A patient presents to their physician with progressive dyspnea on exertion, orthopnea, and fatigue. Physical examination reveals bibasal rales, distended neck veins, an S3 gallop, and an apical holo-systolic murmur. The clinical diagnosis is straightforward - this patient has congestive heart failure.Almost certainly one of the first tests performed in this patientÔÇÖs diagnostic evaluation will be an echocardiogram. Indeed, echocardiography is routinely used in patients with heart failure to answer the following clinical questions:
- Is the problem purely one of ventricular performance or is there other underlying pathology such as congenital heart disease or primary valve disease?
- Is there systolic dysfunction and, if so, what is the ejection fraction?
Providing the answers to these questions is basic echocardiography. However, as will be expanded on in this review, the technique is capable of providing much more. Specifically, a comprehensive echocardiographic evaluation of the patient with heart failure can and should include a detailed assessment of cardiac anatomy, right and left ventricular systolic and diastolic function, valvular function, particularly that of the atrioventricular valves, and in some cases, determination of hemodynamic parameters such as cardiac output and filling pressures.
This information may be integrated to provide prognostic information and guide therapy, the latter being particularly important in patients with QRS prolongation in whom cardiac resynchronization therapy (biventricular pacing) may be an option. Finally, although not directly relevant to the clinician, it is notable that current echocardiographic techniques provide an experimental tool in small animal models of heart failure.
A delineation of cardiac anatomy is an important part of the evaluation of the patient with heart failure. After excluding underlying congenital heart disease or primary valve disease, the echocardiographic evaluation quickly focuses on both ventricles and the atrioventricular valves. The exquisite spatial resolution of echocardiography, its multiple imaging windows and more recently introduced three-dimensional (3-D) applications provide precise methods of assessing ventricular volumes, planar dimensions, left ventricular mass, and wall thickness.
These techniques may be applied to the right ventricle as well as to the left ventricle. Echocardiography has also been important in identifying mitral and tricuspid annular dilation and the left ventricular geometric alterations that cause functional mitral and tricuspid regurgitation. It may also identify complications such as intra-cavitary thrombus.
The assessment of left ventricular systolic function was one of the earliest applications of echocardiography dating back to the M-mode era. Currently, 2-D approaches that incorporate both imaging and Doppler methodology are widely used for this purpose with realtime and reconstructive 3-D techniques recently added.
Echocardiographic methods can provide multiple indices of global function that include widely used but relatively load-dependent ejection phase indices such as left ventricular ejection fraction, fractional shortening and fractional area change. There is also a simple method for determining left and right ventricular dP/dT based on continuous wave Doppler recordings of mitral and tricuspid regurgitant jets respectively. This (dP/dT) is an afterload independent index of ventricular performance.
Until recently, echocardiographic determinations of load-independent indices of myocardial contractility such as end-systolic elastance and preload recruitable stroke work have been based on pressure-volume and pressure-area loops. These have required commercially available ultrasound equipment with automatic boundary detection capability and offline custom computer applications. Importantly, they have also required invasive monitoring of intraventricular pressure. Their use has been largely restricted to the research and intra-operative settings. Recently, however, advances in Doppler tissue imaging have created methods for realtime displays of myocardial strain and strain rate. These approaches overcome the fact that conventional Doppler tissue imaging is unable to differentiate active myocardial contraction from passive motion generated by translation of the heart or tethering of akinetic to contracting segments. Echocardiographic-derived strain and strain rates provide totally non-invasive load-independent indices of ventricular myocardial performance that correlate well with end-systolic elastance.
While echo-Doppler indices of diastolic function are widely used in patients with heart failure and normal systolic function, it is notable that these indices also have value in patients whose primary functional abnormality is systolic dysfunction. Echocardiographic methods for assessing left ventricular diastolic function include those based on pulsed Doppler recordings of mitral and pulmonary venous inflow, Doppler tissue imaging of the mitral annulus, and color Doppler M-mode recordings of mitral inflow.The latter is used to derive the propagation velocity, a load-independent index of relaxation. It is also possible to derive peak negative dP/DT by analyzing mitral regurgitant Doppler spectra.
The technology required to perform all these analyses is available on most commercially available systems and the time required to obtain a detailed assessment of diastolic function is minimal.
Myocardial Performance Index
The myocardial performance index, first introduced by Tei and colleagues, integrates myocardial systolic and diastolic function. It is defined as:
This easily derived index has been applied to both right and left ventricles.
Atrioventricular Valve Function
Functional mitral and tricuspid regurgitation frequently accompany left ventricular systolic dysfunction and may contribute significantly to patient symptomatology. Echocardiography has proved instrumental in delineating the pathophysiology of functional mitral regurgitation. A number of animal and clinical studies using both 2-D and 3-D techniques have demonstrated a path gnomonic pattern of leaflet closure termed apical tethering. One of the key causes of this disturbance of leaflet coaptation is increased tethering forces on the leaflets and chords created by geometric remodeling of the left ventricle and attendant papillary muscle displacement. By exerting traction at the site of leaflet insertion, annular dilation also contributes to pathologic leaflet tethering.
