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Pick the best test for heart health
WITH increasing health consciousness, more people are getting assessed for heart disease. The electrocardiogram (ECG) remains the most common test to screen for heart disease. It is not uncommon for people to say that their ECG is normal and hence they have no heart disease. This is far from the truth.
Tests to assess the heart are usually non-invasive, which means these tests do not require the insertion of tubes for examination. Common outpatient non-invasive tests to assess the heart can be divided into two major categories; indirect tests and direct tests. Indirect tests allow assessment of the function of the heart without visualising the heart arteries. In these tests, doctors cannot see whether the heart arteries are blocked or not but the results allow them to gauge the severity of the blockage.
These tests include ECG, treadmill test and echocardiogram. Direct tests are tests which allow direct visualisation of the heart arteries. These non-invasive tests include computed tomography (CT) of the heart arteries (also called coronary computed tomography angiogram or CCTA) and magnetic resonance imaging of the heart arteries (also called coronary magnetic resonance angiogram or CMRA). These tests allow doctors to visualise images of the heart arteries and assess the severity of the heart artery disease. As non-invasive indirect tests are the most commonly performed of all heart tests, the first step towards you making the best decisions for your heart tests is to have a better understanding of these tests.
Nature of test: The ECG is a record of the electrical activity of the heart and routinely, electrodes will be applied to the chest wall and limbs. The standard ECG record will have electrical activity recorded in 12 positions, 6 in the vertical plane and 6 in the horizontal plane.
Basis of test: If there is damage to the heart muscle, inherited abnormality of the heart, swelling of heart chambers or abnormalities of the electrical circuit in the heart, there will be changes in the electrical pattern.
Understanding results: An abnormal pattern suggests that there may be an abnormality of the heart and further tests will be required for verification. Do not worry if the ECG shows an abnormal pattern as it does not imply that the heart is definitely abnormal; it just means that there is a higher likelihood of finding an abnormal heart.
Conversely, a normal ECG does not mean the heart is completely normal either, it just means that the likelihood of finding any abnormality is low. For example, if there is severe blockage of the heart arteries but no damage to the heart muscle, the ECG can still be normal.
Best for patient: A good ECG machine will reduce artifacts and provide software interpretation of the ECG recordings. Getting a heart specialist to confirm the ECG findings will be best.
Practical implications: If you are young, have no risk factors for heart disease and have no symptoms, a normal ECG will imply that you have a healthy heart.
Treadmill stress test
Nature of test: The ECG is recorded during the entire period of the test when the patient is on a treadmill, which will increase in speed and degree of inclination according to standard pre-set protocols, and the test will be stopped when fatigue sets in, when the target heart rate is reached or if the ECG pattern is abnormal.
Basis of test: The treadmill stress test is most commonly used to assess the likelihood of significant narrowing of heart arteries. During the test, the heart rate will increase with exercise and if there is insufficient blood flow to a segment of the heart muscle as a result of a blocked artery, there will be characteristic changes to the electrical pattern of the ECG as a result of the inability to provide sufficient oxygen to the affected heart muscle.
Understanding results: If the ECG pattern is abnormal, it suggests that there is a high likelihood that there is underlying blockage of the heart arteries. However, there can be false positive results where the ECG pattern is abnormal but there is no underlying blockage of heart arteries; this is more likely to be seen in those with no major risk factors for heart disease.
If the ECG pattern from the treadmill test is normal, it does not mean that there is no blockage of the heart arteries. It just means that the likelihood is low. For every 100 patients with severe blockage of one major heart artery who undergo treadmill testing, only about 60 will have an abnormal ECG results.
Best for patient: Have your treadmill test done in a centre where there is a modern ECG treadmill machine, proper emergency equipment available and, where possible, a heart specialist is present. Patients can develop life-threatening heart rhythms, a heart attack or severe drop in blood pressure during a treadmill test - it is imperative that trained staff and emergency equipment can deal with any event.
Practical implications: If you have no major risk factors and are able to reach Stage 4 of a standard protocol ECG treadmill stress test without chest tightness or abnormal ECG changes, it is usually unnecessary to do further testing.
