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Thursday, January 12, 2012

Echocardiography

Author : Elyse Foster, M.D. Professor of Clinical Medicine and Anesthesia San Francisco, CA

2008-07-28

An echocardiogram is a clinical examination of the heart that uses ultrasound waves to create both still and moving images.  Based on a special property of sound known as the Doppler principle, this technique is also used to map normal and abnormal blood flow signals and to measure blood flow velocity.  Echocardiographic images are most often obtained using a transducer gently applied to the surface of the chest wall, a “transthoracic echocardiogram.”  The transducer is a microphone-like device that both sends and receives the ultrasound waves, which are too high in frequency to be audible to the human ear.  The transducer is attached to the echocardiographic system that can vary in size from a laptop to a small refrigerator.  The sophisticated software within the system calculates the distance of the various cardiac structures from the transducer much like sonar is used to measure the ocean depth, and then compiles the images to form pictures of the heart. These images may be one-dimensional, displayed graphically over time (M-mode images), two-dimensional, or three-dimensional representations, displayed in a repeating single heart beat format known as a cine-loop. Blood flow velocities are measured much the way radar is used to measure the speed of a moving car.  They may be displayed graphically as “spectral Doppler.”  Flow velocities are also color encoded as “color flow Doppler” and superimposed on the moving 2 and 3-dimensional images to provide flow maps.

Types of Echocardiograms


Echocardiograms are classified according to the site from which the images are obtained.  As mentioned above, the most common type is a transthoracic echocardiogram (TTE), in which the images are obtained from the chest wall.  The major limitation of this examination is that the ribs and the air contained within the lungs do not allow the ultrasound waves to penetrate to the heart so most of the images are obtained between the rib spaces. The major advantage to this test is that it is non-invasive, meaning that it is entirely performed from the body surface. Most examinations are performed while the patient is at rest, lying on a specially designed examination table.  This procedure is essentially risk-free.  Ultrasound as used in these examinations has never been shown to harm the body tissues or affect heart function. TTE may also be coupled with a stress test, using either exercise or medication (see below).
Less commonly, an echocardiogram is performed using invasive techniques.  A transesophageal echocardiogram (TEE) is obtained from within the esophagus to improve the penetration of sound by avoiding the lungs and ribs. Very high quality images with superb resolution of cardiac structures are obtained with this technique. The transducer used for TEE is attached to an endoscope, a tube measuring 100 cm in length and about 1 cm in width.  After numbing the back of the throat with lidocaine spray and administering sedating medications through an intravenous line, the tube is passed into the patient’s esophagus.  There are known risks associated with TEE so the procedure is performed with extensive monitoring of blood pressure, heart rate, and respiratory status.  The risks of concern are:
1) Aspiration:  The patient is unable to swallow so there is some risk of aspirating stomach contents into the lungs.  Therefore the patient must not have eaten for a minimum of 6 hours prior to the test.  Because the throat is numb, the patient should refrain from eating for at least 4 to 6 hours after the test.
2) Excessive sedation:  The most commonly used medications to provide sedation to minimize the discomfort associated with the TEE are a benzodiazepine called midazolam and a narcotic called fentanyl. There are alternative medications for patients with known allergies to these medications.  Sedation in patients with lung disease or advanced heart disease can depress respirations so careful monitoring is needed.  They also may cause a fall in blood pressure.  In some rare cases, an anesthesiologist may be in attendance when the patient’s status is tenuous.  Any outpatient having this procedure will need to be accompanied home and is instructed not to drive for 24 hours.
3) Esophageal damage:  This is a rare complication and most common when there is a history of disease in the esophagus such as a diverticulum, or outpouching, of the wall, a stricture or narrowing of the esophagus, or a tumor.  Patients with any history of pain or difficulty with swallowing should have other investigations before a TEE is performed.
Intravascular ultrasound (IVUS) uses miniaturized transducers so small that they can be placed at the ends of the small tubes (“catheters”) employed during cardiac catheterization and are passed through an artery, usually a major artery in the groin.  The most frequent application of IVUS is to measure the degree of stenosis, “blockage,” within the coronary arteries, when more conventional x-ray images, “coronary angiograms,” are inconclusive.  With intracardiac echocardiography (ICE) the images are captured within the heart by obtaining access through a vein in the groin.  The major indication for ICE is to guide special procedures that are performed in the catheterization laboratory.  The risks associated with these procedures are similar to those for a heart catheterization.
A stress echocardiogram is usually performed in conjunction with a treadmill exercise test to detect abnormalities of heart function that are precipitated by or worsen with exercise.  The most frequent example would be abnormal function under stress of part of the heart wall that is supplied by a blocked artery.  The images are obtained before and immediately after exercise.  Imaging can also be performed continuously with the patient cycling while lying on his/her back.  When a patient is unable to exercise, a medication called dobutamine can be used to simulate exercise to stress the heart.   The medication is administered through an intravenous line, beginning at very low doses and gradually increasing the dose to raise the heart rate and blood pressure to levels that are similar to exercise.  A series of echocardiographic images are obtained to detect changes in wall motion.