The other important causative factor is a reduced valvular closing force due to impaired left ventricular contraction. Functional tricuspid regurgitation appears to have a similar pathophysiology. It may occur on the basis of either primary right ventricular systolic dysfunction or due to the right ventricular remodeling that may develop in the setting of primary left-sided failure and secondary pulmonary hypertension.
Over the last decade, the ability of echocardiography to define cardiac hemodynamics has been greatly expanded. One of the earliest applications of echocardiography was the determination of cardiac output based on calculating forward flow across the cardiac valves.This calculation is based on the fact that stroke volume equals the product of cross-sectional area and velocity time interval (the integrated area under the pulsed Doppler spectral curve) for flow per beat through that area.
Although methods exist based on flow across each of the valves, those that relate to either the aortic or pulmonary outflow tracks are considered optimal since the orifice through which flow occurs is relatively constant in size and is easily modeled geometrically.
Echocardiography also provides simple methods for calculating pulmonary artery systolic and diastolic pressure based on the tricuspid and pulmonic regurgitant jets respectively. These methods are widely used clinically and have been extensively validated. Recently, a number of methods have been reported for determining left ventricular filling pressures.1
These methods include approaches that are based on mitral and pulmonary venous inflow spectra, Doppler tissue imaging of the mitral annulus, and color Doppler M-mode of mitral inflow.
A number of parameters that can be determined echocardiographically have been identified as negative prognostic factors in patients with heart failure and systolic dysfunction. Reduced left ventricular ejection fraction (less than 25%) is a strong negative prognostic factor and impaired right ventricular systolic performance is also an important independent predictor of increased mortality and morbidity.
Echocardiographic methods of assessing right ventricular systolic performance are not as well developed as those for the left ventricle, due in part to the complex geometry of the right ventricle.
Two-dimensional echocardiographic techniques include the tricuspid annular plane excursion, fractional area change, and systolic velocities defined by Doppler tissue imaging. Recently developed realtime 3-D echocardiographic techniques promise to be extremely valuable tools for assessing the right heart.
The myocardial performance index as applied to both right and left ventricular function has also been identified as being prognostically important, as has the presence of functional mitral regurgitation and the response of the left ventricle to dobutamine stress. It is notable that patients who do not demonstrate myocardial contractile reserve with dobutamine infusion do less well than those in whom recruitable myocardial systolic performance is demonstrable.
One of the most robust echocardiographic markers of a poor prognosis is a restrictive mitral inflow pattern characterized by a dominant E wave (E to A reversal) and a shortened E wave deceleration time. Persistence of this filling pattern despite aggressive medical management is particularly ominous.
Guide to Therapy
Echocardiography can also be extremely valuable as a tool to guiding therapy. In addition to providing a number of parameters that can be used to measure the impact of medical therapy, it may also identify patients for specific surgical interventions such as palliative mitral valve repair, placement of left ventricular assist devices and/or ventricular remodeling surgery.
Echocardiography plays a unique role in the setting of cardiac resynchronization therapy (CRT) with biventricular pacing. This technique improves function and survival in patients with Class III-IV heart failure despite optimal medical management and is offered to patients with QRS prolongation and a left ventricular ejection fraction of less than 35%. The superb temporal and spatial resolution of echocardiography makes it uniquely able to measure the degree of ventricular asynchrony, which, in turn, appears to identify patients who are most likely to benefit from this expensive technology. Multiple echocardiographic modalities have been used for this purpose including those based on M-mode, 2-D, Doppler tissue imaging and derived strain techniques.
Echocardiography has also been used to guide left ventricular lead placement and for optimization of atrio-ventricular (AV) and interventricular (VV) delays following implantation. Furthermore, a number of echocardiographic parameters have been used to monitor the response to therapy and are included in many of the trials in this field. These include many of the methods discussed in earlier paragraphs of this review.
This is an area of active investigation in the echocardiographic, pacing and heart failure communities, bringing experts from these three fields together.
Echocardiography as a Tool in Small Animal Models
Technologic advances in echocardiography have resulted in the availability of echocardiographic methods of assessing ventricular function and perfusion in small animal models of heart failure.These include transgenic mice in which resting heart rate exceeds 600 beats per minute (bpm).
Echocardiography is enormously valuable in the diagnosis and management of patients with heart failure. At the time of initial patient evaluation, this technique is easily able to determine whether the primary abnormality is systolic or diastolic dysfunction and exclude underlying primary valve dysfunction or congenital heart disease. It is able to provide insight into the pathophysiology of heart failure in individual patients and has proven valuable in defining the natural history of the disease. It can provide a non-invasive assessment of hemodynamics and may help identify patients with a poor prognosis.
Finally, it may help guide the selection of patients for therapeutic intervention, and plays a critical role in cardiac resynchronization therapy, identifying patients most likely to benefit from device implantation and optimizing device settings post-implantation.Ôûá