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| Echocardiography: Assessing Specific Cardiac Related Diseases |
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Adrielyn Marie Watts, BSES
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Abstract:
An echocardiogram (echo) is a noninvasive test that uses ultrasound waves to examine the heart. Echocardiography plays a vital role in the diagnosis and evaluation of many cardiac disorders by providing information about the anatomy and physiology of the heart, determining pumping capability, and detecting blood clots and excess fluid around the heart. This article demonstrates the different modalities of echocardiography and how they are used in the clinical setting to evaluate common cardiac diseases such as coronary artery disease (CAD), cardiomyopathies, and pericardial diseases. This paper also compares the use of echocardiography to other imaging modalities and describes some of its limitations. Echocardiography plays a critical role in the diagnosis, risk stratification, and definition of the most appropriate therapy in patients with specific cardiac diseases. The information obtained from this modality has been known to decrease the time of diagnosis, patient care costs, morbidity, and ultimately the rate of mortality among Americans. |
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Key Words:
cardiac, echocardiography, disease, heart |
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Introduction
There is a common misconception of the true number one killer of Americans. Since 1900, every year excluding 1918, heart disease has put more people in coffins than any other attributable source. In fact, more people in the U.S. die from heart disease than from cancer, AIDS, and all other diseases combined (1). This paper will look at the use of echocardiography in diagnosing and evaluating specific cardiac related diseases. |
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Anatomy and Physiology
The heart, blood and the blood vessels comprise the cardiovascular system. The cardiovascular system functions to maintain homeostasis by circulating oxygen, nutrients, and waste products through the body. The heart rests on the diaphragm near the middle of the thoracic cavity and is about the size of a person’s closed fist. The heart wall is divided into three layers: the epicardium (external layer), myocardium (middle layer), and the endocardium (innermost layer). The myocardium is cardiac muscle tissue, which makes up the majority of the heart and is responsible for its pumping action. The heart is surrounded by a triple-layered sac called the pericardium, which provides protection and prevents overstretching of the heart.
The heart is specifically designed for its task of propelling blood through an estimated 100,000 km of blood vessels. With each beat the heart pumps blood into two closed circuits: the systemic circulation and the pulmonary circulation. The left side of the heart is the pump for the systemic system. It receives freshly oxygenated blood from the lungs and propels it into the aorta, the largest artery in the body. The right side of the heart is the pump for the pulmonary circulation. It receives all deoxygenated blood returning from the systemic system.
The interior of the heart is divided into four chambers. The right and left atria compose the two superior chambers. The two inferior chambers are the right and left ventricles (see Figure 1). After blood fills the atria, they contract simultaneously delivering blood to the ventricles where the deoxygenated blood is sent to the lungs to receive oxygen via the right ventricle and the oxygenated blood is propelled to all other parts of the body by the left ventricle.
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To prevent regurgitation, or back flow of blood, the heart has valves. The tricuspid valve lies between the right atrium and right ventricle. The valve between the left atrium and left ventricle is called the bicuspid valve. The valves between the pulmonary trunk and right ventricle and between the left ventricle and aorta are termed pulmonary and aortic, respectively. The flow of blood through the many vessels that serve the myocardium is called cardiac circulation. The right and left coronary arteries deliver blood to the heart, whereas the coronary veins drain blood from the heart into the coronary sinus (see Figure 2).
For the heart to be an effective pump, it relies heavily on its conduction system. The conduction system is composed of autorhythmic (self-exicitable) cells, which generate an electrical pulse that stimulates the atria to contract and then the ventricles. The conduction system assures that cardiac chambers become excited to contract in a coordinated manner, which makes the heart an effective pump (2, 3).
