Overview of Cardiac Stress Imaging
Cardiac stress imaging is a type of test which helps doctors check the flow of blood to your heart, how well your heart and its valves are working, whether any part of your heart muscle is damaged, and how well you’re able to exercise. Stress to the heart can be caused by a special medication or physical exercise. Different types of heart imaging can be used for this test, including echocardiography (a type of ultrasound), as well as types of nuclear imaging procedures called single-photon emission computed tomography (SPECT) or positron emission tomography (PET). There’s also a simpler type of test called an electrocardiogram (EKG) exercise stress test, which can provide enough information about the heart for some patients.
However, more detailed imaging tests often show more information about the heart and are therefore usually preferred. Each test and method of stressing the heart has its own pros and cons, and the most suitable option will depend on each individual patient’s situation. The certain diagnosis of coronary artery disease (CAD), which is a condition causing blockages in the heart’s blood vessels, is found when a test shows that at least one of the blood vessels feeding the heart muscle is narrowed by at least 50 to 70%. A stress test isn’t enough to diagnose CAD by itself; instead, it can suggest that a blockage might be present. The results of a stress test can help your doctor estimate the chances of you having CAD, and decide whether further invasive testing is needed. The results of stress tests can also be used to predict how serious the condition could be, and to inform the best treatment options.
Your doctor might want to perform a stress test if you have symptoms like chest pain or newly diagnosed heart failure that might suggest CAD. There are different types of chest pain, which can be classified as typical angina, atypical angina, or non-cardiac chest pain. This classification is based on characteristics such as whether the pain is located at the center of the chest, worsens with exertion or stress, and is relieved by rest or a medication called nitroglycerine. Typical angina pain has all three characteristics, and atypical angina has two. If the pain has none or only one of these, it’s classified as non-anginal or non-cardiac chest pain.
Factors like age, gender, and other health conditions can be used to estimate the likelihood that a patient has CAD before the test is performed, and this estimation could be low (less than 10%), intermediate (between 10 and 90%), or high (more than 90%). Stress testing is usually most useful for patients in the intermediate-risk category, because the results of the test have the greatest potential to change the estimated likelihood of having CAD. For patients at high risk, the results of a stress test probably won’t change the likelihood of having CAD significantly, and these individuals are often referred directly for a more detailed imaging test called an angiography without having a stress test first.
Anatomy and Physiology of Cardiac Stress Imaging
Coronary angiography, a type of heart test, is commonly used to diagnose coronary artery disease (CAD). This disease happens when the blood vessels that supply the heart (coronary arteries) become narrow due to a buildup of fatty material, also known as atherosclerosis. The test allows doctors to see these blood vessels and identify any narrowing. If there’s a 50% to 70% or greater blockage in the blood vessel, it suggests that CAD is present.
There are also other ways to diagnose CAD that are less invasive, but these don’t provide a detailed image of the blood vessels. Instead, these tests, such as stress imaging (a test where doctors study how your heart and blood vessels respond to exertion), show the effect of the narrowing on the heart muscle. For instance, if part of the heart’s wall isn’t moving correctly when stressed (for example during exercise), or if there’s a problem with blood flow to particular areas of the heart, it points to the blood vessel supplying that area being blocked.
Why do People Need Cardiac Stress Imaging
The American College of Cardiology (ACC) and the American Heart Association (AHA) have issued guidelines on when to use exercise stress testing. This test helps detect coronary artery disease (CAD) – a condition where your heart’s main blood vessels get clogged or damaged.
According to these guidelines, exercise stress testing is strongly recommended (Class I) for:
1. People with an intermediate likelihood of having CAD based on factors like gender, age, and symptoms. This means people who might have CAD, based on these factors.
2. Patients already diagnosed with CAD who reported changes in their condition.
3. Patients who have experienced unstable angina (sudden chest pain suggesting heart disease) but lack symptoms for 8-12 hours (low risk) or 2-3 days (intermediate risk).
4. Patients who had a heart attack for their condition assessment, to recommend physical activities and evaluate medication performance.
The guidelines also suggest (Class IIa) the test for:
1. Patients who’ve had unstable angina, no initial signs of heart damage, and their electrocardiograms (EKGs, recording the heart’s electrical activity) show no changes.
2. Assessing known or suspected exercise-induced heart rhythm problems.
3. Assessing condition, recommending physical activities and evaluating medication effectiveness in patients released after the heart blood vessels have been reopened.
Further, the guidelines also think it could be reasonable (Class IIb) to use the test for:
1. Patients with a high or low likelihood of CAD based on age, symptoms, and gender.
2. Evaluating exercise capacity in patients with heart valve disease (excluding severe aortic stenosis – a valve condition which restricts blood flow from your heart).
