial contraction is responsible for only 5% of total cardiac output. However, this atrial contribu tion is of particular importance in the setting of impaired diastolic filling such as left ventricular (LV) dysfunction, mitral stenosis, severe aortic stenosis, hypeıtrophic or restrictive cardiomyopathies, to maintain adequate LV stroke volurne.>? Atrial contraction may contribute up to 40% of stroke volume in such situations. The loss of this atrial contribu tion to LV filling and stroke volume with atrial fibrillation can of ten lead to symptomatic deterioration. Bioelectrical function

of atria

Bioelectrical atrial function is expressed by the P wave in standard electrocardiography (ECG)8 More data on the bioelectrical structure of LA and RA can be received by atrial signal-averaged electrocardiogram. In the fina 1 part of P wave the signals of low amplitude and high frequency can be detected if electrical signal detected from atria is intensified, averaged and filtered. Such electrical signals of high frequencyand low amplitude (the atriallate potentials) are characteristic for patients with paroxysmal atrial fibrillation and organic myocardium dıseases.s Hormonal function

of atria

The cardiac natriuretic peptides (NP) are polypeptide hormones produced by cardiac muscle cells in the atria of mammals. There are two major NP: atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP).9 Despite the name, the highest concentration level of BNP is present in ventricular and atrial cardiocytes. Atrial natriuretic peptide is mainly produced in the atria, while BNP both in the atria and the ventricles.w The main actions of NP are: 1) direct and indirect effects on the kidney to alter renal hemodynamics, and to increase fluid and electrolyte excretion; 2) functional antagonism of the rerıinangiotensin- aldosterone system by inhibiting

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synthesis and/or release of renin and aldosterone, and by antagonizing all the known effects of angiotensin; 3) functional antagonism to all modalities of humoral or autonomic induced vas oc onstriction; 4) shift of fluid from the intravascular to the interstitial compartment by increasing capillary hydraulic permeability .10 The volume load-induced stretch of the atria is the major determinant of ANP release. Atrial natriuretic peptide concentrations are increased in situations with increased atrial load like in congestive he art failure, myocardial infaretion and atrial fibrillation.u Atrial natriuretic peptide blood concentration gives a due to the duration and to the progression of the disease. it is believed that blood ANP and BNP measurements together with the echocardiographic estimation of systolic and diastolic ventricular function, especially with the Doppler estimation of ventricular filling pattem give useful indexes for the diagnosis of he art failure progression and for the prognosis.v Metabolic function

of atria

The metabolic function of the atria corısists of cardiocyte, connective tissue and endothelium metabolism. Little is known about the function of these components. In an animal model of LA pressure and volume overload, there was a signifıcant up regulation of ~-myosin heavy chain isoform associate d with increased atrial mechanical work.t> it has also been observed that in persistent atrial fibrillation mitochondria of cardiocytes are overloaded with calcium and it might be one of the possible mechanisms of atrial stunning after cardioversion of atrial fibrillation to sinus rhythm.ıs Methods used for the assessment of atrial function We have a wide range of methods enabling to evaluate hemodynamic features of both left and right atria in dinical practice. These methods consist of: echocardiography, phlebography, scintigraphic methods, computed tomography, magnetic resonance imaging.

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Figure l.-Waves of jugular vein.

Among invasive methods there are: cardiac catheterization with pressure and oxygen saturation measurements in heart chambers and atriography.i+ One of accessible method of RA volume and pressure evaluation is jugular vein phlebography. The changes of pressure in RA are reflected by the pulsation of right jugular vein. The curve presenting this pulsation consists of the following positive waves: a caused by RA contraction, c - caused by tricuspid valve dosure, x - caused by jugular vein collapse during ventricular systole and RA diastole, v - reflecting elevation of pressure while atrioventricular valves are dosed, y - caused by jugular vein emptying during ventricular diastole (Figure 1).14 Undoubtedly, echocardiography is the most accessible and useful method of atrial evaluation in dinical practice. We can use transthoracic, transesophageal, contrast, and three-dimensiorıal, intracardiac as well as Doppler echocardiography. Several echocardiographic parameters have been developed to evaluate atrial function. They indude the peak A wave velocity of transmitral flow in Iate diastole obtained by pulsed wave Doppler and its velocity time integral (VTI).lS From transmitral flow, the fraction of atrial contribution IS,16 estimated as the A wave VTI as a fraction of total mitral inflow VTI, has also been an established marker of atrial function. More recently the a' velocity using Doppler tissue imaging has been used as a global measure of atrial functiorı.ı? Segmental atrial function can be evaluated using colour Doppler tissue imaging (CDTI) 18 and strain