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Instrumentation
Cardiac imaging is achieved by sending high frequency ultrasound waves into the body via a piezoelectrical transducer (see Figure 3). The waves enter the body and are reflected from the tissues back to the transducer, deforming the crystals in the transducer. The distance between the transducer and the reflecting tissues is automatically calculated by the machine and images are produced. Three types of echocardiographic studies are generally performed: two-dimensional (2D), motion-mode (M-mode), and Doppler imaging . Two-dimensional imaging is used primarily to measure cavity size, wall thickness, stroke volume, ejection fraction and to describe anatomy and motion. This allows the physician to view the various heart structures at work and evaluate them. M-mode was the first cardiac application of ultrasonography and is used for measurement of wall thicknesses and chamber diameters. Doppler imaging is utilized to assess and measure the flow of blood through chambers and valves, and can detect abnormal blood flow within the heart. Doppler imaging can be subdivided into continuous wave (CW) and pulsed wave (PW) imaging. Continuous wave Doppler is used to estimate the severity of valve stenoses and pulmonary artery pressure, and measure high velocities. Pulsed wave Doppler is mainly used to describe diastolic behavior of the left ventricle and to calculate stroke volume (4, 5, 6). |
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Common Cardiac Diseases
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Coronary artery disease is the narrowing of the coronary arteries by atherosclerosis, making them vulnerable to blockage by a blood clot or atrial spasm (7). Atherosclerosis, commonly referred to as “hardening of the arteries”, is a disease of muscular arteries, in which the inner layer becomes thickened by fatty deposits and fibrous tissue. Atherosclerosis causes more deaths than any other disease in industrialized society. Ischemic heart disease is a condition in which imbalance between myocardial oxygen supply and demand results in myocardial hypoxia and accumulation of waste metabolics. It is often the result of atherosclerotic disease of the coronary arteries. Myocardial infarction (MI) is the condition of irreversible necrosis of heart muscle that results from prolonged ischemia. Turbulence and stasis of blood flow in regions of impaired left ventricular contraction due to an MI may induce intracavity thrombus formation, especially when an infarction involves the left ventricular apex (8).
Echocardiography has become an established and powerful instrument for diagnosing the presence of coronary artery disease and defining its consequences in patients with chronic coronary atherosclerosis. In patients with chronic ischemic heart disease, echocardiography is useful for a range of indications, including diagnosis, risk stratification, and clinical management decisions. Doppler techniques are also extremely valuable for evaluating both systolic and diastolic ventricular function in patients with chronic ischemic heart disease. Although it may not be easy to distinguish acute ischemia from previous myocardial infarction, prompt initiation of treatment to achieve reperfusion can reduce mortality, morbidity, and patient care costs. Stress echocardiography is useful for evaluating the presence, location, and severity of inducible myocardial ischemia as well as risk stratification and prognostication. An echocardiogram can also be used to rapidly diagnose the presence of regional contraction abnormality resulting from acute myocardial infarction. Echocardiography can be used to evaluate, at the bedside when needed, virtually any complication of acute myocardial infarction. In addition, echocardiography is the definitive test for detecting intracardiac thrombi (see Figure 4) (9). |
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Cardiomyopathies are a group of heart disorders in which the major structural abnormality is limited to the myocardium. Although these conditions may result from symptoms of heart failure, the etiology of these disorders frequently remains unknown. Cardiomyopathies may be classified into three types: dilated cardiomyopathy is characterized by ventricle enlargement with impaired systolic contractile function; hypertrophic cardiomyopathy involves an abnormally thickened ventricle with abnormal diastolic relaxation; and restrictive cardiomyopathy presents with an abnormally stiffened myocardium, whereas diastolic relaxation is impaired, but systolic function is usually preserved (8).
In patients with dilated cardiomyopathy, Doppler techniques are used to evaluate the presence and extent of associated valvular regurgitation to estimate pulmonary pressures, and to gain insight into diastolic function of the left ventricle. Echocardiography also permits reevaluation of ventriclar size and function so that disease progression and response to therapy may be monitored noninvasively. Echocardiography provides a definitive diagnosis of hypertrophic cardiomyopathy, revealing ventricular hypertrophy in patients without other primary causes. It may also be used to evaluate the response to therapeutic interventions, such as dual-chamber pacing (9).
Diseases of the pericardium form a spectrum that ranges from benign, self-limited pericarditis to life-threatening cardiac tamponade. The most common affliction of the pericardium is acute pericarditis, which refers to inflammation of its layers. Pericardial effusion, a larger volume of fluid, may accumulate in association with pericarditis. This increase in fluid volume can create an enormous compressive force on the heart which results in cardiac tamponade. As the pericardial fluid accumulates under high pressure, the cardiac chambers are compressed and severely limits filling of the heart. As a result, ventricular stoke volume and cardiac output decline, potentially leading to hypotensive shock and death (8).
