A working definition of Heart Failure (HF) was recently conceptualized by the Heart Failure Society of America as:
"a syndrome caused by cardiac dysfunction, generally resulting from myocardial muscle dysfunction or loss and characterized by LV dilation or hypertrophy. Whether the dysfunction is primarily systolic or diastolic or mixed, it leads to neurohormonal and circulatory abnormalities, usually resulting in characteristic symptoms such as fluid retention, shortness of breath, and fatigue, especially on exertion."1
Echocardiography is uniquely suited to characterize anatomical and functional abnormalities in patients at risk of developing heart failure, suspected of having heart failure, and with symptomatic heart failure. Furthermore, echocardiography can provide prognostic information and assist in the management of patients with acute, chronic and end-stage HF.
Evaluation of Cause of HF by Echocardiography
By assessing cardiac structure and function echocardiography plays a central role in determining the etiology of HF. An echocardiogram demonstrating a dilated left ventricle (LV) with decreased global systolic function and no significant valvular abnormalities will suggest the presence of a dilated cardiomyopathy. Similar findings with associated segmental LV wall motion abnormalities will suggest ischemic cardiomyopathy. A dilated LV with impaired systolic function and associated severe mitral or aortic insufficiency would suggest the presence of HF secondary to valvular heart disease. And a HF patient with a non-dilated, hypertrophied LV with preserved systolic function will most likely have either hypertensive heart disease Hypertrophic cardiomyopathy or an infiltrative cardiomyopathy (see Figure 1).
Characterization of Functional Abnormalities by Echocardiography HF in Patients with Systolic Dysfunction
M-mode echocardiographic measurements of LV function benefit from high temporal resolution, but are inaccurate in patients with segmental dysfunction or non-elliptical ventricles. Qualitative, 'eyeball' grading of left ventricular systolic dysfunction into mild, moderate or severe categories is widely used in clinical practice, but requires careful standardization and is difficult to obtain.
Quantitative two-dimensional (2-D) visual assessment has been shown to detect depressed LV ejection fraction with good sensitivity and specificity, but this procedure is only reliable with experienced observers. The apical biplane modified Simpson method of discs has also been validated, but requires accurate endocardial definition, and inclusion of the true LV apex on the imaging planes. Left-sided contrast agents can facilitate endocardial visualization and improve volumetric estimations and reproducibility.
Other common echocardiographic measurements of LV function include: fractional shortening, sphericity index, myocardial performance index, and left ventricular wall motion index.
The recently validated2 volumetric quantification of global and regional LV function from realtime three-dimensional echocardiographic (RT3DE) images has the real potential of overcoming the limitations of all previously described echocardiographic methods. The main advantage of 3-D echocardiography over other echocardiographic methods of volumetric assessment is that it eliminates the need for geometric assumptions and allows for incorporation of the true apex of the LV—a frequent shortcoming of two dimensional echocardiography.
Ejection fraction, despite its shortcomings and dependence on the pre-load and after load conditions of the LV, remains the most commonly used marker of systolic performance in patients with HF. The application of automatic border detection on 3-D echocardiography is likely to enhance the accuracy of 3-D echocardiographic volumetric measurements and provide a practical, accurate and reproducible estimate of ejection fraction; 3-D echocardiography is likely to become the gold standard for clinical estimation of ejection fraction in patient with HF (see Figure 2).
HF in Patients with Normal or Near Normal Systolic Function
It is now clear that a significant percentage of patients presenting with the syndrome of heart failure have normal, or near normal, systolic ventricular function.3 Examples of HF with normal systolic function include hypertension, restrictive cardiomyopathy, Hypertrophic cardiomyopathy and non-compaction cardiomyopathy. Echo/Doppler techniques have provided new insights in the characterization of these patients. Doppler echocardiography allows to asses abnormalities in LV relaxation, compliance or stiffness and to approximate left atrial and left ventricular filling pressures.
Comprehensive echo/Doppler techniques permit not only the detection of diastolic dysfunction but grading its severity.4 The use of mitral inflow velocities, pulmonary venous flow velocities, mitral valve flow propagation velocities and mitral annulus tissue Doppler velocities permits differentiation of the milder forms of diastolic dysfunction (impaired relaxation and pseudonormalization) from the more advanced restrictive physiology pattern.
The diastolic pressure differential between the left atrium and left ventricle can be analyzed for measuring the mitral inflow velocities with spectral pulsed Doppler at the level of the mitral valve tips. Patients with impaired relaxation demonstrate predominant late-diastolic filling during atrial contraction expressed as E/A inversion. Patients with restrictive physiology have increased velocities during early diastole resulting in an exaggerated E/A ratio and a very rapid deceleration time. The intermediate form of diastolic dysfunction or pseudo-normal pattern can be distinguished from the normal filling pattern by tissue Doppler velocities of the mitral annulus which are reduced on patients with pseudo-normalization and normal in the absence of diastolic dysfunction.
An estimate of ventricular filling pressures can be obtained with fairly good accuracy by the ratio of the early mitral filling over the early diastolic tissue Doppler velocity of the mitral annulus. By providing information regarding ventricular filling pressures Doppler echocardiography allows for further characterization of diastolic function and can be used to further refine the treatment of patients with symptomatic heart failure.
Patient Selection for Resynchronization Therapy
Cardiac resynchronization therapy (CRT) is now recognized as an effective treatment for patients who have significant heart failure symptoms despite medical therapy.5
Delayed activation of LV segments from conduction delays leads to ventricular dyssynchrony, which compromises ventricular function and promotes ventricular remodeling. Resynchronization can be obtained by simultaneously pacing both ventricles; this in turn leads to improved functional class, exercise tolerance, quality of life, and LV ejection fraction.
Echo and Doppler techniques are most helpful in defining when dyssynchrony is present so that CRT is offered to patients with HF who are likely to benefit. A variety of Echocardiographic parameters have been proposed to identify potential responders to resynchronization therapy.6 These include M-mode derived septal to posterior wall maximal excursion time delay, spectral Doppler derived right ventricular to LV pre-ejection time delay and tissue Doppler velocity derived time to maximal systolic velocity delay from different wall segments.
Although multiple methods have been used to evaluate for ventricular dyssynchrony, currently 3-D echo and color tissue Doppler imaging provide the most reliable information. With 3-D echocardiography is now possible to precisely measure the time to minimal systolic volume in any of 16 LV segments, and derive from this a dyssynchrony index measured as the standard deviation of the difference to minimal systolic volumes of each of these 16 segments. Figure 3 Furthermore, through parametric imaging, it is now possible to obtain a 'bulls eye' view of the dyssynchronous contraction of the LV. This provides a more accurate display of the electric depolarization of the LV that when presented in different hues of color allows a more precise characterization and quantification of the segments of the LV that have delayed contraction (see Figure 4).
The application of color tissue imaging through the analysis of segmental tissue Doppler velocity, tissue strain and strain rate has allowed simultaneous quantification of opposite LV walls facilitating the recognition of segmental intraventricular dyssynchrony. It has the advantage over standard tissue Doppler in that it provides real time and simultaneous analysis of any of the LV segments of interest (see Figure 5).
Conclusions and Future Directions
Echo/Doppler methods are today the most frequently used techniques for diagnosis, characterization and management of HF patients. This is because, today, there is no other portable imaging technique that provides non-invasively detailed anatomic, pathologic and functional information as provided, cost-effectively, by echocardiography.
Because of its ability to precisely characterize cardiovascular size and function, 3-D echocardiography and color tissue Doppler will became the standard imaging tools for the diagnosis and management patients with HF.