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Figure 2.-Biplane left atrial volume measurement. (Largest left atrial volume is measured in diastole before mitral valve opening. The LA appendage, the pulmonary vein origins, and the funnel of the mitral valve are exduded from the measurement). LV:left ventricle

and strain rate imaging as well.ı? Flows in the atria and their appendages and in the veins entering the atria (caval and pulmonary ones) and in atrioventricular ostia (bicuspid and tricuspid ones) can be estimated by Doppler method. Compared with other imaging modalities like magnetic resonance imaging (MRI) and computed tomography (C'l'), the re is a tendency for LA volume to be underestimated by echocardiography.l? However, studies have generally shown excellent inter- and intra observer agreement in the measurement of LA volume by echocardiography.ı? LA size and volunıe

Measurement of anteroposterior LA linear dimension by M-mode echocardiography 20 is simple and convenient but not reliably accurate, given that the LA is not a symmetrically shaped three-dimensional (3D) structure. Enlargement of the LA is often asymmetrical and may occur in the medial-Iateral as well as the superior-inferior axes because enlargement in the anteroposterior axis may be limited by the thoracic cavity. Therefore, LA anteroposterior dimension is not an accu-

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rate reflector of LA volume, especially in those with an enlarged LA.2l In contrast to LA dimension, LA volume by two-dimensional (2D) or 3D echocardiography provides a more accurate and reproducible estimate of LA size, when compared with reference standards such as MRI and CT,22and has astronger association with cardiovascular outcornes.ö Although, LA size can be represented as a 2D measurement of LA area from the apical 4 and 2 chamber views, LA volume is the preferred rneasurement.w Modifications of the ellipsoid model including the biplane dimension-length method and the biplane area-length method have been used, with the latter being the preferred method. The American Society of Echocardiography, in conjunction with the European Association of Echocardiography, recorrunended either an ellipsoid model or the Simpson's rnethod.>' Similar to the measurement of LV volume, the biplane Simpson's method can also be used to measure LA volume. When manually tracing the outline of the LA, the LA appendage and the confluence of the pulmonary veins are excluded from the measurement. A horizontal line is drawn across


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Figure 3.-Schematic representation of the measurement of LA volume. CPLAX was taken in the parasterrıal long-axis view. LAT and SI dimensions were both taken from the apıcal 4-chamber view using inner edgc-to-inner edge measurernent).

the mitral annular plane, and LAarea does not include the funnel of the mitral valve leaflets (Figure 2)'25 At area-length method; linear LA dimensions were measured in 3 orthogonal plarıes. parasternal long axis (PLAX), lateral (LAT), and superoinferior (SI) (Figure 3). All linear dimensions were measured at the end of ventricular systole determined from the peak of the R wave on the accompanying ECG tracing. The PLAX was taken, in the parasternal long-axis view, from the leading edge of the posterior aortic wall just distal to the aortic leaflets perpendicularly to the leading edge of the posterior LA wall. The LATand SI dimensions were both taken from the apical 4chamber view using inner edge-to- inner edge measurement. The SI dimension was defined by aline bisecting the LA extending from the midpoint of the mitral annulus to the midpoint of the superior (cephalad) LA border. The LAT was taken from a perpendicular constructed from the midpoint of the su peroinferior dimension extending to the atrial borders. As suggested by current guidelines.> LA volume (V) was calculated by using the length diameter ellipsoid method computed at ventrieular end-systole, applying the following equation:

