Ordering a non-invasive cardiac diagnostic test can sometimes be confusing and intimidating. This is particularly true or the cardiac echo test, which has evolved into a powerful but complex diagnostic tool over the past 20 years. The Director of the Cardiac Echo Laboratories at the Hermann Heart Center and the University of Texas Medical School, Dr. Eddy Barasch has put together a guide to the merits and applications of the cardiac echo test in an attempt to de-mystify this useful diagnostic modality. Please welcome Dr. Barasch to Cyberounds^®!

-- Richard W. Smalling, M.D., Ph.D., Cardiovascular Moderator

Introduction

Echocardiography is the most comprehensive modern non-invasive technique for cardiac evaluation. The fascinating journey of echocardiography development began almost fifty years ago with M-mode echocardiography. Twenty years ago two dimensional imaging evolved and this was followed by the development of spectral and color Doppler, transesophageal imaging, three dimensional reconstruction, development of transpulmonary contrast agents, Doppler tissue imaging, harmonic imaging and volumetric echocardiography.

The practicing internist or generalist not familiar with all the potential diagnostic capabilities of echocardiography needs to understand those cardiac conditions for which echocardiography may provide, in a timely and cost-effective manner, the most accurate answer to specific clinical questions.

The following are the most common reasons for echocardiographic evaluation in clinical practice:

Evaluation of Murmur

Systolic Murmurs

Mitral Valve Prolapse (MVP)

This is the most frequent etiology of mitral regurgitation (MR) in the United States. In a retrospective study performed in a tertiary care university hospital comparing the utilization of echocardiography by internists and cardiologists in 598 patients, suspected MVP was the reason for an echo in 10% of studies requested by internists and 5% requested by cardiologists.(1) Strict echocardiographic criteria for MVP diagnosis were described about 10 years ago.(2) Patients with "classic" MVP (leaflets thickening >5mm and upward displacement of the leaflets > 2mm) have a higher rate of complications and an increased severity of MR requiring surgical therapy more frequently.(3) In the absence of MR at rest, patients with MVP who develop MR during exercise have a higher risk of morbid events.(4)

The severity of MR, left ventricular (LV) and right ventricular (RV function), LV systolic and diastolic diameter, left atrial (LA) size will guide the decision for surgical intervention and can predict the surgical outcome. Some patients, diagnosed by echocardiography with MVP ten or twenty ago, were found to have normal valve function in a more recent study when the diagnosis was made according to the new diagnostic criteria.

Figure 1.

Figure 2.

Aortic Stenosis

The echocardiogram will identify the location of stenosis (valvular, sub or supravalvular), the anatomy of aortic valve (bicuspid valve being the most frequent etiology of aortic valvular aortic stenosis in individuals < 65 years old), the magnitude of aortic cusp separation (normal is more than 1.5 cm), the degree of cusp calcification and the presence of potentially associated lesions, like vegetations. The degree of left ventricular hypertrophy can indicate the severity and duration of aortic stenosis and left atrial dilatation may suggest the presence of LV diastolic dysfunction and/or associated mitral regurgitation. In those patients with a bicuspid valve, the descending thoracic aorta should be evaluated for the presence of coarctation.

The blood flow velocities measured by Doppler can be transformed into pressure gradients utilizing the simplified Bernoulli equation. Further applying the continuity equation, one can get the value of the functional aortic valve area which is usually smaller than the area calculated at catheterization. The echocardiographic measurement of the anatomic valve area may be measured by planimetry and its correlation with the valve area calculated at catheterization is excellent. If the aortic valve area is not measured, usually a mean transaortic gradient of 50 mm Hg signifies severe valvular stenosis. The accurate measurement of the sino-tubular junction is required for those patients in whom a stentless prosthetic valve is considered. Low dose dobutamine echocardiography was reported to be able to identify those patients with poor LV systolic function and a low mean transaortic gradients that will benefit from surgery.(5)

Hypertrophic Obstructive Cardiomyopathy

The echocardiogram will give an accurate measurement of the degree of hypertrophy of left ventricular walls and eventually RV free wall and will identify unusual forms of hypertrophy (apical, basal, midventricular). Myocardial fiber disarray may be reflected in the "ground glass" acoustic texture. LV size may be decreased due to significant hypertrophy of the ventricular wall. Systolic anterior motion of the anterior mitral leaflet is the hallmark of dynamic obstruction and the LA may be dilated due to the same causes as in aortic stenosis. The Valsalva maneuver and/or amyl nitrate administration are indicated for detection of latent obstruction. In those patients with SAM, MR is present. The impact of medication, surgery or pacemaker therapy on the dynamic gradient across the LVOT is accurately evaluated by Doppler echocardiography.

