The heart has its own natural pacemaker that controls the heartbeat. It consists of a special group of cells in the heart that emits small electrical impulses. These electrical impulses trigger the heart muscle to contract regularly, about 50 to 80 times a minute. With every heartbeat the blood is pumped out of the heart and into the arteries throughout the body. Our pulse, felt in the wrist or neck, is the blood pulsing through the body due to each heartbeat.
The electrocardiogram (ECG) is the machine that records the electrical activity of the heart as it beats. In a normal heart, the ECG will record a distinct, regular pattern of electrical impulses. However, this electrical pattern can change in certain ways that are characteristic of specific types of heart problems.
For example, problems with the heart's natural pacemaker may cause the heart to beat irregularly (an arrhythmia). This will appear as an irregular pattern on the ECG recording. A heart that beats too quickly or too slowly will also show up on the ECG.
Similarly, a heart attack and heart disease also cause certain distinct, abnormal electrical patterns that can be recognized on the ECG. When a person exercises on a treadmill while connected to an electrocardiogram, the physical exertion causes the heart to work harder. This increases the chance of detecting an abnormal ECG pattern that will lead to an accurate diagnosis or evaluation.
Although a stress test may sometimes miss the presence of an abnormality, the abnormalities that it does detect are very useful in providing an accurate diagnosis.