Cardiac Diagnostic Testing

Cardiovascular disease affects more than 71 million Americans and will cost an estimated $403 billion in 2006.1 Each year about 700 000 Americans experience their first myocardial infarction (MI), 500 000 have recurrent MI, and 175 000 have a silent MI.2 With increasing life spans and a growing population, more than 1 million Americans with a congenital heart defect will require an increasing number of cardiac diagnostics.1 From 1979 to 2003, the number of inpatient cardiovascular operations and procedures increased 470% to an estimated 6.8 million in 2003.2 Accurate assessment of cardiovascular disease and risk is crucial to improving patient outcomes. Nurses caring for patients being tested for these disorders must be familiar with imaging techniques. This article reviews developments in cardiovascular diagnostic imaging, with an emphasis on cardiovascular magnetic resonance imaging (MRI) and echocardiography.Invasive coronary angiography has been the gold standard for evaluating normal and abnormal coronary anatomy since its introduction in the 1960s. Conventional coronary angiography (CCA) is a commonly performed procedure and involves the selective injection of a radiographic contrast agent into the coronary arteries under fluoroscopic guidance. In addition to detecting possible coronary artery stenosis, coronary angiography evaluates coronary flow, myocardial blush, presence of congenital coronary abnormalities, the patency of coronary grafts, and previous percutaneous coronary interventions (PCIs).Although CCA remains the best choice for coronary evaluation, limitations and risks exist. The most significant procedural risks include bleeding, vessel dissection, and infection, cerebrovascular accident or MI from plaque embolization, radiation exposure, and the use of contrast media, which can cause renal insufficiency and anaphylaxis.The use of coronary angiography has increased significantly over the last 2 decades; however, the procedure can still be challenging. A typical coronary angiography procedure can cost up to $17 000, which is 5 times as much as some noninvasive modalities. CCA requires 3 to 4 staff members, including a registered nurse, a radiation technologist, and 1 to 2 other staff combinations that may include a cardiovascular technologist, radiation technologist, or registered nurse.Another significant factor is the frequency of occurrence of normal angiograms. Approximately 30% of all coronary angiograms are negative. In women, 40% of CCAs reveal no significant disease (defined as a lesion that occupies >50% of the coronary lumen). Only one third of angiograms lead to coronary intervention.Currently, the best approach to coronary heart disease is prevention and early diagnosis. Each year, more than 1 million people in the United States experience an acute MI: this number includes 150 000 cases of sudden death. Steps toward developing accessible, less-expensive noninvasive diagnostic tools have gained significant momentum. Promising alternative coronary imaging technologies include multislice spiral computed tomography, electron beam angiography, magnetic resonance angiography, and intravascular ultrasound.Over the last decade, computed tomography (CT) has been an excellent noninvasive method for the visualization of the thorax. Since the first multislice CT was introduced in 1990, faster 4-slice, 16-slice, and 64-slice scanners have been developed that reduce scanning time and motion artifact related to cardiac and respiratory movement. CT techniques currently offer high speed that is integrated with electrocardiogram (ECG)-timed images and high spatial resolution.Multislice CT angiography is performed on an outpatient basis and involves the administration of a nonionic contrast agent. Oral and possibly intravenous (IV) beta-blockers (to obtain a heart rate <60 bpm) are administered to decrease the heart rate for optimal imaging during mid-diastole when the heart is between contractions. New ECG gating technology, which restricts the standard reconstruction windows at middiastole, can reduce radiation exposure by 40% to 50%.Multislice cardiac CT angiography involves the use of scanners to pass an x-ray beam through a patient’s body during a 360° rotation while a detector records images of the beam. Each profile is then reconstructed by computer into 2-dimensional images, which are then reassembled into detailed multidimensional anatomical views. Volume rendering, another recently introduced process, can produce computer-enhanced 3-dimensional multislice CT views (Figure 1).3 New multislice CT technology is highly accurate in the detection of lesions obstructing 75% of the coronary lumen with sensitivity, specificity and positive and negative predictive values of greater than 90% in patients without known coronary artery disease (CAD). 