Tricuspid Regurgitation

This valvular abnormality can be found isolated as in carcinoid syndrome or Ebstein' anomaly but usually is associated with other valvular abnormalities such as rheumatic heart disease and, more frequently, is secondary to pulmonary hypertension. The echocardiogram will identify the possible causes for RV dilatation and will measure the RV systolic pressure which, in the absence of pulmonic stenosis, reflects the pulmonary artery systolic pressure.

Atrial Septal Defect (ASD)

In adults, ASD is better evaluated by transesophageal echocardiography (TEE) which accurately identifies the type of ASD, the size, the degree of left to right shunt, the possible associated abnormalities (e.g., sinus venosum type can be associated with partial anomalous pulmonary venous connection) and the degree of pulmonary hypertension. Serial echo studies are recommended to evaluate the progression of RV dilatation and pulmonary hypertension for prompt ASD closure.

Ventricular Septal Defect

VSD can be characterized as follows:

Congenital or acquired (usually after a myocardial infarction), restrictive (loud murmur due to a high transeptal gradient) or not restrictive, its size, location, the degree of shunt and pulmonary hypertension, the presence of associated valvular abnormalities and the degree of LV dilatation.

Systolic Regurgitant Murmurs in Patients with Structurally Normal Hearts by Two-Dimensional Echocardiography

In a number of patients, mild regurgitant murmurs can be heard on examination and found during a routine echocardiographic study without any valvular abnormalities. One retrospective report mentioned a 34% prevalence of mild regurgitation in at least one of the valves in patients with echo-anatomical normal hearts.(6) Of 867 records, trace or mild regurgitation of the mitral valve was found in 19%, tricuspid in 17%, pulmonic in 5% and aortic regurgitation in 3%.

Diastolic Murmurs

Mitral Stenosis

Echocardiography will identify the etiology of mitral stenosis, will define the anatomy of the mitral apparatus and will define the mitral valve score (leaflet thickness, the severity of calcification, mobility and submitral apparatus pathology) which is helpful in determining the feasibility and the procedural success of balloon mitral valvuloplasty. Left atrial size, the presence of thrombus or spontaneous echo contrast ("smoke") in the left atrium or left atrial appendage, even in patients in normal sinus rhythm, might dictate the need for anticoagulation. TEE should be the technique of choice for visualization of left atrial appendage, especially in patients with atrial fibrillation who may undergo elective electrical or chemical cardioversion. Both anatomical and functional valve area are usually reported. However, one should be careful in utilizing the pressure half-time method in patients who have significant aortic regurgitation or poorly relaxing LV (underestimation and overestimation of severity, respectively). Pulmonary artery pressure should be reported. In patients with discrepancies between the calculated mitral valve area and symptoms, exercise Doppler might provide important additional data.(7)

Figure 3.

Aortic Regurgitation

In a high number of patients, TTE may identify the mechanism of regurgitation (e.g., congenital or acquired cusps, root dilatation, intimal dissection, etc.), its severity and will give accurate measurements of the aortic annulus, sino-tubular junction aortic root (critical information for those patients in whom stentless prosthesis is envisioned), aortic arch and proximal descendent aorta. The evaluation of LV size, volumes and function is important for surgical timing. TEE and MRI have a similar accuracy for detection of aortic dissection. It is worthwhile to perform annual Doppler echocardiographic examinations in those patients with aortic regurgitation who, although asymptomatic, are approaching the size and function of the LV which brings a less successful postoperative outcome (LVEF < 55%, LVDS > 55 mm).(8)

Infective Endocarditis

Systolic, diastolic, or both systolic and diastolic murmurs can be heard in endocarditis. An echocardiogram can identify the presence of vegetations, abscess, dehiscence of prosthetic valves, mycotic aneurysm or arterial-cameral fistula. Although there is still controversy regarding the predictors for embolization, vegetations larger than 10 mm have a high propensity to embolize.(9) Although transthoracic echocardiography (TTE) and TEE have a similar specificity for identification of vegetations, the sensitivity of TEE is higher than that of the TTE, especially in the prosthetic valves. On those patients with a strong suspicion of infective endocarditis and a normal TTE, TEE should be recommended.