If you have a family history of heart disease and/or have risk factors for heart disease, and have chest tightness or abnormal ECG changes during treadmill testing, you will usually need further tests to confirm severe heart artery disease.
Nature of test: The echocardiogram or "echo" is an ultrasound scan of the heart which provides two- or three-dimensional images of the heart and physiological information such as pressure readings and flow pattern across valves.
Basis of test: The test uses sound waves to construct images of the heart and Doppler waves to calculate pressure and flow data. The test is usually performed using a transducer ultrasound probe placed on the chest with real-time images on a screen. Sometimes, the test is combined with a treadmill test or infusion of dobutamine (dobutamine stress echo) to increase the heart rate.
During a stress echo, if the heart artery is severely blocked, the affected heart muscle will not be able to contract as well as normal segments of the heart muscle when the heart rate is high. This is because the oxygen supply cannot meet the demand during faster heart rate situations when the blocked artery restricts the flow of blood to the affected heart muscle segment.
Understanding results: If the heart muscle cannot contract normally during the stress echo, it is very likely that there is severe blockage of the heart artery. A normal stress echo suggests that the presence of blocked arteries is unlikely. However, studies have shown that for patients with severe blockage of the left heart artery, stress echo may only be abnormal in about 60 per cent of the patients. A routine echo without the stress component can provide information on the size of heart chambers, function of valves, heart pump function, and structural abnormalities of the heart.
Best for patient: The echo findings are dependent on the practical experience of the technologist performing the echo, and hence a highly experienced technologist in a high-volume centre is more likely to be able to provide better images.
Practical implications: An echo provides structural and functional information of the heart and is an extremely safe test. If you have multiple risk factors and have an abnormal stress echo, further testing is advised to confirm the presence of heart artery blockage.
Nuclear stress test
Nature of test: A nuclear stress test involves the injection of a radioactive isotope to assess the blood flow to the heart muscle at rest and during exercise. Nuclear stress tests include single photon emission computed tomography (SPECT) scan or myocardial perfusion scan using MIBI (Technetium 99m sestamibi) or thallium isotopes.
Basis of test: The radiation emitted by radioactive isotopes injected through the arm veins is recorded when the isotopes are distributed throughout the heart muscle at rest and at stress. If the distribution of the isotope is decreased at a particular segment of the heart muscle at stress but returns to normal at rest, it suggests that there is impaired blood flow and hence, by inference, there is presence of a significant heart artery blockage.
Understanding results: An abnormal result implies blockage of heart artery but false positive results can occur especially in women. If there is good collateral blood flow from other arteries to the area of the heart muscle which is supplied by a narrowed artery, the cross flow may be sufficient to ensure that there is even distribution of the isotope. In this situation, you will get a false negative result.
Best for patient: Young people and women of child-bearing age should avoid this test as it involves exposure of the entire body to radiation as the radioactive isotope travels throughout the blood stream. Have a discussion with your doctor about safer alternatives and the amount of radiation exposure.
The radiation from a full thallium scan can be as high as about six times the background radiation an average person is exposed to annually. If the test is ordered, you should discuss with your doctor about having the test done with the lowest radiation possible.
Practical implications: This is a functional test and does not enable visualisation of the heart artery. Hence, if the test result is abnormal, a test to visualise the heart artery should be considered.So the next time you want to get your heart checked, find out what is included in the heart tests provided by the centre and you can discuss with your doctor as to what is best for you. If any of these non-invasive tests are abnormal, you will need a test to visualise the heart arteries. This will be discussed in the next article.
- This is the first of two parts on Best Test For The Heart. .Dr Lim is medical director at the Singapore Heart, Stroke & Cancer Centre. He is a professor at Fudan University, Shanghai and vice-president of both the World Chinese Doctors Association and the World Federation of Chinese Cardiovascular Physicians. He is also founding editor of Heart Asia, a journal of the British Medical Journal Publishing Group
- This series is brought to you by Heart, Stroke and Cancer Centre. It is produced on alternate Saturdays.
- To read more of Dr Lim's previous health-related articles, visit: http://www.btinvest.com.sg/specials/shscc