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V=4n/3X (PLAX/2) X (LAT/2) X (SI! 2) More recently, LA volume has been measured using real-time 3D echocardiography that has shown good agreerrient with other imaging modalities including MRI.26LA volume assessment by 3D echocardiography has the most favorable test-retest variation with the least intra- and inters observer variability compared with other echocardiographic techniques with good correlations between biplane 2D and 3D measurements.s? However, studies on 3D echo measurement of LA volume have been small, and there is no consensus on the methods or comparisons with established normal values. Thus, 2D biplane LA volume would be considered an accurate measure in routine elinical practice.?" LA PHASIC

VOLUMES

The size of the LAvaries during the cardiac cycle.28 Generally, only maximum LA size is routinely measured in elinical practice. However, various LA volumes 29 can be use d to deseribe LA phasic function: 1) maximum LA volume occurs just before mitral valve operıing; 2) minimum LA volume occurs at mitral valve closure;


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3) total LA emptying volume is an estimate of reservoir volume, which is calculated as the difference between maximum and minimum LA volumes; 4) LA passive emptying volume is calculated as the difference between maximal LA volume and the LA volume preceding atrial contraction (at the onset of the P-wave on electrocardiography); 5) LA active emptying (contractile) volume is calculated as the difference between preatrial contraction LAvolume and minimum LA volurne; 6) LA (passive) conduit volume is calculated as the difference between LV stroke volume and the total LA emptying volume. LEFT ATRIAL SIZE: PHYSIOLOGIC

CORRELATES

Eady size and gender.-LA size increases with increasing body size and should be indexed to body size to allow meaningful comparisons. LA size indexed to the body surface area is considered the most appropriate adjustment for body size. Men have been shown to have a larger LA size compared with women.ö However, this apparent gender difference in LA size has been largely attributed to the differences in relative weight between males and females.e' Age.-It has been suggested that increasing age is associated with an increasing LA size. However, LA enlargement is not considered part of the normal aging process.e' In a study of normal individuals over a wide age range, total atrial emptying volume, maximum and minimum LA volumes were not significantly different between younger and older study populations.e' There was a decrease in passive LA emptying and conduit volumes together with an increase in active atrial emptying. These changes are probably due to compensatory mechanisms to overcome the normal age-related decrease in LV relaxation. Thus, previously observed changes in atrial volume with aging are more likely attributable to unrecognized or 'sub clinical' pathological processes.» When either the extent or duration of LV diastolic abnormalities exceeds what is observed with normal "healthy aging", shifts in the percentage

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of active and passive LA filling are observed.e with a subsequent increase in the total LA volume. Normal reference values for LA size have been measured in groups of healthy individuals without a history of cardiovascular disease and in population-based studies with close agreement in the normal reference values of LA volume indexed to body surface area between studies.» Published reference values for maximum and minimum LA volumes are 22±6 ml.zm- and 9±4 ml./me, respecrively.v In a study of LAfunction, mean total LA emptying volume was 13.5±4.3 ml.zrn- (representing 37±13% of LV stroke volume), fractional emptying of the LA was 65±9%, and conduit volume was 23±8 mL/m2.32 LA SIZE AS A PARAMETER OF LV OIASTOUC OYSFUNCTION

- LA size increases in two broad conditions: pressure and volume overload. Left atrial pressure overload occurs in mitral stenosis or increase in LVventricular filling pressures seeondary to systolic or diastolic dysfunction. LA volume overload results from mitral valvular regurgitation, high cardiac output states, left to right shunts, or arteriovenous fistulae. Thus, "pathological" LA enlargement may be viewed as an adaptive response with an initial increase in LA volume, and LA emptying fraction serves to maintain LV stroke volume and cardiac output. However, continued LA enlargement may ultimately exceed its optimal FrankStarling relationship resulting in decreased LA cornpliance, reduced reservoir and contractile pump functions, and eventually increased ineidence of atrial arrhythmias.v In subjects with normal diastolic function, the relative contribution of the reservoir, conduit, and contractile function of the LA to the filling of the LV is approximately 40%, 35%, and 25%, respectively.ss With ab normal LV relaxation, the relative contribution of LA reservoir and contractile function increases and conduit function decreases. However, as LV filling pressure progressively increases with advancing dias-


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Relaxation deficit pattern

Normal diastolic function pattern

O,75 1) (peak E velocity in mitral flow deereases as well). Ar peak velocity may, but not necessarily, increase.» At the stage of pseudo-normal-


