Electrocardiography is a technique to study your heart’s electrical activ- ity by recording the path of an electrical impulse from its origin in the sinoatrial node through your heart as it causes the heart to contract (see page 11). The printout of this activity, an electrocardiogram, is a graph of the electrical activity of each heartbeat over time and the rhythm of successive beats.

Electrocardiogram

The electrocardiogram (ECG) is a safe, inexpensive way to get a wealth of information: it tells your doctor about your heart rate and heart rhythm. The ECG may also suggest whether a heart attack has occurred and whether there are potential problems with blood supply to the heart. It is a routine, painless test.

What to Expect

You do not have to prepare in any special way to have an ECG, except perhaps to wear clothes that you can take off easily. The ECG may be done in your doctor’s of?ce or in a hospital. For the test, you may be asked to change into a hospital gown and sit or lie on an examina- tion table. In order to conduct the electrical impulse, electrodes will be attached to various parts of your body: your chest, back, wrists, and ankles. To ensure a good connection between your skin and each elec- trode, which is mounted on a sticky patch, the technician will clean these areas, perhaps shave areas of the chest on a man, and apply a conducting gel. Then he or she will hook up the electrodes and enter

some data into the electrocardiograph machine. You will not feel anything during the testing, which usually lasts a minute or less. There is no electrical energy being passed into your body, and there is no danger of electrical shock. The ECG simply records your heart’s activity.

What the Results Mean

In a healthy person, the electrical impulse during a heartbeat follows a regular, sequential path. The electrodes over different parts of your heart follow the path of the impulse and record it on the ECG. The most basic piece of information it gives is your heart rate, which is usu- ally measured by your pulse, but the ECG can give a more accurate rate if your pulse is unusually irregular or hard to feel. Normal heart rates range between 60 and 100 beats per minute.
The ECG also indicates your heart rhythm, which should be regu- lar. The test may reveal either an abnormally fast beat (tachycardia) or an abnormally slow one (bradycardia). It can also demonstrate an elec- trical blockage in the heart that alters the rhythm and causes an irregu- lar ECG tracing. Each type of arrhythmia causes a distinctive type of tracing pattern.
In addition, the ECG may tell whether you have had a heart attack, because damaged muscle or scar tissue doesn’t transmit the electrical impulse the same way as healthy tissue would. It can indicate approxi- mately where the damage is in the heart ventricle. Often the ECG reveals evidence of a past heart attack that you didn’t even know occurred. It can also indicate if you are having an attack during the test.
A component of the wave on the ECG can be affected by an inade- quate supply of blood or oxygen to your heart, particularly if the test is obtained during chest pain symptoms. Further tests may be necessary to determine why this is happening and under what circumstances.
The ECG can provide information about structural abnormalities, the effects of medications on the heart rhythm and electrical conduc- tion, hypertension, kidney problems, or hormonal problems that affect the wave pattern in speci?c ways. Although a normal ECG does not always exclude heart disease, it still is a reassuring ?nding. Also, if there is a heart problem, the ECG may give clues that indicate what type of testing is needed to further isolate and identify the problem.

Holter Monitoring

Because a conventional ECG records only a brief period (6 seconds) of your heart activity, a continual recording over a period of 24 hours or longer may be useful to identify changes in your heart’s rate or rhythm. To accomplish this form of ambulatory ECG, you will wear a battery-powered recording device called a Holter monitor. About the size of a small paperback book, the Holter monitor is portable enough to wear around your waist or on your belt. The monitor connects to electrodes placed on your chest via wires (leads) that pass under your clothes.
Being ?tted for a Holter monitor is a painless procedure. It is a good idea to bathe or shower before you go to your doctor’s of?ce, because you cannot get the monitor wet once you are wearing it. The technician will prepare your skin just as for an of?ce ECG. At least one electrode and lead may be taped down to secure it as you move around. You will wear the device usually for 24 hours, including while you sleep. You will also be asked to keep a log as you go about your usual day: what you were doing and whether you experienced any symptoms, and at what times. Every heartbeat will be recorded and analyzed for information.
After the designated monitoring period, you will return to your doc- tor’s of?ce to turn in the device. Having the electrodes removed might be uncomfortable, like tearing off a bandage. The tracings for the mon- itoring period will be analyzed, and correlations will be made between the Holter recording and the times of unusual symptoms or events in your log.