Identification of Intracardiac or Aortic Source of Systemic or Pulmonary Embolism

This represents an important indication for both Trans-Thoracic Echocardiography and TEE. Besides left or right sided vegetations, tumors or thrombus, TTE, and especially TEE, can identify interatrial septal abnormalities [atrial septum aneurysm, patent foramen ovale (PFO), ASD], spontaneous echo contrast and ascending aorta or aortic arch atherosclerotic debris which when +/> 4 mm are significant predictors of recurrent brain infarction or other vascular events.(10) A saline contrast study during the Valsalva maneuver may detect a PFO responsible for a paradoxical embolism. However, in patients with a normal TTE, the TEE has a higher sensitivity for interatrial septum pathology and should be recommended in younger patients (<45 y/o) with otherwise unexplained events in whom a PFO is more prevalent and has potential higher importance.(11)

Patients with atrial fibrillation > 48 hours and no contraindications for anticoagulants should be anticoagulated for two to three weeks before cardioversion, unless a thrombus is not identified in the atria or atrial appendages. TEE is instrumental in these cases. Patients with chronic atrial fibrillation and enlarged left atrium ( anterio-posterior diameter > 4.5 cm) have a reduced chance to maintain a normal sinus rhythm after cardioversion. The finding of a calcified mitral annulus in elderly individuals increased the stroke incidence twofold.(12)

Figure 4.

Evaluation of Patients with Congestive Heart Failure (CHF)

The left and right ventricular size, the atria size (biatrial dilatation is a feature of restrictive cardiomyopathy) the myocardial acoustic texture changed in infiltrative cardiomyopathies (e.g., amyloidosis), the presence of associated valvular diseases which may explain CHF (e.g., critical silent aortic stenosis in the elderly), the pulmonary artery pressure, the presence of thrombus or spontaneous echo contrast and the degree of regional and global systolic dysfunction are critical parameters for the diagnosis, treatment, follow-up and prognosis of such patients. LV diastolic dysfunction of a restrictive type (E wave deceleration time < 115 ms) is an independent predictor of poor outcome or need for transplantation in patients with dilated cardiomyopathy.(13) Patients with a LV ejection fraction > 40% and otherwise unexplained symptoms and/or signs of LV dysfunction should be specifically assessed for the presence of LV diastolic dysfunction identifiable by Doppler interrogation of the mitral inflow, pulmonic veins, measurement of isovolumic relaxation time and, lately, by color M-mode flow propagation and Doppler tissue imaging.

Arterial Hypertension

The presence of left ventricular hypertrophy (LVH) (concentric remodeling) and/or increase of LV mass are considered independent risk factors for adverse outcome in these patients. Echocardiography can also furnish information about LV systolic and diastolic function, which may be important information for the management of these patients. It may be useful to perform an echocardiogram in patients with stage I hypertension before commencing treatment. The regression of LVH, secondary to weight loss and blood pressure control, can also be documented by echocardiography although its relevance regarding the impact on outcome has to be determined. Left atrial dilatation, secondary to diastolic dysfunction, may precede the development of atrial fibrillation and, therefore, in this group of patients, more aggressive control of blood pressure is warranted.

Coronary Artery Disease (CAD)

An abrupt occlusion of a coronary artery will affect both the relaxation and contraction myocardial properties, the last, translating into regional wall motion abnormalities. Therefore, patients with acute chest pain of coronary origin can be diagnosed by echocardiography which, from the pooled data, has a modest positive predictive value (50%) and very high negative predictive value (95%) for diagnosing an acute coronary event.(14) In patients with acute myocardial infarction (MI), a rest echocardiogram can identify the location, the extent and the consequences of the MI on the global LV systolic and diastolic function, its mechanical complications [ventricular septal defect, acute mitral regurgitation due to disruption of the sub-mitral apparatus, free wall (subacute or acute rupture)] and even the patient's prognosis. The echocardiographic follow-up of these patients after the acute event is important for evaluation of the LV remodeling, thrombus formation, the improvement of regional and global LV function and the evolution of mitral regurgitation.