Performance, Interpretation, and Quality of Echocardiograms

A skilled technician, known as a cardiac sonographer, will perform transthoracic echocardiograms, while transesophageal, intravascular, and intracardiac examinations are performed only by physicians.   A standard TTE in a patient with mild cardiac abnormalities usually requires about 30 minutes while a study of a patient with complex diseases may last as long as 1.5 hours. To ensure that complete information is acquired, the sonographer should follow a standard examination that has been defined by the American Society of Echocardiography.  Images are obtained from a series of positions on the chest wall, the notch above the sternum (breastbone) and from the upper abdomen.  The patient may be asked to hold their breathing momentarily during the acquisition to improve image quality.  All of the studies are recorded, previously on videotape, now predominantly on digital media.  Clinical echocardiographic laboratories may be housed in hospitals, doctors’ offices, and free-standing cardiac imaging centers.  These laboratories may choose to receive certification from an organization known as the Intersocietal Commission for the Accreditation of Echocardiographic Laboratories (ICAEL).  Certified laboratories have undergone a rigorous review process demonstrating consistent quality.

The images are always interpreted by a cardiologist, who generates a report of the findings, which is usually available within 7 days of the test.  Cardiologists vary in their level of expertise in the interpretation of the findings.  There is now a special examination that certifies a cardiologist’s expertise in echocardiography. (ref National Board of Echocardiography)  However, many cardiologists without this certification can skillfully interpret these studies.

General Indications for Echocardiography

Echocardiograms provide a wealth of anatomical and functional information and are employed in the diagnosis and monitoring of most forms of heart disease.  After the physical examination and an electrocardiogram (EKG), the echocardiogram is usually the next step in the diagnosis of suspected heart disease. Occasionally a patient with a strong family history of heart disease will have a screening examination to detect early disease, not yet associated with physical signs or symptoms.  However, most echocardiograms are ordered by a physician to investigate symptoms suggesting heart disease or abnormal physical findings on the cardiac examination. The most common symptoms associated with heart disease are shortness of breath, especially that which occurs during exercise, fatigue, chest pain, and syncope (fainting).  Findings on physical examination suggesting heart failure or disease of a heart valve are also further investigated with an echocardiogram. Other less common indications for echocardiograms are:
1)    Hypotension: an abnormally low blood pressure
2)    Embolic stroke: as many as 25% of strokes arise from a clot or “thrombus” within the heart 
3)    Suspected endocarditis: an infection of the structures that line the inner surfaces of the heart, including the heart valves in patient with fevers and evidence of bacteremia (blood infection)
4)    Pulmonary hypertension: abnormally elevated pressures in the circulation to the lungs

Echocardiograms may be repeated in patients with established heart disease to monitor its natural progression and to assess the effectiveness of medications or non-pharmacologic therapy. 

