Page 320 - Cardiac Nursing

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296 P A R T III / Assessment of Heart Disease
to move or conduct energy into heat, either of which can cause se- There are several different types of iodinated IV contrast agents.
rious bodily injury. Therefore, all patients undergoing an MRI Many of the contrast agents differ by the osmolality of the prepa-
should first undergo a formalized written screening process by a ration that is injected: hyper-, hypo, iso-osmolal in comparison to
knowledgeable provider, nurse, or certified technician. Resources human serum osmolality.
are available regarding the safety of various external and internal
objects. One such Web site is http://www.mrisafety.com. 47 Diagnostic Indications for CT
While most patients tolerate MRI procedures well, some may
experience claustrophobia and require sedation. Positioning a Cardiovascular CT is most commonly used for anatomical imag-
family member or friend in the same room as the patient may re- ing 49–51 (Table 14-1). It provides excellent descriptions of vascu-
duce anxiety. The family member or friend must also undergo lar structures (arteries and veins), chambers within the heart, and
MRI prescreening because they also are exposed to the external the pericardium. 52 CT is commonly used for the localization and
magnet. During imaging, loud noises are typically emitted from characterization of cardiac masses and tumors. Although myocar-
the magnet, so ear protection is required for safety. In order to dial perfusion and ventricular and valvular function can be evalu-
minimize artifact, patients should not move inside of the MRI ated by CT, 53 CT is not routinely done as the first test of choice
and may be asked to hold their breath for short periods. Blood for these functions.
pressure, heart rate, respirations, pulse oximetry, and an ECG CT is playing an increasingly important clinical role in the vi-
rhythm strip are monitored. sualization of coronary arteries. Calcification of coronary arteries
If IV contrast is anticipated, an IV will need to be placed. Most is indicative of coronary atherosclerosis; both the presence and
contrast injections are performed via a power injector. Specialized, quantity of coronary calcification strongly predict future cardiac
MRI-safe power injectors, IV pumps, and vital sign monitoring events. 54–58 While not all coronary plaque is calcified, coronary
equipment must be used since all of this equipment is usually calcium is a measure of overall plaque burden. Most patients pre-
within the same room as the magnet. senting with acute coronary syndromes will have detectable coro-
Gadolinium chelates are safe and generally well tolerated. nary calcification in amounts substantially higher than matched
59
However, minor adverse reactions include headache, nausea, ab- controls. Thus, studies to evaluate the utility of coronary CT in
dominal pain, flushing, or transient metallic taste. Recently, the the evaluation of patients presenting with acute chest pain syn-
Food and Drug Administration warned of an increased risk of dromes are underway.
nephrogenic systemic fibrosis in patients with acute or chronic re- A 64-detector CT with IV contrast injection (CT angiography)
nal insufficiency when exposed to gadolinium. This reaction may results in images in which the lumen of a coronary artery can be vi-
be delayed. Clinicians and patients should be informed of this risk sualized (Fig. 14-5). Reconstruction of the images is rendered and
when gadolinium use is anticipated; risks and benefits should be allow for visualization and detection of significant stenoses. 60
weighed on an individual basis. Although improvements in scanner technology have occurred in the
last 5 years, temporal resolution is still a limiting factor. Heart rate
and rhythm dramatically affect the resolution of images. At low,
61
regular rates resolution is reasonable. Therefore, most patients are
COMPUTED TOMOGRAPHY given -blockers prior to imaging. Patients who have rhythm dis-
turbances, such as atrial fibrillation, or contraindications to -
Modern day CT scanners are composed of a moving table and a blockers provide great challenges to coronary CT imaging.
circulating gantry surrounding the table, containing x-ray tubes CT provides an accurate anatomical description of the size, lo-
(x-ray source) and detector arrays. The table moves through the cation, and connection of all four cardiac chambers and valves.
center of the circulating gantry to expose the body to x-rays. Tubes Similar to MRI, CT is of increasing importance in patients with
within the gantry generate x-rays, and detector arrays measure the congenital heart disease. 62 CT with three-dimensional recon-
transmission of x-rays through the body. CT is performed in a structions provides accurate and detailed descriptions of bypass
360-degree radius around the patient, as the table is moving. graphs and large vessels including the aorta. CT with contrast in-
Complex mathematical and computer algorithms analyze the data jections clearly show LV and RV geometry, wall thickness, and
from the detector arrays assimilating timing and localization to function. 63
produce an image.
CT scanner technology has advanced significantly in recent Patient Preparation for CT
years. The latest 64-detector scanners are faster with improved Significant renal insufficiency is a contraindication to contrasted
spatial and temporal resolution. 48 It is this improvement in reso- cardiovascular CT imaging. Although there is not necessarily a
lution that has allowed for greater clinical use in cardiovascular specific glomerular filtration rate at which a study is contraindi-
imaging. cated, the risks of worsening renal insufficiency versus the benefits
As with MRI, compensation for respiratory and cardiovascular of the diagnostic information must be weighed on an individual
motion is required to yield diagnostic quality images. Respiratory basis by the provider ordering the procedure. Before any IV con-
motion is compensated for by asking patients to hold their breath. trast injection is given, the patient’s creatinine clearance should be
Cardiovascular motion is compensated for by acquiring the data calculated.
during the cardiac cycle by triggering acquisition to the patient’s To minimize the effects of contrast on renal function, admin-
ECG. ister IV fluids before and after exposure. The contrast is elimi-
Contrast is a pharmaceutical agent given during a CT study to nated from the body mainly via the kidney. Some research has
highlight vascular structures. Contrast agents increase the absorp- suggested that use of acetylcysteine for 24 hours before and after
tion of x-rays. Areas with contrast uptake are usually depicted as exposure to contrast minimizes the deleterious effects on renal
white (bright). For cardiovascular imaging, IV contrast is used. function.