Event Monitoring

If you are having symptoms that are unpredictable or infrequent, you may have to use another ECG device called an event monitor or transtelephonic monitor, which records your heart rhythm. You are usu- ally asked to use this monitor for one month. You can take it off to bathe, or for other brief periods, but it’s best to wear it as often as possible. A small recorder is attached to a bracelet or ?nger clip. If you experience a symptom, such as light-headedness, you attach the recorder (if you are not wearing it) and push a button on the monitor that triggers a memory of what was recorded for several minutes before and after you pushed the button. This data can be transmitted over the telephone, or you can bring the monitor to the of?ce. This helps identify any rhythm disturbances that occur while you have symptoms. If a dangerous rhythm problem is identi?ed, you may be instructed to seek medical attention urgently.
Another type of event monitor, this one implantable, has been devel- oped to capture your heart’s activity during infrequent symptoms that occur only a few times a year. Called an implantable loop recorder, it is inserted in your chest, and you wear it for as long as 18 months.

Exercise Stress Testing

An exercise stress test is a continuous ECG that shows how your heart performs during exercise, when the body is demanding more blood and oxygen. It shows the adequacy of the blood sup- ply to the coronary arteries and how well the heart muscle functions. You also might hear it called a treadmill test, an exercise tolerance test, or an exercise ECG. It is a common diagnostic tool for detecting coronary artery disease and the origin of symptoms such as chest pain, because it shows whether the blood supply in the coronary arteries is reduced. It can identify a safe level of exercise for any heart patient, checks the effectiveness of medications, helps predict the risk of heart attack, and checks the effectiveness of procedures done to improve circulation in a person with coronary artery disease.
The test is designed to place stress on your heart—about as much as a fast walk or a jog up a hill in a carefully controlled environment with trained staff close at hand. During the test, the technician will carefully monitor your heart rate, breathing, blood pressure, heart rhythm, and how tired you feel.

Having an exercise stress test
An exercise stress test is a type of ECG that shows how the heart performs when you exercise. Usually the test is done while you walk on a treadmill, and then the speed or slope are gradually increased to make your heart work harder. The test is like a carefully supervised workout, with a warm-up, a gradual increase in the level of exercise, and a cool-down period. A doctor or technician will watch you closely throughout the test.

The most typical stress test is done by having you walk on a treadmill or ride a stationary bicycle. If the test shows that your heart doesn’t function normally during exercise, you may need to repeat the treadmill test combined with echocardiography  or nuclear tech- nology  to better identify the problem. Often these tests are done with the initial exercise stress test to improve the accuracy of diagnosis, especially in women. If you are unable to exercise because of illness, you may undergo a chemical stress test (see page 128), for which you will be given a drug that mimics some of the effects of exercise on your heart rate while ECG tracings or nuclear images are made.

What to Expect

You will be asked not to eat for 12 hours before the test, because a meal can make you uncomfortable or nauseous. You can drink a small amount of liquid such as water, but no beverage such as coffee, tea, soda, or chocolate that contains caffeine. Be sure to ask your doctor about any medications you take and whether you should stop taking them before the test. If you have diabetes, you will be given speci?c instructions about taking insulin.
A technician will prepare your skin for the placement of electrodes, similar to the preparation for a regular ECG. You will also be ?tted with a blood pressure cuff. A resting ECG will be taken before you start exercising, and then you will get on a treadmill or a bicycle. The ?rst 2 or 3 minutes you will exercise at a slow, warm-up pace. Then every 2 or 3 minutes, the speed or slope will be increased gradually to simulate going uphill. The doctor will probably encourage you to continue until you are too tired to go on, or until you have a symptom such as pain, dizziness, or shortness of breath. After this pro- cedure, you lie down or sit quietly for about 10 minutes. Your doctor or technician will monitor your heart and blood pressure throughout this period. He or she will ask you questions about how tired or out of breath you feel. If the ECG reveals any potential problems, the doctor or technician will ask you to stop exercising. After the test is complete, you can return to your normal day.

What the Results Mean

The doctor reading the ECG may be able to tell you preliminary results immediately, but a complete analysis will probably take several days. If the test shows that your heart functions normally during exer- cise, the results can be used to help you plan a ?tness program. If the results indicate that your heart functions abnormally during exercise, you may need to have more tests, such as an echocardiographic stress test  or a nuclear stress test, to determine more precisely where the blood supply is being blocked. On occasion, you may go straight to having an angiography . If you already have coronary artery disease, the test can reveal a new blockage or one that is worsening.
The choice of stress testing will depend in part on your medical history and your doctor’s preferences. Exercise stress testing is less specific—and therefore, less helpful—than thallium or echocardio- graphic stress testing; however, exercise stress testing is much less expensive and thus is often used as a ?rst step in screening for heart dis- ease. Those who already have heart conditions that may in?uence the ECG result may need nuclear stress testing. Echocardiographic stress testing may be better for women because women, especially young women, are prone to false positives on ECGs.