The Normal Echocardiogram


All of the cardiac structures can be displayed in an echocardiogram.  These include:
 1)    The four cardiac chambers:
a.    The left ventricle (LV) is the main workhorse of the heart, pumping the oxygen-rich blood to the entire body through the aorta.  The muscular walls of the heart are called the myocardium, normally about 1 cm in thickness in the left ventricle.  The blood filled cavity is roughly shaped like a cone and accommodates approximately 100 ml at the end of diastole (the filling portion of the cardiac cycle).  At the end of systole (the active pumping portion of the cardiac cycle), approximately 40 ml remains in the cavity.  The percentage of the blood pool ejected, known as the ejection fraction, is therefore 60%.  The normal ejection fraction ranges from 55% - 70%.  The tasks for echocardiography in the evaluation of the left ventricle are to:
                                               i.     Measure the size of the chamber and the thickness of the walls
                                             ii.     Calculate the ejection fraction
                                            iii.     Ensure that all of the walls of the ventricle are contracting in a uniform and synchronous manner
                Most of these tasks can be accomplished with transthoracic echocardiogram. 
b.    The right ventricle (RV) is the other pumping chamber, which pumps oxygen-poor blood through the pulmonary artery to the lungs, where oxygen is replenished.  The right ventricle normally pumps at lower pressures and the walls are thinner, less than 0.5 cm.  Its shape is more like a crescent and it is located in front of the left ventricle, directly under the sternum.  While echocardiography also evaluates the shape, size, and wall thickness of the right ventricle, these qualities are usually described rather than measured.
c.     The left atrium (LA) is the reservoir for the oxygen-rich blood returning from the lungs through the pulmonary veins.  It holds about 50 ml and partially empties into the left ventricle during diastole.  The major task of the echocardiogram is to measure the size of the left atrium. 
d.    The right atrium (RA) is the reservoir for the oxygen-poor blood returning from the body through the major venous channels, called the inferior and superior vena cava. It holds about 50 ml and partially empties into the right ventricle during diastole.  The major task of the echocardiogram is to measure the size of the right atrium.
2)    The four heart valves are thin, mobile structures that are classified as semilunar valves (aortic and pulmonary valves) or atrioventricular valves (mitral and tricuspid valves).  The semilunar valves separate the ventricles from the great vessels (aorta and pulmonary artery) and open in systole when the ventricles are contracting. The atrioventricular valves separate the atria from the ventricles and open in diastole to permit the ventricles to fill.  The opening of the normal valves is adequate to permit smooth blood flow and closure is adequate to prevent backflow between the respective structures.
a.    The aortic valve is comprised of three cusps or leaflets. It separates the left ventricle and the aorta.  When fully open in systole, the area of the opening is 3 – 4 cm2.  During systole the pressures in the aorta and the left ventricle are the same and roughly equal to the systolic blood pressure  measured with a cuff on your arm (normal is approximately 120 mmHg).  During diastole, the aortic valve is closed.
b.     The mitral valve is comprised of 2 leaflets and it separates the left atrium and left ventricle.  When fully open in diastole, the area of the opening is 4 - 6 cm2.  During systole, the valve is closed but small amounts of back leak known as “mitral regurgitation,” can be detected in more than 25% of totally normal patients.
c.     The pulmonary valve separates the right ventricle and the pulmonary artery.  When fully open in systole, the area of the opening is 3 – 4 cm2.  During systole the pressures in the pulmonary artery and right ventricle are the same and roughly equal to the pulmonary artery systolic pressure (normal is approximately 20 mmHg).  During diastole, the pulmonary valve is closed.
d.    The tricuspid valve is comprised of 3 leaflets and it separates the right atrium and right ventricle.  When fully open in diastole, the area of the opening is 4 - 6 cm2.  During systole, the valve is closed but small amounts of back leak known as “tricuspid regurgitation,” can be detected in more than 50% of totally normal patients.
3)    The great vessels include the pulmonary artery and the aorta.  They cannot be visualized in their entirety on an echocardiogram.  Usually the root adjacent to the respective ventricle and the initial 4 – 6 cm of the vessel can be seen. 
a.    The aortic root is clover shaped in short axis and gives rise to the coronary arteries, which supply the heart with blood.  Although the origins of the coronary arteries can be seen in most people, visualization is insufficient, even with TEE, to detect blockages.  The aortic root is the widest part of the aorta, measuring approximately 3 cm, and narrows after 1 – 2 cm to form the tubular portion. 
b.    The pulmonary artery has a large trunk (the main PA), measuring about 3.5 cm, that branches to form the right and left pulmonary arteries, one to each lung.  Only the initial 1 cm of the branches is usually seen.
4)    The venous structures that connect to the atria include:
a.    The inferior vena cava, which often is best visualized from the upper abdomen in the so-called sub-costal view.
b.    The superior vena cava is not routinely visualized in the adult patient, but can be seen when necessary.
c.     There are four pulmonary veins, which enter the right atrium and usually the upper veins are visualized.
5)    The pericardium surrounds and encloses the heart.   It is comprised of two paper-thin layers, one adherent to the heart, the “visceral pericardium,” and the other second outer layer, the “parietal pericardium.”  Normally 20 – 50 ml of fluid between the two layers is present to lubricate cardiac motion.  When normal, the layers are barely visible.