3Introduction of the faster, more sophisticated 64-slice CT device in 2005 allowed quality imaging in a patient who has difficulty with breath-holding. It provides highly accurate and detailed images of the entire coronary tree, atheromatous plaques, and calcification. The widespread availability of the 64-slice scanners has greatly advanced the role of CT angiography in clinical practice. It is efficient in evaluating repeat coronary artery bypass graft (CABG) patients (prior to sternotomy) for graft adhesion, patients on blood thinners, and those with possible PCI restenosis. Other multislice CT applications include guidance prior to biventricular pacemaker implantation in congestive heart failure patients, atrial ablation, and procedures involving myxomas and aneurysms (Table 1).3Limitations and risks of multislice CT include higher radiation exposure than CCA and imaging difficulties for patients with any of the following: body mass index >40 kg/m2, heart rate >65 bpm, distal stents <3 mm in diameter, distal CABG anastomoses containing metal clips, or heavily calcified areas overlying a targeted vessel lumen.The incidence of CAD has been directly correlated with the presence of calcific deposits in patients. From the standpoint of prevention, the use of cardiac CT for calcium scoring is of significant benefit in identifying patients’ probability for CAD. Detection of calcium and the amount of vascular calcification is calculated as the total calcium burden. Elevated results that are indicative of excessive plaque burden will ultimately lead to nuclear stress testing and/or CCA. If the total calcium burden test result is negative, then the likelihood of CAD is extremely low.The choices of imaging methods vary with physician training, available technology, and patient preferences. For example, in an asymptomatic patient with diabetes and a family history of CAD and a calcium score suggesting a moderate plaque burden, most cardiologists would perform a nuclear stress test and, if it were positive, subsequent CCA. For a patient with chest pain of unknown origin and a negative nuclear stress but no cardiovascular (CV) risk factors, multislice CT is an efficient, minimally invasive modality (Figure 2).4There are infrequent instances where a cardiologist may employ the assistance of a radiologist when evaluating patients with continuous anginal symptoms but no recognized coronary blockages via CCA and/or intravascular ultrasound (IVUS). Using this modality gives the physician additional images of the coronary arteries and may reconfirm CCA or IVUS findings.Feuchter and colleagues5 used cardiac CT as a noninvasive imaging modality for serial monitoring of coronary artery plaque in patients on statin therapy. One year after statin therapy, overall there was a significant regression of plaque area and an increase in mean CT densities (suggesting an increase in mean fibrotic and reduction in lipid components of plaque). Thus, the combined use of cardiac CT angiography and IVUS-generated calcium scores of coronary lesions can be useful for assessing atherosclerotic risk.Until CT angiography becomes more reliable in detecting significant disease, the use of invasive coronary angiography is preferable in high-risk patients.Echocardiography, also known as cardiac ultrasound, is a noninvasive diagnostic technology that uses high- frequency sound to penetrate non–air-containing structures and construct an image with the sonic reflections returning to the skin surface. This technically demanding technology offers good diagnostic accuracy and provides images with functional and anatomical information. Types of echocardiographic techniques include motion (M) mode, pulsed wave and continuous wave tissue Doppler, real time 2- and 3-dimensional IVUS, and intracardiac echocardiography (ICE).6 In addition, techniques include transthoracic echocardiography (TTE) and transesophageal echocardiography (TEE).Standard pulsed-wave Doppler provides an image of the temporal distribution of blood flow velocities in a specific location. M mode Doppler provides images with spatio-temporal distribution of these velocities across a vertical line as if multiple simultaneous pulse Doppler tracings were being obtained.7 With M mode TTE or TEE, a single ultrasound beam is directed from the anterior chest wall or esophagus, respectively, into the heart, allowing the technician to obtain images of the heart structures along that single beam at different angles. Returning signals are recorded onto a moving strip of paper that displays cardiac structures and their unique motion patterns, as well as possible alterations that identify pathology and its severity.8 Generally, M mode images are simultaneously generated with 2-dimensional TEE. For example, M mode 2-dimensional TTE can reveal decreased contractility in patients with nonischemic and ischemic cardiomyopathy compared to individuals with normal heart function. During exercise stress, M mode echocardiography reveals a left ventricular wall motion response to increased stress in a healthy person and in patients with cardiomyopathies. Areas with scar and tissue damage are associated with reduction in wall motion and reduction in contractility.Two-dimensional echocardiography uses primary ultrasound imaging methodology to produce real-time tomographs by sweeping an ultrasound beam across cardiac structures. Two-dimensional echocardiography can provide quality 2- and 4-chamber apical views of the heart.9 Doppler echocardiography uses information about the velocity and direction of moving red blood cells within the cardiac chambers to give an indirect measurement of the velocity and the direction of blood flow. The Doppler technology allows excellent visualization of the early dumping of blood