Page 457 - Cardiac Nursing

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C HAPTER 1 9 / Exercise Testing 433
Myocardial Perfusion Imaging addition to measures of ejection fraction, measures of specific re-
gional wall motion can also be taken. 75,77,79
The most commonly used technique to evaluate myocardial per- The limitations of thallium, sestamibi single photon emission
fusion is the application of the radionuclide thallium-201. When computed tomography, and technetium imaging include their
thallium-201 is injected intravenously at maximal exercise, it is higher cost and exposure of the patient to ionizing radiation. Addi-
rapidly extracted from the blood by living cells in the my- tional equipment and personnel are also required for image acqui-
ocardium. Uptake of thallium-201 is similar to that of potassium sition and interpretation, including a nuclear technician to admin-
in living cells. Radiologic images are then taken, which reveal ar- ister the radioactive isotope and acquire the images and a physician
eas of absent, poor, or moderately poor uptake of thallium-201. trained in nuclear medicine to reconstruct and interpret the images.
When exercise images are compared with resting images, the dif-
ferences in uptake of thallium-201 indicate areas of decreased
blood flow. At the same time, if areas absent of thallium-201 up- Exercise Echocardiographic Imaging
take occur at rest, it can be assumed that this represents areas of Echocardiographic imaging of the heart is being increasingly used
myocardial scarring and not ischemia with exercise. This informa- during exercise and pharmacologic stress testing. This technique is
tion, along with the exercise test, can be more definitive in the frequently combined with an exercise electrocardiogram to in-
evaluation of the extent and localization of ischemia. crease the sensitivity and specificity of exercise testing. Typically, a
Perfusion imaging with technetium-99m sestamibi has be- two-dimensional image is taken at rest, and repeat images are ob-
come common. This imaging agent permits higher dosing with tained at peak exercise or immediately afterward. If images are
less radiation exposure than thallium, resulting in improved im- taken after exercise, they must be obtained within 1 to 2 minutes
ages that are sharper and have less artifact and attenuation. Ses- because abnormal wall motion begins to normalize after this
tamibi is the preferred imaging agent for obtaining tomographic point. Rest and stress images are compared side-by-side in a cine-
images of the heart using single photon emission computed to- loop display that is gated during systole from the QRS complex.
mography. Single photon emission computed tomography images Myocardial contractility normally increases with exercise, whereas
are obtained with a gamma camera, which rotates 180 degrees ischemia causes hypokinesis, akinesis, and dyskinesis of the af-
around the patient, stopping at preset angles to record the image. fected segments. Therefore, a test is considered positive if wall mo-
Cardiac images are then displayed in slices from three different tion abnormalities develop in previously normal territories with
axes to allow visualization of the heart in three dimensions. Thus, exercise or worsen in an already abnormal segment. 80
multiple myocardial segments can be viewed individually, without Some advantages of exercise echocardiography over nuclear
the overlap of segments that occurs with planar imaging. 77,79 As imaging include the absence of exposure to ionizing radiation and
with thallium-201 imaging, perfusion defects that are present dur- a shorter amount of time required for testing. Like standard exer-
ing exercise but not seen at rest suggest ischemia. Perfusion defects cise testing and radionuclide techniques, the diagnostic accuracy
that are present during exercise and persist at rest suggest previous of echocardiography depends primarily on the specific methodol-
MI or scar. In this manner, the extent and distribution of ischemic ogy used and the pretest probability of CAD in the subjects tested.
myocardium can be identified. The accuracy of echocardiographic testing also depends on ob-
Perfusion imaging of the coronary anatomy has been shown server experience. Reviews of studies published since the advent of
to be somewhat more sensitive and specific than the exercise elec- exercise echocardiography in the early 1980s suggest that the av-
trocardiogram for detecting CAD. An extensive review of the lit- erage sensitivity and specificity of this technique for detecting
erature suggested that the sensitivity and specificity of exercise coronary disease are both approximately 85%. 81 The limitations
thallium scintigraphy for detecting coronary disease were in the of exercise echocardiography include dependence on the operator
76
order of 84% and 87%, respectively. This modality also permits for obtaining adequate, timely images, and some variation exists
the localization of ischemia, which is not possible with ST-segment in image interpretation. In addition, as many as 20% of patients
depression on the electrocardiogram. This technique is especially have inadequate echocardiographic windows secondary to body
helpful in patients with equivocal exercise electrocardiograms, habitus or lung interference. 82
those using digoxin, or those with left bundle-branch block, in
whom the interpretation of electrocardiographic changes is more
problematic. 4,17 Pharmacologic Stress Techniques
It is advantageous to use pharmacologic stress techniques for pa-
Ventricular Function Studies tients who are unable to exercise on a treadmill or cycle ergometer
to an adequate level. These include patients who have orthopedic
Ventricular function is commonly evaluated with the use of the limitations, peripheral vascular disease, and chronic obstructive
radioisotope technetium-99m. This radioisotope is administered pulmonary disease or other limiting pulmonary diseases; elderly
as an intravenous bolus, and its transit through the ventricles is patients with low functional capacity; diabetic patients with severe
measured by special cameras. Technetium-99m is also used to la- neuropathy; and patients with neuromuscular conditions. For
bel red blood cells for equilibrium blood pool studies. Both of these patients, pharmacologic methods can be extremely useful for
these methods have been used extensively in the evaluation of left evaluating coronary blood flow and myocardial function. Pharma-
and right ventricular function after acute MI and other cardiac cologic stress is a relatively new area with important applications
events. This technique can be performed at rest as well as at max- for echocardiographic and nuclear techniques, but only limited
imal exercise. When performed at maximal exercise, it has the ca- data are available directly comparing pharmacologic stress testing
pability of determining decreased ventricular function compared with standard exercise testing.
with rest measures. This can help in the diagnosis of ischemic ab- Two types of pharmacologic stress agents have been used: those
normalities as well as exercise-induced ventricular dysfunction. In that increase coronary blood flow through coronary vasodilation