The Abnormal Echocardiogram

In many patients with heart disease, the echocardiogram is abnormal.  An important exception is the patient with coronary artery disease who has stenoses (“blockages”) in the coronary arteries but has never had a myocardial infarction, otherwise known as a “heart attack.”  Other imaging techniques are needed to visualize the coronary arteries to detect significant disease (see below).  This section will discuss the major categories of heart disease that are often diagnosed primarily with echocardiography and provide images diagnosing some of these diseases.
1.    Heart Failure:  Heart failure is a syndrome with many different causes that can lead to shortness of breath, fatigue, exercise intolerance, and leg swelling.  There are excellent treatments for heart failure so proper diagnosis is critical.  Cardiologists classify heart failure according to its underlying mechanism as systolic or diastolic heart failure.  Diastolic heart failure is also known as heart failure with preserved systolic function.
1.    Systolic heart failure:  In systolic heart failure, the pumping function of the left ventricle and often the right ventricle are impaired.  The pump function is primarily measured as the “ejection fraction,” described above.    The ventricles and ultimately the atria enlarge (dilate).  The role of the echocardiogram is to measure the ejection fraction and to categorize the severity of dysfunction:
                                            ·  Mildly reduced (40 – 54%)
                                            ·  Moderately reduced (30 – 40%)
·  Severely reduced (< 30%)
The LV function may be reduced globally, involving all of the walls, most commonly seen in a “dilated cardiomyopathy.”  Segmental dysfunction is non-uniform and often a manifestation of coronary artery disease, in which particular walls that are fed by blocked coronary arteries are individually affected.  There also may have been infarction (death of the wall tissue) during a heart attack.  These findings on echocardiography will often lead a cardiologist to recommend a cardiac catheterization.
2.    Diastolic heart failure: By definition, this entity is heart failure in the presence of an ejection fraction > 45%.  In this case, symptoms occur because the heart is stiff.  For any given volume in the ventricle, the pressure called the “filling pressure” is elevated.  Often the heart is hypertrophied meaning that the walls are thicker than normal and the left ventricular cavity is small.  The atria may become very large and flow patterns in the heart are used to infer rather than directly measure the pressure.  The most common causes of diastolic heart failure are hypertension, aging, and diabetes, but it is also a manifestation of hypertrophic cardiomyopathy.  Diastolic dysfunction may also coexist with systolic heart failure.