Stress echocardiography has continuously been refined during its twenty years of history and in competent hands has a similar sensitivity, specificity and accuracy as myocardial nuclear perfusion techniques for the detection, localization and extension of CAD.(15) Both techniques are superior to exercise electrocardiography. Post-treadmill echocardiography and dobutamine echocardiography are the most utilized protocols for exercise and pharmacological stress, respectively, in the United States. The greatest diagnostic benefit will be for those patients with an intermediate pretest likelihood of CAD. Identification of the viable myocardium by low dose dobutamine echocardiography in patients with significant LV dysfunction before revascularization procedures (and, therefore, predicting the post-procedural functional recovery) is another important indication of stress echocardiography.

Evaluation of Patients with Syncope

Our impression is that in a patient with no history of heart disease and with normal clinical examination, the role of echocardiography in determining the cause of syncope is quite limited. In a recent prospective study, the echocardiogram was diagnostic only in 7.8% of 258 patients presented with syncope.(16)

Pericardial Diseases

Echocardiography is invaluable in identification, semiquantification and, at times, in establishing the etiology of pericardial effusion. Cardiac tamponade is a clinical diagnosis and an echocardiogram is performed in order to validate the clinical impression. In patients with a clinical history suggesting constrictive pericarditis (e.g., prior open heart surgery, infections, etc.), although the echocardiography may not be the most accurate method to measure pericardial thickening (TEE is superior to TTE), the Doppler signs of constrictive physiology, which can differentiate it from restrictive cardiomyopathy, are very reliable. This differentiation has paramount importance because the therapeutical options are totally different: surgery in constriction and medical therapy in restriction. In patients treated with radiation therapy, the constrictive and restrictive physiology can co-exist making the diagnosis more difficult but not impossible.

Follow-Up After Different Surgical Cardiovascular Procedures

These include valvular surgery (especially the assessment of the quality of mitral valve repair), aortic repair for aneurysm of the thoracic aorta (these patients should have an annual imaging procedure -- TEE is the least expensive -- in order to identify new aneurysm formation), surgery for obstructive cardiomyopathy (assessment of residual LVOT gradients), and after cardiac tumor resection which can re-occur (e.g., myxoma). In patients with intra-cardiac mechanical devices suspected of infection, TEE should be the test of choice.

Cardiac Evaluation for Noncardiac Surgery

Rest echocardiography and dobutamine stress echocardiography are shown to be valuable tools for risk stratification before noncardiac surgery. A LVEF at rest of < 35% imposes a great risk of perioperative cardiac complications.(17) A positive stress echocardiogram may require further investigation by coronary angiography.

Other Indications for Echocardiography

Adult patients with congenital heart disease who need to be followed indefinitely, patients with pulmonary diseases for the assessment of the severity of pulmonary hypertension, the influence of different medicines on myocardial function (e.g., doxorubicin), or valves (e.g., phen-fen), the screening of families of those patients with connective tissue disorders and the evaluation of potential donor hearts are other indications for Doppler echocardiography. Screening athletes and families of those patients with cardiomyopathies is still in debate.

In most of the clinical situations discussed above, TTE performed in an established high volume echo lab is sufficient. However, TEE is usually employed with a very low complication rate (1.3%)(18) in the work-up of patients with stroke, endocarditis, native or prosthetic valve dysfunction (especially in mitral position) and aortic dissection.

Conclusion

In a paper analyzing the utilization of echocardiography in internal medicine practice,(1) the most frequent reason for an echocardiogram ordered by an internist was suspected valvular dysfunction, while, for cardiologists, in contrast, the evaluation of the LV function was the first indication for an echocardiographic study.

I would like to emphasize that, in spite of the tremendous progress of cardiac ultrasound, echocardiography did not and will not replace careful history taken and knowledgeable and meticulous physical examination. Echocardiography will validate or refute your clinical impression and will assist you in taking further steps in a patient's evaluation or in deciding upon a certain medical or surgical therapeutic option in the patient's best interest.