Just because your heart condition turns out very different from what non-invasive tests suggest, it is not because your doctor did not do his best. By Michael Lim
WHEN I was working as a junior doctor in the 1980s, managing patients with heart disease meant taking a good history and listening carefully to the heart sounds and the chest with my stethoscope. Other than a 12 lead electrocardiogram (ECG) to try to determine whether there was damage to the heart, a chest X-ray to determine whether the heart was enlarged, and a treadmill ECG stress test, there were not many tests that we could do to assess the patient’s heart condition. Ward rounds were spent trying to determine who heard the abnormal heart sounds correctly, and whenever there was disagreement, the most senior physician always had the last say. Health care was inexpensive as we had very few diagnostic tools and many people who had a heart attack did not make it out of the hospital.
The late 1980s saw the availability of ultrasound tests that allowed us to see moving images of the heart and assess the heart function and the heart size more objectively. It mattered less as to what heart sounds you heard than what you saw on the ultrasound test, which was also called echocardiography.
There was also the availability of heart nuclear scans where a radioactive isotope was injected into the blood stream and taken up by the heart muscles. In areas of the heart muscle where there was insufficient blood flow or damaged heart muscle, there will be less uptake of the isotope on the scan. An abnormal test meant that it was likely that there was significant blockage of the heart (coronary) arteries. Then, finally, we had invasive coronary angiography which meant that we could see the heart arteries by inserting a tube into the patient’s groin artery and injecting a contrast dye into the heart arteries and take moving X-ray images.
The 1990s showed further advancements in non-invasive heart testing as doctors tried to use non-invasive tests to help them identify patients who were likely to have significant blockage of the coronary arteries and will be required to undergo invasive coronary angiography. Guidelines were developed to provide guidance to physicians to determine which patients will require coronary angiography. This was because coronary angiography was not only a relatively expensive procedure but it carried a real risk of stroke, heart attack and even death, although the risk was small.
Studies published on the various non-invasive tests including treadmill testing, stress echocardiography and nuclear scans of the heart showed different sensitivities, specificities and accuracies, varying from more than 70 per cent to more than 90 per cent accuracy depending on the modality of testing and the centre performing the tests.
On this basis, can we assume that a large majority of the patients who had an abnormal non-invasive test and had undergone coronary angiography will have significant blockage of the coronary arteries? Well, the answer is not as simple as it seems. In a large study of almost 400,000 patients from more than 600 US hospitals who had undergone non-urgent coronary angiography for suspected blockage of the coronary arteries that was published last year in the New England Journal of Medicine by Patel et al, only slightly more than one-third of the patients had significant blockage of the coronary arteries. Almost 40 per cent had no coronary artery disease. What was also very surprising was that for those patients who had an abnormal result on a non-invasive test, only 41 per cent had significant blockage of the coronary arteries. This was only slightly better than patients who did not undergo any testing prior to angiography where 35 per cent had significant blockage of the coronary arteries.
What this means is that based on the results of published data on the accuracy of non-invasive tests, we would have expected that the large number of patients with an abnormal non-invasive tests to have significant blockage of the coronary arteries. But in real world practice, the accuracy of non-invasive tests were very different from the more than 70 per cent to more than 90 per cent accuracy in published studies. Various factors such as experience, staff training and methodology vary from centre to centre and may account for these differences.
Most of these non-invasive heart tests are functional tests that help to predict the likelihood of coronary artery disease but although their accuracy has been demonstrated to be relatively lower in real world practice, they do still play a role in highly experienced centres. The main disadvantage of most of these tests is that they do not allow doctors to visualise the coronary arteries.
In the last few years, development in imaging technology has resulted in new computed tomography (CT) X-ray scanners that can image the coronary arteries within seconds. The newer generation CT scanners can scan the heart arteries without exposing the patient to the risk of complications of invasive coronary angiography and can complete the scan with lower X-ray radiation dose than invasive coronary angiography. Published data from high-volume centres show a high accuracy for detection of coronary artery disease. However, like any other tests, the accuracy varies from centre to centre.
More recently, other than invasive coronary angiography and CT scans which are able to provide visualisation of coronary arteries, imaging of the coronary arteries using magnetic resonance imaging (MRI) has been made possible with new generation heart MRI scanners. The main advantage of this new scanning technique is that there is no X-ray radiation and no injection of contrast is required.
The authors of the New England Journal of Medicine study, Patel et al, concluded that better strategies are needed for identifying patients who are likely to have significant coronary artery disease and to increase the diagnostic yield of coronary angiography in routine clinical practice.
Diagnostic testing using non-invasive techniques to detect significant blockage of the coronary arteries has evolved from functional testing, such as treadmill test, stress echocardiography and nuclear scans, to newer modalities that allow visualisation of the coronary arteries, such as CT scans and MRI scans. The application of technologies such as CT and MRI imaging of coronary arteries may help to address the need to reduce the incidence of coronary angiograms with normal coronary arteries or no significant blockage.
The next time you have a test for your heart, do remember that there is no test that is 100 per cent accurate all the time. All technologies, no matter how good or new they are, have their limitations. Even in a developed country like the US, in real world practice, accuracy of these non-invasive tests can be much lower than published data from experienced centres. These tests can only serve to help your doctor identify your likelihood of coronary artery disease and help him come to a decision on your management.
So the next time your doctor tells you that you have significant coronary artery disease on the basis of the results of non-invasive testing, and you are subsequently found to have either no significant disease or more severe disease than was originally thought, it is not because your doctor did not do his best but he was making his decisions on the basis of non-invasive test results which in this day and age continue to have their limitations and are definitely not 100 per cent accurate all the time.