 
2)    Valvular Disease:  Each of the four heart valves may develop abnormal function, categorized as either stenosis or regurgitation.  Valve disease is detected on physical examination by the presence of a heart murmur, which is one of the most common indications for an echocardiogram.  Many heart murmurs are innocent, meaning that they do not signify important valve disease. The echocardiogram may be ordered to distinguish an “innocent murmur” from a “pathologic murmur.” The information on the echocardiogram is often sufficient to recommend a patient for surgical or non-surgical treatment of the valve disease. However, cardiac catheterization may be recommended before surgery to confirm the results of the echocardiogram and to detect co-existing coronary artery disease.
1.    Stenosis is a narrowing of the valve and generally becomes clinically significant when the valve area falls below 1 cm2.   With stenosis the pressure in the chamber proximal to the valve increases to drive flow across the valve.  The pressure drop across the valve is called the gradient and is derived from the Doppler flow velocities.  The roles of echocardiography in valve stenosis are to:
·  Define valvular anatomy and to detect valve thickening and reduced opening on the 2-dimensonal and 3-dimensional images
·  Measure the flow velocities across the valve to calculate the pressure gradient
·  Calculate the valve area
·  Determine the effect of the stenosis on the function of the heart
2.    Regurgitation is sometimes also called valvular insufficiency.  With regurgitation, the valve closure is incomplete and there is back leak through the valve.  It is usually graded on a 1 to 4 scale from mild to severe.  The roles of echocardiography are to:
·  Examine the valve to detect anatomic abnormalities that cause it to leak;  an example would be a perforation or hole in the valve
·  Determine the severity of the back leak using color Doppler
·  Determine the effect of regurgitation on the size and function of the heart
3)    Pericardial Disease:  Abnormal accumulation of fluid between the two layers of the heart can occur due to a number of different causes.  Among the most common are cancer, viral infection, and kidney disease.  In blunt chest trauma, a bloody effusion may develop.  When there is a large amount of fluid, it can prevent the heart from filling and the output of the heart eventually falls, causing a drop in blood pressure.  This syndrome is called “cardiac tamponade.”  The fluid in the pericardial space must be drained to prevent death.  The roles of echocardiography are to:
·  Detect the fluid surrounding the heart
·  Evaluate the effect of the fluid on the filling of the heart
·  Guide drainage of the fluid
4)    Congenital Heart Disease:  There are so many different types of congenital heart disease that a detailed description is beyond the scope of this section.  However, echocardiography is extremely useful in diagnosing inborn defects of the heart such as ventricular septal and atrial septal defects.
5)    Pulmonary hypertension:  The normal pressure in the vessels to the lungs, the “pulmonary circulation,” is expressed as a systolic pressure over a diastolic pressure, similar to the systemic blood pressure. Normal pulmonary artery pressure is approximately 25/10 mm Hg with a mean pressure of 12 – 15 mmHg.  High pressure in the lungs is called pulmonary hypertension and is diagnosed when the mean pulmonary artery (mPAP) pressure is greater than 25 mmHg, roughly corresponding to a PA systolic pressure (PASP) of 40 mmHg.  There are many causes of pulmonary hypertension and it is being increasingly diagnosed.  Echocardiography can provide an estimate of pulmonary artery systolic pressure if there is some regurgitation of the tricuspid valve.  Mild tricuspid regurgitation (TR) is exceedingly common so an estimate is possible in most people.  Pulmonary hypertension will be incorrectly diagnosed if the interpreter does not distinguish between the mean pressure and the systolic pressure.  Based on the pulmonary artery systolic pressure derived from the TR jet we classify the severity as follows:
·  Mild: PASP = 40 – 50 mmHg
·  Moderate: PASP =  50 – 70 mmHg
·  Severe: PASP > 70 mmHg
Other roles for the echocardiogram in pulmonary hypertension are to:
·      Evaluate the right ventricular size and function
·      Detect possible causes of pulmonary hypertension
However, the diagnosis of pulmonary hypertension, which requires specific treatment, should be confirmed by cardiac catheterization.