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ization, in which the transmitral Doppler variable s ap pe ar normal, pulmonary venous diastolic forward flow velocity is increased, systolic forward flow velocity is usually decreased CS/D30 msec). Estimation of pulmonary venous flow velocity curve enables to distinguish a pseudo-normalized filing pattem from normal LV filling in middle-aged healthy subjects. In the restrictive type of LV filling D wave (like E wave in transmitral flow) further increases and S decreases CS/D«l) (Figure 4).57 Atrial function parameters from tissue Doppler imaging Tissue Doppler imaging allows the quantification of the low-velocity, high-amplitude, and long-axis intrinsic myocardial velocities (Figure 4).51 Tissue Doppler imaging provides incremental prognostic value in a wide range of cardiac diseases.» Several studies have demonstrated that the peak velocity at the mitral annulus in Iate diastole secondary to atrial contraction Ca', A' or Am velocity) can be used as an accurate marker of global atrial function.v Unlike the early diastolic tissue Doppler velocities Ce',E' or Em), there was no significant difference between the basal septal and basal lateral peak a' velocities.» Similar to the transmitral inflow A wave velocities, there is an age-related increase in the a' velocity in healthy subıects.» A few studies have demonstrated that measurement of a' velocity is a rapid and an accurate marker of atrial funcrıon.» The a' velocity correlates with other traditional parameters of atrial function induding the peak A velocity, atrial fraction, and the atrial ejection force.>? it was also demonstrated that the tissue Doppler a' velocity correlated with LA fractional area and volume change.w Furthermore, a' velocity was reduced in diseased states associate d with atrial dysfunction. Although systolic (S' or Sm) and early diastolic tissue velocities at the mitral annu-

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lus (e') have been studied more extensively as prognostic markers, a' velocity has also been shown to be a significant prognostic indicator. In a study of 353 patients with cardiac diseases and 165 normal controls, an a' velocity >7 cm/s compared with an a' , velocity :54 cm/s was predictive of cardiac death with ahazard ratio of 11 .53, whereas an a' velocity >4 cm/see but s? cm/see was predictive of cardiac death with ahazard ratio of 4.28.54 TISSUE DOPPLER LUS IN

VELOCITIES OF THE MITRAL ANNU-

LV DIASTOLlC

DYSFUNCTION

When the pulsed wave Doppler cursor is placed at the mitral annulus from the apical window, the recorded velocities represent the longitudinal contraction (positive systolic wave, Sa) and relaxation (early negative diastolic wave, e', and Iate negative diastolie wave, a').55 The e' has been shown to be a good index for LVrelaxation.» it correlates strongly with the time constant of isovolumic relaxation.» The e' is related to age, since there is normal age dependent reduction in diastolic funetion. An e' value of less than 10 cm/s implies impaired relaxation in individuals >45 years of age.56 In normal individuals mitral E wave and e' occurs simultaneously, whereas it was shown that in patients with restrictive eardiomyopathy peak velocity of e' occurs after mitral E wave (Figure 4). it was also reported that in patients with delayed relaxation and pseudonormal filling e' is delayed and the time interval between the onset of E and onset of e' may be use d as an index for diastolic dysfunction.X Color Doppler tissue imaging of atria Atrial segmental hınction using Color Doppler tissue imaging was reeently described.>? Based on previous studies that estimated segmental ventricular function,58 the atrium divided into multiple segments at the annular, mid-atrial and superior leveL. From the apical four-chamber view, measurements were ma de from five segments of the left atrium and three segments of the right atrium. Using the two-chamber view, measurements were made from five segments of


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Figure 5.-Atrial segments from the two apical echocardiographic views. CA)Apical four-chamber view cı, septal annular segment; 2, scptal mid segment; 3, superior segment; 4, lateral mid segment; 5, lateral annular segment; 6, lateral annular RA segment; 7, lateral mid RA segment; 8, sııperior RA segment). CB)Apical rwo-chamber view (9, posterior annular segment; 10, posterior mid segment: 11, superior segment; 12, anterior mid segment; 13, arıterior annıılar segment.