The ACC and the American Society of Nuclear Cardiology suggest that stress nuclear MPI testing, a type of cardiovascular stress test, is appropriate for patients with various degrees of likelihood for CAD. The ACC and the American Society of Echocardiography also provide recommendations on when to use stress echocardiography, a test that uses ultrasound imaging to show how well your heart muscle is working to pump blood to your body.
Choosing the best stress test method and imaging technique often depends on the patient’s likelihood of having CAD, their ability to exercise, and the interpretability of their EKG. In some cases, imaging can improve the accuracy of identifying CAD and can generally be done with exercise stress. However, pharmacological agents can also be used to induce stress if a patient cannot exercise due to issues like arthritis or peripheral neuropathy. The choice of these methods often depends on a variety of conditions, including a patient’s gender, the accuracy of the tests, and the specific information needed for the patient’s case.
When a Person Should Avoid Cardiac Stress Imaging
There are certain situations where a patient should not undergo cardiac stress imaging, which is a test used to check the condition of the heart. These situations include:
1. If the patient has recently experienced a heart attack (also known as an “acute myocardial infarction”) particularly if it has happened within the last 2 days. It’s also not recommended for those suffering from inflammation in the heart due to infections (endocarditis), inflammation around the heart (pericarditis), or if there is tear in the large blood vessel branching off the heart (aortic dissection).
2. For those suffering from acute decompensated heart failure, a severe health condition where the heart is weak and cannot pump enough blood to meet the body’s needs.
3. If the patient has symptomatic, severe aortic stenosis. This is a condition where the heart’s aortic valve narrows, preventing normal blood flow and causing symptoms like chest pain and shortness of breath.
4. If the patient has a heart rhythm disorder (arrhythmia) that is affecting their stability (hemodynamic instability), this procedure is not advisable.
5. Ongoing unstable angina also bars a patient from the test as this condition can cause heart-related chest pain.
6. Specific to dobutamine stress tests: this type of test isn’t suitable for those with hypertrophic obstructive cardiomyopathy (a disease in which the heart muscle becomes abnormally thick) and a history of glaucoma (a condition that damages the optic nerve due to high eye pressure).
Equipment used for Cardiac Stress Imaging
The standard equipment used includes:
1. An ultrasound machine with a special feature called Doppler. This machine is used for creating a picture of the heart, known as an echocardiogram.
2. Alternatively, a doctor might use a SPECT scanner or a PET scanner. These special machines are used to create detailed images of the heart’s activity, a test known as Myocardial Perfusion Imaging or MPI.
3. A twelve-lead EKG and telemetry system are used for constant heart monitoring, which means keeping an eye on the heart’s rhythm and activity.
4. A blood pressure cuff, which can be manual or automatic, is used to check your blood pressure.
5. A treadmill or a stationary bike could be used if the doctor wants you to exercise a little during the test.
6. Also, a system for delivering medication directly into a vein (an intravenous medication system) might be used, as well as the appropriate medications. This would be the case if the test can’t be done while you exercise, and a medication is used to make your heart work harder instead.
7. If a MPI test is conducted, an intravenous medication system will be used along with a radioactive tracer. The tracer helps create a clear picture of how well blood is flowing to your heart.
8. Lastly, in case of emergencies, there would be emergency equipment like carts with life-saving equipment (crash carts) and machines used to restore normal heart rhythm (defibrillators) nearby.
Who is needed to perform Cardiac Stress Imaging?
Imaging tests to check for stress, like those that monitor the heart’s activity during exercise, are often done by healthcare providers who have special training in these areas. This could be a doctor, or it could be a nurse, exercise specialist, or medical technologist, who all work under the guidance of a doctor. The people who actually take these images are usually sonographers or nuclear technicians. They are the experts in using the machines that capture these images.
A cardiologist, which is a heart doctor who has specific training in echocardiography (images of the heart’s chambers and valves) and/or nuclear imaging (uses radioactive substances to see organs), will review these images. This specialist interprets what the images show, meaning they help explain what is seen in the images and what it might mean for your health.
Preparing for Cardiac Stress Imaging
Doctors usually give certain instructions to patients before a stress test. One common rule is that patients should not eat or drink anything for 4 to 8 hours before the test. This helps ensure accurate results. For a specific type of stress test called a nuclear MPI regadenoson stress test, patients should also avoid caffeine and a type of medication called phosphodiesterase inhibitors for 48 hours before the test. In addition, patients shouldn’t take beta-blockers, a type of heart medication, on the day of a different kind of stress test called a dobutamine stress echocardiogram.