Alternative Imaging Techniques

The many roles of echocardiography have been described above.  When properly performed and interpreted, it is a safe, effective, relatively inexpensive, and widely available technique.  In some patients, no other imaging modality is required.  In other patients, the more invasive techniques listed below are necessary.
1)    Nuclear Scans use small amounts of radioactive substances to create images of the heart.  They require intravenous administration of these agents, usually in association with exercise or a second medication that “stresses” the heart.  The main roles of these tests are to:
a.    Measure cardiac function, specifically left ventricular ejection fraction using a “gated blood pool scan”
b.    Measure blood flow to the heart using a “perfusion scan”
c.     Detect myocardial viability, that is abnormal function of a wall of the heart that can be restored to normal if blood flow is improved, such as with bypass surgery.
In general, echocardiography is less effective in measuring blood flow and viability than nuclear scans but equally effective at evaluating heart function.  Nuclear scans do not provide information about the valve function, pericardium, and congenital heart defects.
2)    Cardiac catheterization is an invasive technique in which small tubes are threaded into large arteries and veins, usually in the groin, to reach the heart.  The major roles of cardiac catheterization are:
a.     Coronary angiography, the most common type of catheterization, during which a contrast agent or “dye” is injected into the coronary arteries to visualize blockages or stenoses.  This is often followed immediately by opening the artery with a balloon and/or a stent.  Echocardiograms do not visualize the coronary arteries adequately to determine the presence of a stenosis.  Coronary artery disease can only been inferred from an echocardiogram by the presence of abnormal contraction of a wall of the left ventricle at rest or with exercise.
b.    Hemodynamic catheterization is performed to directly measure pressures within the heart and to diagnosis valvular disease and pulmonary hypertension.  Echocardiography is usually highly effective in making these diagnoses, but when results are uncertain, catheterization may be necessary.                                                                                                                                   
3)    Coronary CT Angiography is a relatively new x-ray technique that visualizes the coronary arteries without a catheterization although it does require intravenous contrast administration.  Its accuracy is still being tested. CT Angiography of the great vessels is highly effective in diagnosing aortic aneurysms, aortic dissection, and clots within the pulmonary arteries (pulmonary emboli) and is generally superior to echocardiography for these applications.  However, transesophageal echocardiography is an excellent technique for diagnosing aortic dissection.
4)    Cardiac Magnetic Resonance Scan: There are many established and evolving applications for cardiac MR.  Much of the information obtained with this technique is comparable to that of an echocardiography, although CMR may have some advantages.  It does require an intravenous contrast agent.

Conclusion

Echocardiography is highly effective in screening for heart disease, establishing a diagnosis and for monitoring many treatments.  It is safe and readily available. 

Links

www.uptodate.com

 References

1. Biykem Bozkurt and Douglas L. Mann   Shortness of Breath Circulation, Jul 2003; 108: e11 - e13.
2. Gunjan J. Shukla and Peter J. Zimetbaum Syncope  Circulation, Apr 2006; 113: e715 - e717.
3. Carol Flavell and Lynne Warner Stevenson Take Heart With Heart Failure Circulation, Oct 2001; 104: e89 - e91
4. Lisa Ku, Jennie Feiger, Matthew Taylor, Luisa Mestroni on behalf of the Familial Cardiomyopathy Registry Familial Dilated Cardiomyopathy Circulation, Oct 2003; 108: e118 - e121
5.  Rick A. Nishimura, Steve R. Ommen, and A.J. Tajik Hypertrophic Cardiomyopathy: A Patient Perspective   Circulation, Nov 2003; 108: e133 - e135.
6. Stern S, Behar S, Gottlieb S.  Aging and diseases of the heart.  Circulation. 2003 Oct 7;108(14):e99-101
7. Thomas Biancaniello  Innocent Murmurs: A Parent’s Guide  Circulation, Mar 2004; 109: e162 - e163
8.  Zoltan G. Turi  Mitral Valve Disease  Circulation, Feb 2004; 109: e38 - e41
9. Rick A. Nishimura  Aortic Valve Disease  Circulation, Aug 2002; 106: 770 - 772.
10.  Christopher H. Cabell, Elias Abrutyn, and Adolf W. Karchmer  Bacterial Endocarditis: The Disease, Treatment, and Prevention  Circulation, May 2003; 107: e185 - e187.
11. Jacques I. Benisty  Pulmonary Hypertension  Circulation, Dec 2002; 106: e192 - e194.
12.  Richard A. Lange and L. David Hillis  Diagnostic Cardiac Catheterization Circulation, May 2003; 107: e111 - e113.
13.  Raymond Y. Kwong and E. Kent Yucel  Computed Tomography Scan and Magnetic Resonance Imaging  Circulation, Oct 2003; 108: e104 - e106.
14.  Marcelo F. Di Carli and Sharmila Dorbala  Exercise Testing and Nuclear Scanning   Circulation, Apr 2003; 107: e100 - e102.