Your doctor may order one or more heart tests as part of your cancer therapy. Some tests are performed to monitor the heart and may be ordered even though you feel well and have normal, reassuring assessments by your health care provider. Some tests are performed to investigate a symptom or finding during a medical assessment by your health care provider.
Each type of heart tests looks at a specific part of the heart. Different heart tests are used to answer different questions. For example, the pumping strength of the heart is measured using tests such as an echocardiogram (echo) or a multigated acquisition (MUGA) scan. A treadmill stress test looks for major blood flow problems during exercise. Some stress tests look at blood flow problems and pumping strength of the heart. An electrocardiogram (ECG) records the electrical signals in the heart. The following section discusses some heart tests in greater detail.
An echocardiogram is a heart test that uses ultrasound to take pictures of the heart. It takes approximately 45 minutes. During the test, a technologist applies some electrodes to the skin to measure the heart rate, and uses a small probe on the surface of your chest to take pictures of the heart.
The most common reason for your health care provider to order an echocardiogram is to measure the strength of the heart muscle by calculating an “ejection fraction”. Echocardiogram pictures also tell your doctor about other problems with the heart structures including: heart valves that can either leak or get stuck, fluid buildup around the heart, pressures inside the heart chambers, and whether any heart chambers are larger than normal.
An echocardiogram does not hurt, and usually does not require an intravenous injection. Some patients have “difficult echocardiogram pictures”, which means that something makes the pictures fuzzy. This can happen in patients who are overweight, have lung disease, or who have had surgery such as reconstruction of the left breast. In some of these cases, an intravenous catheter is placed and a medication called echocardiogram contrast (Definity) is given to improve picture quality. This medication is not X ray dye – it is a solution of microscopic bubbles that enhances the quality of echocardiograms. The solution disappears from your system within a few minutes of administration, and does not cause side effects.
The advantage of echocardiograms for measuring the strength of the heart is that they usually do not require an intravenous catheter, they do not require X ray dye or radiation, and they sometimes reveal important extra information about other parts of the heart (e.g. a leaky valve). The disadvantages of echocardiography are that if the pictures are difficult, the accuracy of the test can be poor, and even with perfect pictures echocardiography is not the most accurate test to measure the strength of the heart.
A normal ejection fraction on echocardiography is over 55 %. This number represents the fraction of blood that is pumped out to the body which each heartbeat. The accuracy of ejection fraction measured using echocardiography depends on picture quality. Patients with the best echocardiogram pictures can have an ejection fraction accurate to within 7 units, but this can fall to 15 units of variability when the pictures are difficult.
Multigated Acquisition Scan (MUGA Scan)
A MUGA scan is a heart test that measures the strength of the heart muscle. MUGA stands for “multiple gated acquisition”. Other terms for MUGA scans include left ventricular gated scan and radionuclide ventriculography – these all refer to the same test which measures how strong the heart muscle is.
A MUGA scan takes about 45 minutes. During a MUGA scan, a nuclear medicine technologist injects a small amount of radioactive material into your veins. Some electrodes are applied to the skin to measure the heart beat. You then lie under a camera. The camera takes pictures of your heart muscle and collects information about how strong each heart beat is. The camera collects as much information as it needs to get the most accurate ejection fraction possible. Sometimes, additional computer processing is performed on the pictures to look at “slices” of the walls of your heart to get extra information.
The advantage of a MUGA scan is that it is a very accurate test for ejection fraction, with an accuracy of within 5 units. MUGA scans are less affected by patient characteristics such as obesity and previous breast surgery, compared with echocardiograms. The disadvantages of a MUGA scan include the need for intravenous injections and exposure to radiation. The amount of radiation is equal to the radiation that we all get over a 2 year period of time (most of this comes from outer space and some naturally occurring radioactive rocks).
Another way of describing how much radiation you receive from one MUGA scan is that it is equal to one fifth of the annual amount of radiation that health care workers, such as a nuclear medicine technologist, are allowed to safely receive during a year. Due to the accuracy of ejection fraction measured by MUGA, they are commonly used to look for small changes in ejection fraction before and after chemotherapy.
An electrocardiogram (ECG) is a recording of the electrical signals in your heart. It tells your doctor if your heart has an abnormal rhythm. Electrocardiograms are also used to diagnose serious emergency situations such as heart attacks. Electrocardiograms do not tell us about the strength of your heart, and they cannot tell us if you have a narrowing in an artery.
A stress test is used to evaluate whether a symptom during exercise (such as shortness of breath) is coming from the heart, and is used to discover if you have narrowed or blocked arteries.
There are several types of stress tests. A treadmill test is the simplest and easiest. It involves walking at a progressively faster pace up a gradually increasing incline on a treadmill while a health care professional observes your symptoms, heart rate, blood pressure and an electrocardiogram. The advantage of a stress test is that it does not require an intravenous catheter, is widely available, inexpensive, and does not require contrast dye or radiation. The disadvantage is that it only has about a 65% accuracy for detecting blocked arteries. In spite of this relatively low accuracy number, stress tests are very good at “ruling out” narrowed arteries when the probability of artery problems is low.