from the left atrium to the left ventricle following opening of the mitral valve, the so-called E wave. Doppler also distinguishes the E wave from the A wave of atrial contraction, during which the last 10% to 30% of blood flows to the ventricles during late diastole after atrial contraction. Using both the M mode and the pulsed Doppler for velocity and direction of blood flow, both cardiac anatomy and function can be evaluated. For example, diastolic dysfunction due to delayed relaxation can be distinguished from restrictive filling abnormality in which the pulsed Doppler reveals changes in blood flow consistent with stiff ventricles.6An important limitation to 2-dimensional TTE is that a complete examination is not always possible. This limitation occurs in patients with hyperinflated lungs or chronic obstructive pulmonary disease, obesity (fat conducts ultrasound poorly), or with patients in a critical care setting in which ventilation and dressings limit views or projections. Geometric assumptions are not reliable when cardiac chambers are distorted or enough views cannot be obtained.6Three-dimensional echocardiography is a newer technology that uses a modified ultrasound probe that tracks in 3-dimensional space by means of an electromagnetic field. With 3-dimensional echocardiography, images are reconstructed off-line to create 3-dimensional data sets that closely resemble the true anatomy of the heart.10 Real-time 3-dimensional echocardiography obtains data via multiple modes: real-time, full volume, and 3-dimensional color.11 The advantage of this technology is shown by its ability to generate a 3-dimensional reconstructed view of a flail leaflet in a damaged tricuspid valve.12Most commonly, echocardiography is used to determine ventricular ejection fraction in patients suspected of having heart failure (Table 2). Echocardiography is applied in diagnosing and managing heart failure because it can predict structural abnormalities including mechanisms of regurgitation and stenoses and response to costly cardiac resynchronization therapy.13 Additionally, echocardiography is used to optimize catheter placement after cardiac resynchronization therapy leads are inserted.5What is the rationale for using TEE instead of TTE? In TEE, the echocardiographic catheter is placed down the esophagus, and, thus, within an airless ultrasonic window posterior to the heart, directly behind the left atrium (Figure 3). TEE is preferred when TTE provides an inadequate image, as with some ventilated and intubated critical care patients. In cardiac surgery, TEE is popular for intraoperative monitoring of left ventricular size, function, and wall motion. Other situations in which TEE can be advantageous include examination of aortic pathology, observation of left atrial thrombi (in atrial fibrillation [AF]) or tumors, assessment of prosthetic valve regurgitation and endocarditis, and native valve disruption, vegetation, or ring abscesses.IVUS is based on the property of piezoelectric crystalline material’s ability to expand and contract to produce sound waves when electrically excited. In IVUS, a catheter similar to a left heart catheter but with an ultrasound transducer on its tip is used to image different structures within the layers of an artery. The target segment of an artery is interrogated by obtaining multiple cross-sectional views of the vessel, including a proximal and distal section for reference. It is used to assess the severity as well as the morphology of lesions in patients with CAD. Thus, IVUS is effective in identifying pathophysiology of coronary atherosclerosis and providing both direct and derived measurements of arterial disease. IVUS can be used to obtain stent measurements and assess optimal stent deployment and coronary artery vascular remodeling.4,14 IVUS-obtained cross-sectional segments of a coronary artery may reveal vulnerable plaque that refers to a plaque with a large plaque burden and a normal coronary artery lumen size, because the artery has undergone compensatory enlargement or remodeling.15 A coronary angiogram of a patient with large vulnerable coronary artery plaque may appear normal because the lumen appears normal in size throughout the length of the artery. IVUS allows the direct measurement of the arterial lumen area at the lesion. Measurements of the maximum and minimum atheroma thickness, the lesion maximum and minimum luminal diameter, remodeling index, lesion, and atheroma eccentricity can be obtained. In addition, IVUS can find arcs of calcium and determine coronary artery calcium burden.4,14The clinical uses of IVUS include determining lumen measurements and lumen characteristics including plaque versus calcium burden, extent of vessel size, lesion length, dissections,11 and assessment of stent restenosis after PCI.14,16The IVUS procedure remains largely a research tool because it is costly and time-consuming. Thus, its widespread clinical use is not justifiable until further studies suggest it provides improved clinical outcomes in a large number of patients.17 Still, despite its drawbacks, niche applications for IVUS exist. One is in characterizing left main coronary stenosis, which is notoriously difficult to characterize with traditional coronary angiography. 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