One of the earliest clinical applications of echocardiography was in the detection of pericardial effusion, and it remains the procedure of choice for evaluating this condition (see Figure 5). Differentiation among types of pericardial fluid cannot be made, but fibrous strands, tumor masses, and blood clots can often be distinguished. Echocardiography can define the distribution of the fluid so that the safest and most effective approach can be planned for a pericardiocentesis. It is also the most useful noninvasive technique in evaluating whether pericardial effusion has led to cardiac tamponade. An accurate and early diagnosis of tamponade can often be made using echocardiography. The echocardiographic findings may provide an opportunity for early therapeutic intervention (9). |
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Echocardiography vs. other Modalities
When faced with a patient needing cardiovascular evaluation and testing, the clinician must choose among a variety of available tests. Echocardiography, nuclear testing, magnetic resonance imaging (MRI), and positron emission tomography can yield overlapping if not identical information, often with similar or comparable accuracy. A decision concerning which technique to use is based on factors such as test availability, cost, and patient preference. The ability of echocardiography to provide unique noninvasive information with minimal discomfort or risk without using contrast material or ionizing radiation, coupled with portability, immediate availability, and repeatability, accounts for its use in virtually all categories of cardiovascular disease. Patients’ acceptance of this noninvasive technique for initial and reevaluation observation is high. MRI has the capability to detect the presence of stenotic and regurgitant lesions and has several advantages. However, MRI instrumentation is substantially more expensive and not as widely available (9). As with any diagnostic procedure, echocardiography too has its limitations. The sensitivity of resting echocardiography for detecting wall-motion abnormalities progressively decreases the longer the time between resolution of chest pain and the acquisition of images. Also, it cannot accurately distinguish acute ischemia or infarction from previous myocardial infarctions. Finally, application of echocardiography requires mobilization of special personnel and equipment and cannot be justified from a practical, cost-effective standpoint in all patients (10). |
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Conclusion
The use of echocardiography in establishing cardiac diagnoses and making therapeutic decisions is well established. With advances in medical technology, such as echocardiography, physicians can accurately and expediently diagnose and develop a treatment plan for specific cardiac related diseases. The major advantages of echocardiography include its widespread availability, portability, noninvasive nature, and the accurate anatomic, hemodynamic, and prognostic information it provides (10). The information obtained from this modality has been known to decrease the time of diagnosis, patient care costs, morbidity, and ultimately the rate of mortality among Americans (9). |
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References
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6. The Nebraska Medical Center. Available at: http://www.nebraskamed.com/heart/prech02.cfm#Whatisecho
7. Johansson P. Heart Disease. The Plague of Modern Society. Berkeley Heights, NJ: Enslow Publishers, Inc; 1998: 15-25.
8. Lilly LS. Pathophysiology of Heart Disease. Atherosclerosis. Ischemic Heart Disease. The Cardiomyopathies. Diseases of the pericardium. New York: Lippincott Williams & Wilkins; 1993: 101:119-120: 145: 167: 217: 289: 295-297.
9. ACC/AHA Guidelines for the Clinical Application of Echocardiography. Available at: http://circ.ahajournals.org/cgi/content/full/96/6/1686 Accessed April 9, 2004.
10. Roldan CA, Abrams J. Evaluation of the Patient With Heart Disease. The Patient With Coronary Artery Disease. New York: Lippincott Williams & Wilkins; 2002: 65-103.
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Illustration References
1. Interior View of the Heart. Available at: http://www.nebraskamed.com/heart/anatomy.cfm
Accessed April 28, 2004.
2. Anterior View of the Heart. Available at: http://www.nebraskamed.com/heart/anatomy.cfm
Accessed April 28, 2004.
3. Echo and Patient. Available at: http://www.heartandvascularinstituteofnj.com/CardiacCare/echo.htm
Accessed April 28, 2004.
4. Left Ventricular Thrombus.
Available at: http://www.medical.siemens.com/webapp/wcs/stores/servlet/PSProductImageDisplay?storeI d=10001&langId=-1&catalogId=- 1&productId=17968&parentName=ACUSON%20Aspen%20Echo&catTree=100001,12805, 12761*2617418120
Accessed April 28, 2004.
5. Pericardial Effusion. Available at: http://www2.umdnj.edu/~shindler/pericardium.html
Accessed April 28, 2004.
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