Electrocardiography graphically demonstrates this electrical activity. The elements of an electrocardiogram (ECG) include the P wave, PR interval, QRS complex, T wave, and QT duration (Fig. 4-4). The P wave is the graphic display of the spread of the electrical impulse from the atria. The PR interval shows the amount of time required for the electrical impulse to travel from the SA node to the ventricular muscle fibers. The spread of the electrical impulse through the ventricles is displayed by the QRS complex, and the period in which the ventricles recover from the spent electrical impulse is graphically displayed by the T wave. The QT duration represents the total time from ventricular depolarization (QRS) to ventricular repolarization (T).

FIGURE 4-4 Electrocardiogram demonstrating P wave, QRS wave, and T wave.

Circulatory Vessels

Arteries are blood vessels that carry blood away from the heart and are generally named for their location or the organ they supply (e.g., splenic artery). They are composed of three layers. The outermost layer is termed the adventitia, the middle layer is the media, and the innermost layer is the intima. The internal, tubular structure of the vessel is termed the lumen. Veins are blood vessels that carry blood to the heart. They are composed of the same three layers; however, venous walls are thinner than arterial walls, and veins contain valves at set intervals to help with blood return to the heart. Capillaries are microscopic vessels that connect arteries and veins (Fig. 4-5). They are responsible for the exchange of substances necessary for nutrient and waste transport.

FIGURE 4-5 General systemic circulation.

Imaging Considerations

Radiography

Several imaging modalities play an important role in the evaluation of the cardiovascular system and the management of cardiovascular disease. Chest radiography provides information about heart shape and size. Chest radiography is also excellent for demonstrating the great vessels and vascular changes within the lung fields. Radiographers do need to be aware that many factors may affect the cardiac image.

Factors that the radiographer can control include patient posture, degree of inspiration, correct positioning, geometric factors, and exposure technique selection. Whenever possible, chest radiographs should be taken with the patient in the erect position. If a patient is semirecumbent or recumbent, the heart appears to be enlarged because abdominal organs push the diaphragm and the heart up into the thoracic cavity. It is important to identify cases in which the patient is not able to assume the erect position to aid the physician in diagnosis and interpretation.

Chest radiographs obtained without a good inspiration also distort heart shape and size (Figs. 4-6 and 4-7). Keep in mind that at least 10 posterior ribs should be visible within the lung fields on a chest radiograph taken with a good inspiration. The sternoclavicular joints should be an equal distance from the spine, and the scapulae should be rolled forward out of the lung fields on a well-positioned PA chest radiograph. To position a patient for a lateral chest radiograph, the arms and shoulders should be placed above the patient’s head to ensure that they are above the apices.

FIGURE 4-6 Posteroanterior chest radiograph with adequate level of inspiration.

Geometric factors affecting heart shape and size include source-to-image receptor distance (SID) and object-to-image receptor distance (OID). Conventional chest radiographs are generally obtained using a 72-inch SID to decrease magnification of the heart to an approximate factor of 10%. Again, it is important to document variations in SID to aid in the proper diagnosis of heart disorders. Because the heart is fairly anterior in the mediastinum, it is preferable to obtain PA chest images, whenever possible. This places the heart closest to the image receptor, allowing for the smallest OID. Positioning the patient for a PA projection also helps decrease magnification of the cardiac silhouette. A third geometric factor that is frequently overlooked is the anode-heel effect. Radiographers can use this phenomenon to their advantage by placing the anode over the apical region and the cathode toward the base of the lungs, when possible, thus distributing the radiographic density more evenly throughout the chest radiograph.

Adequate penetration of the mediastinal structure is also critical in chest radiography and requires the use of a relatively high kilovoltage. A minimum of 100 kilovolts peak (kVp) should be used. Vascular markings within the chest help the physician assess ventricular function. The pulmonary vessels also provide information about pulmonary artery pressure. Dilatation of these vessels often indicates problems with the right ventricle. Exposure times of one tenth of a second or less should be used, whenever possible, to decrease involuntary cardiac motion. It has been documented that heart motion may increase the size of the cardiac shadow. The heart may look larger if the radiograph is exposed during diastole.

In most institutions, chest radiography is the most commonly performed procedure, and radiographers all too often underestimate the importance of these basic radiographic principles. Well-positioned diagnostic chest radiographs are crucial in the diagnosis and treatment of cardiovascular disorders. In a normal adult, the transverse diameter of the cardiac shadow should be less than half the transverse diameter of the thorax on a PA erect chest radiograph. An enlarged heart is termed cardiomegaly (Fig. 4-8), which is indicative of many cardiovascular disorders and is a nonspecific finding.

FIGURE 4-8 Posteroanterior chest radiograph demonstrating cardiomegaly secondary to congestive heart failure.

Factors affecting cardiac shape and size that are not under the technologist’s control include patient body habitus, bony thorax abnormalities, and pathologic conditions such as pneumothorax or pulmonary emphysema. Bone abnormalities of special concern include scoliosis and pectus excavatum. Individuals with pectus excavatum present a funnel-shaped depression of the sternum. The abnormal placement of the xiphoid causes displacement of the heart to the left and distortion of the cardiac shadow. Vascular lines and tubes are discussed in Chapter 3 of this text.

