More tests to diagnose coronary disease
Dateline: 07/20/97
In last week's article, we discussed the electrocardiogram (EKG) and angiogram as tests used to detect disease of the coronary arteries. This week, I will briefly explain some other tests which also are useful in diagnosing coronary artery blocks or disease.
STRESS EKG
While in the presence of severe disease the electrocardiogram may be abnormal even when recorded at rest, when the extent of coronary artery block is not so severe, the resting EKG may be perfectly normal. This does not mean that everything is fine.
How then can we test for mild disease, at an earlier stage of its evolution ?
When there is still a reasonable doubt of coronary atherosclerosis, the cardiologist recommends a stress test (Stress EKG). The reasoning behind a stress EKG is simple. In coronary atherosclerosis, the problem is due to the fact that a narrow coronary artery is not capable of delivering blood flow (with oxygen and nutrients) to the heart muscle. The amount of oxygen and blood flow the heart muscle requires depends directly on the amount of work it is called upon to do. So during exercise (stress) the heart requires more blood. When the coronary artery is normal, this increased requirement is met comfortably. In the presence of even mild disease, this increased need cannot be satisfied and as a result the heart becomes incapable of normal function. The EKG recorded at the time of this exercise thus becomes abnormal.
A stress EKG is recorded while the patient is being exercised by walking on a treadmill, or asked to climb up and down a set of two steps, or while pedalling a stationary bicycle. The entire test is performed under expert supervision, and is stopped if any symptoms or untoward complications develop during the test.
The EKG changes that develop during exercise are analyzed. The severity of these EKG changes, and the exercise level at which they first appear give an indication of the severity of coronary artery obstruction.
ECHOCARDIOGRAPHY
The echocardiogram is an ingenious imaging method that uses ultra-sonic waves (high frequency sound waves) to provide a picture of the heart. While currently available machines are not sensitive enough to actually image the coronary arteries, the echocardiogram can still be useful in detecting coronary disease. When the coronary branch providing blood flow to a segment of the heart muscle becomes blocked, that part of the heart cannot contract normally. This decreased contractility can be viewed on an echocardiogram, providing indirect evidence of a blocked artery. Other complications of coronary atherosclerosis, including ventricular aneurysms, ruptured mitral valve papillary muscle or interventricular septum, and global loss of contractility of the heart can all be detected on an echocardiogram.
RADIO-ISOTOPE PERFUSION SCAN
A radio-active isotope is one that emits radiation that can be detected using a sensor. The most common isotope used to measure coronary perfusion is Thallium. Newer alternatives are also available, and have specific advantages over Thallium. The radio-isotope perfusion scan involves the intravenous injection of the isotope, which then circulates in the blood stream and is distributed to the ventricle wall. If the coronary artery to a portion of the ventricle is blocked, that area will not receive Thallium, and will appear as a dark area on the scanned pictures. The speed at which the Thallium is then washed out from the ventricle gives yet further information about the coronary arteries being normal or narrowed from disease.
COMPUTERIZED TOMOGRAPHY (CT SCAN)
Computerized Tomography has now evolved to the level that it can be used to visualize the coronary arteries, and even pick up areas of disease or narrowing. While not yet widely used for this purpose, it has a small role in diagnosing heart disorders.
MAGNETIC RESONANCE IMAGING (MRI SCAN)
Recent modifications in the MRI equipment has now permitted visualization of coronary arteries, and even imaging of atherosclerotic plaques within it. With further improvements likely in the future, this may become a much more useful diagnostic test. Research work suggests that MRI may even be used to quantify blood flow within a coronary artery.
POSITRON EMISSION TOMOGRAPHY (PET SCAN)
A PET scan is unique in that in contrast to other tests that image the structure of the heart and arteries, it actually evaluates viability (or living status) of heart tissue. A PET scan does this by measuring metabolism of tissues, with only living tissue showing metabolic activity. It's advantage in coronary atherosclerosis is that it helps in deciding whether a particular badly diseased portion of the heart has tissue that might improve after surgery or angioplasty of the blood vessels to that area. However, it is not a routine test for this condition.
This week's article then concludes the tests used to diagnose coronary artery disease. From next week on, we will discuss the different treatment options available for narrowed coronaries.
I would greatly appreciate receiving feedback from you about this and other previous articles. Mainly, I wish to know if these notes are too technical and difficult to understand, and if they answered all your questions about the topic they address. If you have any comments or suggestions, please write back to me.
