Tests to detect coronary atherosclerosis
Dateline: 07/13/97
Last week, we discussed the disease process called atherosclerosis, which causes hardening, thickening and narrowing of arteries including the coronary arteries, and took a peek at how this may affect one's health. In this article, I will try to explain the two major tests used to detect disease in the coronary arteries - the Electrocardiogram (EKG) and the Coronary Angiogram
ELECTROCARDIOGRAM (EKG)
The electrocardiogram is also called the EKG or ECG. It is nothing but a graph that records electrical activity in the heart.
"What ! Does my heart have an electric current running through it ?"
It sure does ... but a very weak one ! In earlier articles, I have written about how the heart "contracts" and pumps blood to the rest of the body. This contraction is started off by a weak electric current. This current begins in a part of the heart called the pacemaker (or sino-atrial node). It is named so because it "sets the pace" for the heart to beat. From the pacemaker, normally, this current follows a well-defined path through the rest of the heart. This movement can be recorded by "electrodes" - which are simply metal plates placed on the chest wall and limbs to detect current flow inside the heart. The graph recorded in this way is the EKG.
How is the EKG recorded ?
The EKG is an absolutely painless test. The nurse or doctor or technician who records the EKG ties the "electrodes" (or pastes them) onto both wrists and ankles, and on the chest, after first applying "jelly" to the site. Just as in an Echocardiogram, the jelly helps better conduct the electric current in the heart to the machine for recording. By choosing a specific set of electrodes, the technician records the electrical activity from specific portions of the heart. The entire process only takes a few minutes, or maybe even less !
What does the EKG tell my doctor ?
The EKG is not specific for detecting any disease by itself. But when viewed along with the other tests like an X-ray and Echocardiogram, the EKG can be very informative. Mainly, the EKG helps detect disturbances in the pattern of electrical activity in the heart - a condition called arrhythmia. The EKG also tells the doctor if any chamber of the heart is abnormally huge or thick walled.
In coronary atherosclerosis, the reduced blood flow to heart muscle can cause a variety of changes in the EKG, depending on the location, number and severity of arterial blocks. Many criteria are combined to make a diagnosis of coronary artery obstruction based on the EKG.
CATHETERIZATION
While the EKG is a "non-invasive" test - that is, there are no injections or intravenous tubes to be inserted - the catheterization test, however, is "invasive".
Catheterization is usually done under "local" anesthesia - in other words, some medicine is injected in the groin (or fore-arm), which makes the skin numb and pain-free. Sometimes in very small children, it may have to be done under general anesthesia - or "total unconsciousness". A special thin plastic tube, called a catheter, is threaded into an artery in the groin (or fore-arm) after a needle-stick. The catheter is then guided into the heart. The pressure of the blood inside the different heart chambers is measured. The amount of oxygen - the life-giving gas in air - that is present in the blood is also measured. Then an angiogram is performed.
What is an angiogram ?
It is a special kind of X-ray of the heart taken after injecting a dye into the coronary arteries. This dye is "opaque" to x-rays, and does not let them through, casting a shadow. Since the dye is injected inside the blood vessel, the shadow that is cast will be in the shape of the artery ! By studying this shadow, the cardiologist or surgeon is able to get an idea of the nature of the arteries, locate and grade severity of coronary artery blocks due to atherosclerosis and make decisions about the need for surgery or angioplasty and even plan the nature of these interventions. In a fanciful way, a coronary angiogram may be considered the "road map" of the coronaries !
While the angiogram is an extremely useful test to evaluate the coronary arteries for disease, there are some risks involved in the procedure. Some people have an allergy to the dye used in angiography. Rarely, catheterization can cause irregularity in heart beats, injury to the heart chambers or sudden worsening of coronary artery disease. In extremely rare instances, particularly in very sick patients, death has occurred.
Before a patient undergoes a catheterization and angiogram, a detailed discussion with the attending physician is helpful. Any questions about the procedure must be answered, and the risks to a particular patient could be explained and understood. The aim is to have a well-informed, calm and quiet patient coming to the catheterization laboratory.
In next week's feature, we'll discuss two other major tests for coronary atherosclerosis - the Stress or Exercise EKG and the Radio-isotope Perfusion scan, and take a brief look at newer alternative tests for this condition. If you have any comments about this week's feature, or would like to suggest any other topics you'd like to see on this column, please drop me an e-mail.
