How does a Chemical Stress (Isotope Stress) Test work?
A chemical or pharmacological stress test combines an intravenous medication with an imaging technique (isotope imaging or echocardiography) to evaluate the LV. In these cases, the medication serves the purpose of increasing the heart load instead of using exercise. Stress causes normal coronary arteries to dilate, while the blood flow in a blocked coronary artery is reduced. This reduced blood flow may decrease the movement of the affected wall (as seen by echo), or have reduced isotope uptake in a nuclear scan. Agents that are commonly used in pharmacologic stress testing include dipyridamole, dobutamine and Adenosine (Trade name).
Indications of Chemical Stress Test ?
Treadmill stress testing is the test of choice when a patient is able to exercise because of the physiologic effect that exercise has on the blood pressure and heart rate. It also helps give the physician an idea about the patient's exercise tolerance and whether or not exertion has any adverse effects on the patient's symptoms or irregular heart beats. Additionally, one does not have to contend with any potential side-effects of chemical stress, even if they are usually minor.
However, exercise may not be possible because of physical limitations like back trouble, joint disease, marked fatigue, unsteady gait, prior stroke, dizziness, shortness of breath, etc. In such cases, chemical stress testing is employed. In other words, pharmacologic or chemical stress test is performed in situations where patients are unable to perform more than moderate exercise due to severe arthritis, prior injury, reduced exercise tolerance (as a result of debilitating illnesses, etc.), or in patients who are unable to increase the heart rate (as in some with heart pacemakers or in the setting of certain diseases that keep the heart from speeding up).
Preparing for an Isotope Stress Test : The following recommendations are "generic" for all types of cardiac stress tests:
* Do not eat or drink for three hours prior to the procedure. This reduces the likelihood of nausea that may accompany strenuous exercise after a heavy meal. Diabetics, particularly those who use insulin, will need special instructions from the physician's office.
* Caution about asthma: The use of dipyridamole is generally avoided in patients with asthma. Please be sure to notify your physician if you have a history of asthma, bronchitis or emphysema.
* Specific heart medicines may need to be stopped one or two days prior to the test. Such instructions are generally provided when the test is scheduled.
* Wear comfortable clothing and shoes that are suitable for exercise.
* An explanation of the test is provided and the patient is asked to sign a consent form.
Is it safe?
The patient is exposed to a very small amount of radiation and the risk is minimal, if any. The risk of the chemical stress portion of the test is very small and similar to what you would expect from any strenuous form of exercise (jogging in your neighborhood, running up a flight of stairs, etc.). As noted earlier, experienced medical staff is in attendance to manage the rare complications like sustained irregular heart beats, unrelieved chest pain or even a heart attack. In such cases, the patient is better off having the problem in the presence of experienced staff, rather than have it happen when they are exercising alone. Also, the stress medicine like Dobutamine can be immediately stopped if there are problems, The effects of dipyridamole (which can occasionally cause nausea or a headache can be reversed by aminophylline (an anti-asthma medication). Please also see the caution about asthma under the "Preparing..." section.