How is Cardiac Stress Imaging performed
Cardiac stress testing is a set of procedures that helps doctors see how your heart works when it’s under stress, such as during physical exercise. There are three main types of tests. All of these primarily aim to monitor how your heart functions during rest and exercise.
Exercise Tolerance Testing (ETT)
This test uses a widely applied exercise routine, often known as the Bruce protocol. This involves gradually increasing your walking speed and the incline of a treadmill every three minutes. The aim is to get your heart working up to 85% of its maximum possible rate. Your maximum possible heart rate is calculated as 220 minus your age. Throughout the test, the electrical activity of your heart and your body’s response to the exercise are monitored using an electrocardiogram (EKG), which is a device that records the heart’s electrical signals.
Stress Echocardiography (Echo)
This test is usually achieved by making your heart work harder either through exercise or by giving you a medication called dobutamine, which increases your heart rate. If the dobutamine is used, the dose will be increased gradually every three minutes until your heart reaches 85% of its maximum rate. Possible side effects of dobutamine include chest pain, nausea, vomiting, and heart rhythm abnormalities. The way your heart looks while at rest (called a baseline), during exercise, and within a minute after reaching your target heart rate is then examined using a heart ultrasound called an echocardiogram.
Stress Myocardial Perfusion Imaging (MPI)
Again, stress on the heart in this test can be generated either through exercise or by using a medication called regadenoson. This medication is given to you as an injection over 10 seconds. You might experience bronchospasm (tightening of the airways), flushing, headache, nausea, or chest discomfort from the medication. After creating stress on your heart, the blood flow to your heart is then visualized by injecting small amounts of radioactive substances and capturing images of your heart. This test may be spread over two days for people who are overweight.
Possible Complications of Cardiac Stress Imaging
Problems can sometimes happen during a cardiac stress test, which is a test to check how your heart works when you’re exercising. Some of these problems include:
1. Cardiopulmonary arrest: This is a serious event where the heart and lungs suddenly stop working.
2. Myocardial infarction: This is the medical term for a heart attack, where part of the heart doesn’t get enough blood and is damaged as a result.
3. Hypotension/Hypertension: These terms refer to unusually low (hypotension) or high (hypertension) blood pressure.
4. Arrhythmias: These are abnormal heart rhythms, which means your heart beats too slow, too fast, or irregularly.
5. Bronchospasm: This refers to the narrowing of the airways in your lungs, which can make it hard for you to breathe.
6. Unexpected or allergic reactions to isotope/pharmacologic agent: Sometimes, the substances used in the test to help the doctor see how well your heart is working can cause an allergic reaction or other unwanted side effects.
What Else Should I Know About Cardiac Stress Imaging?
An Exercise EKG Stress Test, often referred to as a treadmill test, helps doctors understand how well your heart handles work. It makes use of an electrocardiogram or EKG examines heart rhythm, along with symptoms brought on by effort, blood pressure changes, and exercise capacity. It can signify blocked arteries, which can lead to heart disease, if you experience chest pain, low blood pressure, arrhythmias, or specific EKG changes during the test.
The Duke treadmill score (DTS) can summarize the results of the test by calculating your exercise time, ST deviation (changes in your EKG measurement), and exercise angina (chest pain during exercise). Your DTS score can determine whether you’re at low, moderate or high risk of a heart event in the next year.
Stress Echocardiography is a type of test where doctors observe your heart at rest and during exercise. If your heart muscle works less during exercise, it could show a lack of blood supply due to blockage. On the other hand, if it improves at minimal stress and worsens again at peak stress or improves continuously as stress levels increase, it may suggest two different types of reversible heart muscle weakness.
SPECT-MPI and PET-MPI stress tests involve injecting a small amount of radioactive substance to visualize blood flow in your heart muscles. They give an idea about how well your heart muscle uptake is and can identify if there is any reduction in blood flow during exercise, a fixed defect (scar tissue from a heart attack), or changes in wall motion, all of which can lead to heart diseases.
However, these tests come with some limitations. Certain heart conditions can lead to false positives and make it seem like there’s a problem when there isn’t. On the flip side, they can also miss signs of issues – false negatives, especially in older women with specific heart conditions. So, a positive test doesn’t always mean there’s a problem; conversely, a negative test doesn’t always mean you’re in the clear.
If you test positive, your doctor will talk you through your next steps. It might involve more testing, treatment options, or adjustments to your medication regime. The goal is to confirm the results and then make the best plan for your heart health.