Stress tests are also very good at detecting very serious artery problems. For milder or less serious artery problems, or if your doctor suspects that your abnormal stress test is a “false alarm”, a more sophisticated stress test (nuclear stress, stress echocardiogram, cardiac computed tomography) may be necessary.
Nuclear Stress Test
During a nuclear stress test, a patient exercises on a treadmill. At maximum exercise, a small dose of radioactive material is injected which then stays in the heart in a pattern of blood flow that tells us what your heart was doing at peak exercise. About 20-30 minutes later, you lie under a camera to take pictures of blood flow and heart muscle strength.
These stress pictures are compared to pictures of resting blood flow to identify blood flow problems that are a result of narrowed or blocked arteries. The test takes up to 2 hours for the stress pictures, and up to one hour for the rest pictures. Sometimes rest and stress pictures are done on separate days, versus all on the same day. The advantage of a stress nuclear scan is higher accuracy (80-90%) than a stress test. For patients who cannot exercise on a treadmill, it is possible to use a drug called persantine to accelerate blood in the heart in the same way as exercise, except you don’t have to exercise.
Nuclear material is then injected and stress pictures can be acquired in the same manner as an exercise nuclear test. A persantine nuclear stress test is commonly used to look for blood flow problems in patients who are unable to exercise on a treadmill. The disadvantage of nuclear stress testing is that it requires an intravenous catheter and also involves radiation. The amount of radiation is equal to the radiation that we all get over a 2 year period of time (most of this comes from outer space and some naturally occurring radioactive rocks). Another way of describing how much radiation you get from one nuclear stress test is that it is equal to one fifth of the annual amount of radiation that health care workers, such as a nuclear medicine technologist, are allowed to safely receive during a year.
Cardiac Computed Tomography (Cardiac CT)
Cardiac computed tomography involves administering X-ray dye and then taking a CT scan of the arteries of your heart. It can detect very early deposits of cholesterol in your arteries, even before the arteries have narrowed enough to cause heart symptoms or to show up on a stress test. The advantage of cardiac CT is that it detects even early cholesterol build up. The test has good overall accuracy (up to 90%). The disadvantage is that it involves radiation. The amount of radiation is equal to the radiation that we all get over a 2 year period of time (most of this comes from outer space and some naturally occurring radioactive rocks).
Another way of describing how much radiation you get from one cardiac CT scan is that it is equal to one fifth of the annual amount of radiation that health care workers, such as an X-ray technologist, are allowed to safely receive during a year. In addition, a Cardiac CT scan requires intravenous X-ray dye, which rarely causes an allergic reaction, and may be harmful to kidneys in patients with kidney disease or diabetes. Sometimes, if your heart beat is too fast, or even slightly irregular, a cardiac CT cannot be performed.
Stress Echocardiogram (Stress Echo)
During a stress echocardiogram, you perform exercise either on a treadmill or a bike, and pictures of your heart muscle are taken at peak exercise and compared to resting pictures. If the doctor observes part of your heart becoming weaker during exercise compared to rest, this signals a narrowing in the artery that supplies that part of the heart.
For patients unable to exercise, a drug called dobutamine can be given to make the heart pump faster and stronger. This permits a stress echocardiogram to be performed without having to exercise. The advantages of a stress echocardiogram include better accuracy (80-90%) than a stress test, no radiation exposure, and usually no intravenous catheter. The disadvantage is that sometimes the pictures can be fuzzy, which makes them difficult to interpret and may reduce the accuracy of the test.
Ejection Fraction (EF)
Ejection fraction is a common measurement of how strong your heart is. The normal heart fills up with about 100 milliliters of blood per beat, and pumps out about 60 milliliters per heart beat. That means that the normal ejection fraction is 60 / 100 = 60%. By studying tens of thousands of people using various heart tests, we have established normal values for ejection fraction.
These normal values vary slightly between different types of heart test. For example, a normal echo ejection fraction is over 55%, while most centers consider a normal MUGA ejection fraction to be 50%. We consider a heart to be mildly weak if the ejection fraction is 40% – 50% on a MUGA, or 45% to 55% on an echocardiogram. We consider an ejection fraction of 30%-40% on MUGA or 30% – 45% on an echocardiogram to be a moderately weakened heart muscle.
Some patients have ejection fractions below 30 % due to a severely weakened heart muscle. The accuracy of an ejection fraction can vary. For example, an ejection fraction that is 55% today and 60% tomorrow means that there is really no change in the heart – it is simply that ejection fraction varies with each heart beat, and varies day to day.
Your doctor will be able to tell you if you need to see a cardiologist based on a given ejection fraction number. In general, small changes in ejection fraction do not require any action. Large changes in ejection fraction (e.g. drops of 15% or more, especially when the ejection fraction has fallen below normal), usually require that you see a cardiologist.