the left atrium. Sampling volume was used and a tissue velocity profile throughout the cardiac cycle was displayed at each location (Figure 5).59 Thus, the atrium adjacent to the annulus had the maximum movement while the superior segment is relatively fixed. A significantly increased segmental velocity was noted uniformly in the annular and basal segments in the older age group.w ln a previous study, analysis of segmental atrial contractility in a cohort of patients cardioverted to sinus rhythm demonstrated a low segmental velocity immediately postcardioversion with a temporal increase in segmental atrial contractility.60 Differential recovery of function was noted between the right and left atrium with right atrial function normalizing in four weeks. LA function remained subnormal even at six months.w Atrial ejectionforce Atrial ejection force is the force exerted by the LA to propel blood across the mitral valve into the left ventricle during atrial systole. Based on Newtonian principles, the atrial ejection force is calculated as the product of

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-the mass and acceleration of blood from the LA during atrial systole.s! LA ejection force has be en used as a marker of recovery of atrial mechanical function after successful cardioversion.w with the atrial ejection force being depressed immediately after, with gradual improvement over weeks in patients who main ta ine d sinus rhythm.w In the Strong Heart Study, which enrolled 2808 patients, atrial ejection force correlated with age, hypertension, body mass index, serum creatinine, glucose, and insulin levels. After adjusting for elinical covariates, atrial ejection force was independently associated with larger LV, higher LV mass, stroke volume, and cardiac output. A greater atrial ejection force was an independent predictor of subsequent cardiovascular events.w However, there are technical limitations on the use of atrial ejection force as the measurements of the peak transmitral A velocity and the mitral annular diameter are not simultaneous. In addition, the robustne ss and the variability in the measurement of atrial ejection force and, more importantly, the value of atrial ejection force incremental to LA volume assessments have not been investigated.s!


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Atrial sırairi and strain rate imaging Doppler strain and strain rate imaging are derivatives of tissue Doppler velocities. Strain measures the myocardial deformation during a cardiac cycle, and strain rate measures the tissue velocity gradient within the myocardium. Strain and strain rate imaging have been extensively used in the assessment of LV systolic and diastolic function, myocardial ischernia and viability, and valvular he art disease.se Strain and strain rate imaging may be applied in the study of LA function. Atrial strain rate demonstrated that the longitudinal shortening and lengthening of the atrium are discordant with ventricular longitudinal motion because the atrium fills during ven-: tricular systole and empties during ventricular diastole. This illustrates the improved site specificity of strain and strain rate imaging. A temporal increase in atrial strain rate similar to other parameters has be en demonstrated with the restoration and maintenance of sinus rhythm in patients with chronic atrial fibrillation. However, unlike the peak A velocity, peak atrial strain rate did not normalize at 6 months follow-up.v' Arecent study demonstrated that the atrial strain and strain rate were independent predictors for the maintenance of sinus rhythm after successful cardioversion.vt LA speekle ıraeleing eehoeardiography Newly developed speckle tracking echocardiography is useful in assessment of left ventricular strain and strain rate 65 Speckle tracking has be en use d for assessment of regional systolic function of the left ventricle and for the left ventricular rotation.e- 66 It has also been used for assessment of LA 67 and is feasible for assessment of LA function with acceptable variability. The advantages of strain imaging derived from speckle echocardiography compared with TDI for the assessment of LA are angle independence and avoidance of tethering by the left ventricle.e? LA mechanical function assesses by speckle tracking echocardiogra phy. 68

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Atrial functions in disease states Systolie heartfailure Congestive heart failure (CHF) is characterized as myocardial function failure. Without neglecting the autonomous nervous system hormonal mechanisms or the other adapting or deleterious bimolecular implications, the most relevant aspect is that, the cardiac structııre does not provide contraction and ejection with sııfficient systolic volume. Elevation of filling presslife is uniformly found in the presence of symptomatic CHF. Because the majority of individuals in the community with LV dysfunction (systolic or "isolated" diastolic) are in a preclinical phase of the disease, methods to quantify the risk of progression to symptomatic heart failure would be clinically useful.s? Evidence for a prognostic role for LA volume to predict incident CHF is emerging.w In a large prospective, population-based study, subjects with incident CHF during follow-up had slightly higher baseline LA line ar di ameters (39 mm vs. 37 mm for women [P