Echocardiography

Echocardiography encompasses a group of noninvasive sonographic (ultrasound) procedures that can provide detailed information about heart anatomy, function, and vessel patency (Fig. 4-9). Sonographic imaging may be performed using M-mode, two-dimensional (2-D) imaging, spectral Doppler, color Doppler, or stress echocardiography.

FIGURE 4-9 A, Echocardiographic visualization of heart anatomy of a 47-year-old man as seen in a parasternal sagittal view. B, Coronal view of the same heart demonstrated by echocardiography.

M-mode echocardiography uses a stationary ultrasound beam to provide an examination of the atria, ventricles, heart valves, and aortic root, allowing evaluation of left ventricular function. The “M” refers to motion, as this technique allows for the recording of the rate of motion and the amplitude of moving objects. This technique has the advantage of being able to demonstrate subtle motion of the cardiac structures. M-mode cardiac sonography is also used to measure the thickness of the ventricular walls; however, it has largely been replaced by 2-D echocardiography. Use of 2-D imaging allows for spatially correct, real-time imaging of the heart. It provides multiple tomographic projections of the heart and great vessels in a cinelike (dynamic imaging) presentation. In addition, it is an excellent modality for visualizing the ascending and abdominal aorta in cases of suspected aneurysm. Both M-mode and 2-D images are obtained by placing the transducer over the thorax at the sternal borders, at the cardiac apex, between the ribs, or at the suprasternal notch. Smaller transducers have been developed to allow for transesophageal echocardiography (TEE), in which the patient swallows a mobile, flexible probe containing the transducer. With TEE, the heart’s structure can be readily visualized without interference from such structures as skin, the rib cage, and chest muscles. It is especially helpful in imaging the aortic arch and aortic root. Smaller transducers are placed on intravascular catheters to assess vessel anatomy and blood flow.

Stress echocardiography combines an exercise test with an echocardiogram to check the heart’s contraction ability and its pumping efficiency. If exercise is not possible, a drug, dobutamine, may be used to increase cardiac output to assess how well the heart pumps during infusion.

Doppler sonography is an adjunct, noninvasive procedure used to study the peripheral vasculature. It has been a mainstay of vascular imaging since the 1970s and is used to determine the direction and velocity, as well as the presence or absence, of blood flow in both arteries and veins. The Doppler Effect is the principle that the sound coming toward you has a higher pitch than the sound going away from you. The return of pulsed ultrasound allows calculation of the shift in the direction of blood flow and creates a spectral display from which velocity is calculated (Fig. 4-10). The spectral signal is displayed on a strip chart or videotape. With Doppler sonography the flow of blood is not affected until any obstruction present is at least 60% complete. The percentage of stenosis present dictates the treatment of vascular disease, and usually this consists of surgery (e.g., endarterectomy). Such vascular imaging is said to be duplex in that it helps reveal physiologic characteristics, and the imaging component defines anatomy (e.g., plaque morphology). The most common conditions imaged by 2-D Doppler sonography are carotid stenosis (significantly reducing carotid angiography) (Figs. 4-11 and 4-12), lower extremity arterial stenosis, and deep venous thrombosis (Figs. 4-13 and 4-14), largely supplanting traditional venography. Blood flow may be encoded with color to differentiate blood flowing toward the transducer (red) from blood flowing away from the transducer (blue); this is termed color Doppler echocardiography.

FIGURE 4-10 Doppler effect with normal arterial flow in the internal carotid artery depicted above a graph of the arterial flow.

FIGURE 4-12 Doppler sonogram of a normal carotid bifurcation from the common carotid into the internal and external carotid arteries.

Nuclear Cardiology

Nuclear medicine procedures used in the assessment of cardiovascular disease include myocardial perfusion scans, gated cardiac blood pool scans, and positron emission tomography (PET). They are useful in assessing coronary artery disease (CAD), congenital heart disease, and cardiomyopathy.

A myocardial perfusion scan is the most widely used procedure in nuclear cardiology. It may be performed on patients with chest pain of an unknown origin, to evaluate coronary artery stenosis, and as a follow-up to bypass surgery, angioplasty, or thrombolysis. It is especially useful in detecting regions of myocardial ischemia and scarring (Figs. 4-15 and 4-16). In this study, a radionuclide, usually radioactive technetium sestamibi or thallium, is injected through a vein. It concentrates in the areas of the heart that have the best blood flow. Those areas lacking blood flow demonstrate filling defects, visualized between images taken at rest and under stress. Stress may be induced by exercise on a treadmill or by the use of pharmaceuticals such as regadenoson. Myocardial perfusion scanning is performed using single photon emission computed tomography (SPECT), allowing the camera to rotate around the patient to obtain tomographic images of the heart parallel to the short and long axes of the left ventricle. SPECT myocardial perfusion scans can detect significant CAD in 90% of patients presenting with CAD. PET may also be used for imaging myocardial perfusion employing a variety of positron perfusion and metabolic agents. A PET unit is more sensitive than conventional nuclear medicine cameras, and spatial resolution is superior to that of conventional cameras. It is highly accurate for detecting CAD that interferes with blood flow to the heart muscle and can identify injured but viable heart muscle. PET can also provide quantitative data about the distribution of the radionuclide within the body.