Contrast echocardiography is used to delineate structures not readily seen (superior and inferior vena cava, descending aorta, right ventricular outflow tract, pulmonary arteries) as well as to evaluate intracardiac shunts, regurgitant lesions, and complex congenital heart problems. Contrast echocardiography uses microbubbles by agitating approximately 8 cc of saline between two 10-ml syringes connected to a three-way stopcock and an intravenous line. The agitated saline is injected with extreme force into the intravenous line while recording in the four-chamber view or focusing on the suspected site. Apical or subcostal four-chamber views allow visualization of a small number of microbubbles crossing a right-to-left shunt as well as a negative jet of left-to-right flow that causes a defect in the otherwise bubble-filled right chamber.
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If a large shunt is suspected, the number of injections and amount of contrast should be limited. An appropriate routine is first to inject 3 cc, record and evaluate; if a large shunt is not present, 8-10 cc are then injected. If the study is negative at this point, reinjection, with the patient coughing when the right atrium is filled, increases right-sided pressure and allows even the smallest shunt to be identified. This technique is extremely sensitive and specific for shunt diagnosis, which can be confirmed with only a few microbubbles; it is even more sensitive than oximetry and dye dilation and can detect shunts as small as 3%. It is also sensitive enough to evaluate patency of the foramen ovale or surgical repairs of shunts. The noninvasive laboratory at our institution uses contrast in the echocardiographic evaluation of all patients with stroke or transient ischemic attacks (TIAs).
Normal contrast echocardiography. Four-chamber view shows complete filling of right atrium and ventricle (to left) with microbubbles and no bubbles in left atrium or ventricle (to right).
How do echocardiography and Doppler imaging help in the evaluation of the patient with suspected ischemic heart disease?
Echocardiography is useful in evaluating possible ischemic heart disease, chest pain syndromes, and left ventricular function and in establishing risk stratification and complications of acute myocardial infarction. Assessment of regional wall motion and absence of systolic thickening of the myocardium may implicate coronary artery disease. Serial studies can be evaluated in side-by-side formats to compare wall motion abnormalities or to evaluate myocardial remodeling. Also, exercise echocardiography adds sensitivity and specificity to routine stress electrocardiography and has been advocated as especially useful in diagnosing coronary artery disease in women, with a sensitivity and specificity comparable to radionuclear stress studies. Whereas only 50% of ECGs are diagnostic for acute myocardial infarction, the detection of regional wall motion abnormalities is much more sensitive (though less specific). A negative echocardiogram during chest pain predicts a very low risk of ischemia. Another advantage of using echocardiography with chest pain syndromes is the additional diagnostic information about other causes of chest pain, such as aortic stenosis, aortic dissection, mitral valve prolapse, pericardial effusions, or hypertrophic cardiomyopathy.